Co-reporter:Mengmeng Dong, Jun Gao, Chengbu Liu, and Dongju Zhang
The Journal of Physical Chemistry B November 9, 2017 Volume 121(Issue 44) pp:10276-10276
Publication Date(Web):October 12, 2017
DOI:10.1021/acs.jpcb.7b07191
To illustrate the formation mechanism of normal and abnormal N-heterocyclic carbene–carbon dioxide adducts (NHC–CO2 and aNHC–CO2), we implement density functional theory calculations on the reactions of two imidazolium-based ionic liquids ([C2C1Im][OAc] and [C2C1Im][CH3SO3]) with CO2. The reaction of [C2C1Im][OAc] with CO2 is mimicked using the gas phase model, implicit solvent model, and combined explicit–implicit solvent model. In the gas phase, the calculated barriers at 125 °C and 10 MPa are 12.1 kcal/mol for the formation of NHC–CO2 and 22.5 kcal/mol for the formation of aNHC–CO2, and the difference is significant (10.4 kcal/mol). However, the difference becomes less important (1.5 kcal/mol) as the solvation effect is considered more realistically using the combined explicit–implicit solvent model, rationalizing the experimental observation of aNHC–CO2 adduct in the [C2C1Im][OAc]–CO2 system. The anion of the ionic liquid is shown to play a substantial role, which can adjust the reactivity of imidazolium cation toward CO2: upon replacement of the basic [OAc]− anion with a less basic [CH3SO3]− anion, the reaction becomes very difficult, as indicated by high free energy barriers involved (41.4 kcal/mol for the formation of NHC–CO2 and 39.2 kcal/mol for the formation of aNHC–CO2). This is in agreement with the fact that neither NHC–CO2 or aNHC–CO2 is formed in the [C2C1Im][CH3SO3]–CO2 system, emphasizing the important dependence of the reactivity on the basicity of the anion of imidazolium-based ionic liquids for the formation of NHC– and aNHC–CO2 adducts.
The Journal of Physical Chemistry A February 9, 2017 Volume 121(Issue 5) pp:1133-1139
Publication Date(Web):January 10, 2017
DOI:10.1021/acs.jpca.6b11610
To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from N-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of N-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) IL. The reaction is shown to proceed via an SN2 transition state with a free energy barrier of 34.4 kcal/mol in the gas phase and 27.6 kcal/mol in toluene solvent. The reaction can be remarkably promoted by the presence of ionic products and water molecules. The calculated barriers in toluene are 22.0, 21.7, and 19.9 kcal/mol in the presence of 1–3 ionic pairs of [Emim]Cl and 23.5, 21.3, and 19.4 kcal/mol in the presence of 1–3 water molecules, respectively. These ionic pairs and water molecules do not participate directly in the reaction but provide a polar environment that favors stabilizing the transition state with large charge separation. Hence, we propose that the synthesis of imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons is an autopromoted process and a polar microenvironment induced reaction, and the existence of water molecules (a highly polar solvent) in the reaction may be mainly responsible for the initiation of reaction.
Co-reporter:Yaru Jing, Rongxiu Zhu, Chengbu Liu, and Dongju Zhang
The Journal of Organic Chemistry December 1, 2017 Volume 82(Issue 23) pp:12267-12267
Publication Date(Web):October 30, 2017
DOI:10.1021/acs.joc.7b02102
DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of α-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazolium-based ionic liquid (IL). The two catalytic reactions involve an intrinsically consistent molecular mechanism: a rate-determining, concerted nucleophilic substitution followed by a facile proton-transfer process. The histidine derivative or the IL anion is shown to play a decisive role, acting as a Brönsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated free energy barriers of two reactions (15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction) together with their respective characteristic features, the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL, rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time.
The Journal of Physical Chemistry C April 16, 2009 Volume 113(Issue 15) pp:6215-6220
Publication Date(Web):2017-2-22
DOI:10.1021/jp8108489
While nanoscale gold particles show exceptional catalytic activity toward the water−gas shift (WGS) reaction, not much is known about the detailed reaction mechanism and the influence of charge state of Au nanoparticles on the reactivity. We here report a systematic theoretical study by carrying out density functional theory calculations for the WGS reaction promoted by cationic, neutral, and anionic Au dimers, which represent three simplest prototypes of Au nanoparticles with different charge states. The reaction mechanism is explored along two possible entrances: one involves the complexes of the dimers with CO and the other is related to the complexes of the dimers with H2O. In all cases, it is found that the catalytic cycle proceeds via the formate mechanism and involves two sequential elementary steps: the rupture of the O−H bond in H2O and the formation of H2 molecule. The calculated results show that the reaction mediated by Au2+ is energetically most favorable compared to those promoted by Au2 and Au2−, indicating that the charge state of Au dimers plays an essential role for the catalyzed WGS. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic WGS by Au-based catalysts.
•The experimentally proposed carbonyl insertion mechanism is not favorable.•The solvent has decisive influence on the hydrogenation mechanism.•The substituent group on an ester has little influence on the mechanism.•The substituent group on an ester greatly affects the reactivity.Density functional theory (DFT) calculations have been implemented to clarify the hydrogenation mechanism of a non-active ester: ethyl benzoate (PhCOOC2H5) to ethanol (C2H5OH) and benzyl alcohol (PhCH2OH) catalyzed by RuIIPNN. The calculated catalytic cycle involves two similar hydrogenation processes with the persistent participation of the catalyst: hydrogenation of PhCOOC2H5 to C2H5OH and intermediate PhCHO, and further hydrogenation to PhCH2OH. Three potential mechanisms, named as carbonyl insertion mechanism, stepwise double-hydrogen-transfer mechanism, and direct reduction mechanism, have been investigated in details in different solvents. It is found that the solvent has decisive influence on the hydrogenation mechanism. In a less polar solvent, the reaction prefers a stepwise double-hydrogen-transfer hydrogenation mechanism consisting of dihydrogen activation, stepwise double-hydrogen-transfer, and hydrogen abstraction rather than the carbonyl insertion mechanism proposed in the experimental work. However, in a more polar solvent, the reaction proceeds via the direct reduction mechanism involving the separated ions (PhCH(OC2H5)O− and monocation) instead of the experimentally proposed hemiacetal intermediate PhCH(OH)OC2H5. The dihydrogen activation as common starting point of the reaction in all three potential mechanisms can be facilitated by the products (C2H5OH and PhCH2OH). The substituent group on an ester has little influence on the reaction mechanism while it greatly affects the reactivity: the electron withdrawing group favors the hydrogenation, while the electron donating group makes the reaction more difficult. These theoretical results are expected to provide valuable guidance for the experimental study of the hydrogenation of non-active esters.Download full-size image
•The adsorption of TCDD on the pristine and Ni-doped BNNTs is investigated by DFT calculations.•The Ni-doped BNNTs present much higher reactivities toward TCDD than the pristine BNNTs.•The doping of Ni remarkably improves the electronic property of the BNNT.•Ni-doped BNNTs are expected to be potential materials for detecting or removing TCDD.Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic to humans and the environment. Developing efficient methods to detect or remove these pollutants is particularly important and urgent. Boron nitride nanotubes (BNNTs) with low dimension and high surface-to-volume ratio might be one of promising materials for the adsorption of PCDD/Fs. Here we present a density functional theory (DFT) study on the interaction of the pristine and Ni doped (8,0) single-walled BNNTs with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener among PCDD/F family. The calculated results show that the pristine BNNT intrinsically interacts with TCDD via physisorption with π-π stacking interaction, in contrast, the Ni-doped BNNT presents much higher reactivities toward TCDD. The impurity Ni atom plays a crucial role for capturing TCDD molecule. We also find that the Ni doping introduces the local electronic states within the band gap of the BNNT and induces magnetism in the doped systems. The present results are expected to provide useful guidance for the potential application of BNNTs as adsorption materials for detecting or removing dioxin pollutants.The Ni-doped BNNT presents stronger interaction toward TCDD pollutant than the pristine BNNT and is expected to be a potential novel resource for detecting or removing dioxin.Download high-res image (254KB)Download full-size image
The Journal of Physical Chemistry B 2017 Volume 121(Issue 9) pp:
Publication Date(Web):February 14, 2017
DOI:10.1021/acs.jpcb.6b11820
To better understand the efficient transformation of glucose to fructose catalyzed by chromium chlorides in imidazolium-based ionic liquids (ILs), density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl2 in 1,3-dimethylimidazolium chlorine (MMImCl) ionic liquid (IL). The reaction is shown to involve three fundamental processes: ring opening, 1,2-H migration, and ring closure. The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol. Throughout all elementary steps, both CrCl2 and MMImCl are found to play substantial roles. The Cr center, as a Lewis acid, coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate, and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property, while the Cl– anion is identified as a Bronsted/Lewis base and also a H-bond acceptor. Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of O2–H hydroxyl group, which is in nature different from the stepwise mechanism proposed in the early literature. The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs.
New Journal of Chemistry (1998-Present) 2017 vol. 41(Issue 17) pp:8714-8720
Publication Date(Web):2017/08/21
DOI:10.1039/C7NJ00878C
This work presents a density functional theory (DFT) study on the molecular mechanism for conversion from 5-hydroxymethylfurfural (HMF) to levulinic acid (LA) catalyzed by a representative of SO3H-functionalized imidazolium-based ionic liquids (ILs), 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4). The calculated overall barrier of the catalytic conversion is 34.7 kcal mol−1, which is compatible with the efficient transformation of 5-HMF into LA in the IL under mild heating conditions. The present calculations emphasize the dependence of the catalytic performance of the IL on the acidity and nucleophilicity of its constituent ions.
Density functional theory calculations have been performed to understand the intriguing experimental observations on the hydration of propargylic alcohols to α-hydroxy ketones catalyzed by task-specific ionic liquids (ILs) and CO2. Focusing on a representative propargylic alcohol, 2-methylbut-3-yn-2-ol, we explored its hydration mechanism and the catalytic reactivities of different ILs towards the reaction in detail. The calculated results show the electrostatically controlled character of the reaction, where the reactivity depends on not only the anion's own nature but also its counterion cation that can regulate and control the anion basicity via electrostatic and H-bonding interactions. The reaction is proposed to proceed via an energetically viable mechanism that features the initial addition of CO2 to the hydroxyl group of the propargylic alcohol with assistance of the IL anion as a proton acceptor. The different catalytic performances of several ILs are attributed to their different proton-accepting capabilities. The best catalytic performance of [Bu4P][Im] is ascribed to its most efficient proton-accepting properties. The theoretical results provide a foundation for exploiting the controlled synthesis of α-hydroxy ketones as well as cyclic carbonates and oxazolidinones from the hydration of propargylic alcohols or propargylic amines.
Co-reporter:Jingjing Li, Jinghua Li, Dongju Zhang, and Chengbu Liu
ACS Catalysis 2016 Volume 6(Issue 7) pp:4746
Publication Date(Web):June 13, 2016
DOI:10.1021/acscatal.6b00564
DFT calculations have been performed to gain insight into the mechanism of formic acid (HCOOH) decomposition into H2 and CO2, catalyzed by a well-defined bifunctional cyclometalated iridium(III) complex (a Ir–H hydride) based on a 2-aryl imidazoline ligand with a remote NH functionality. It is shown that the reaction features the direct protonation of the Ir–H hydride by HCOOH with the hydrogen shuttling between the NH group and the carbonyl group of HCOOH. Importantly, the simultaneous presence of two HCOOH molecules is proposed to be important for the dehydrogenation, where one works as a hydrogen source and the other acts as a hydrogen shuttle to assist the long-range intermolecular hydrogen migration. The dehydrogenation mechanism is referred to as HCOOH self-assisted concerted hydrogen migration. With such a mechanism, the energetic span, i.e. the apparent activation energy of the catalytic cycle, is calculated to be 17.3 kcal/mol, which is consistent with the observed rapid dehydrogenation of HCOOH under mild conditions (40 °C). On one hand, the effectiveness of the self-assisted catalytic system is attributed to the d–pπ conjugation between the Ir center and the proximal nitrogen, which increases the electron density at the Ir center and hence promotes the Ir–H bond cleavage. On the other hand, the effectiveness is also closely related to the hydrogen-shared three-center–four-electron (3c-4e) bond between formate and formic acid, which stabilizes the transition states and hence reduces the free energy barriers of the reaction. In addition, calculated results also emphasize the importance of the concerted catalysis of the bifunctional catalyst: when the γ-NH functional group does not participate in the reaction or is replaced by an O atom, the reaction becomes remarkably less favorable. The present work rationalizes the experimental findings and provides important insights into understanding the catalysis of the bifunctional cyclometalated iridium(III) complexes.Keywords: dehydrogenation of HCOOH; DFT; hydrogen shuttle; iridium catalysis; metal−ligand cooperation; self-assisted concerted mechanism
Co-reporter:Xueli Mu, Xiaodeng Yang, Dongju Zhang, Chengbu Liu
Carbohydrate Polymers 2016 Volume 146() pp:46-51
Publication Date(Web):1 August 2016
DOI:10.1016/j.carbpol.2016.03.032
•DFT calculations illustrate the mechanism of the graft reaction of chitosan with the epoxy compound.•The polar environment of [Amim]Cl ionic liquid favors the graft reaction.•The ionic liquid also serves a catalyst of the reaction to promote the ring-opening of the epoxy compound.The molecular mechanism of the graft reaction of 2,3-epoxypropyl-trimethyl quaternary ammonium chloride with chitosan monomer was investigated by performing density functional theory (DFT) calculations. The calculated results show that the −NH2 group of chitosan monomer is more reactive than its −OH and −CH2OH groups, and the graft reaction on the −NH2 group is calculated to be exothermic by 20.5 kcal/mol with a free energy barrier of 42.6 kcal/mol. The reaction cannot benefit from the presence of the intruded water molecule, but can be considerably assisted by 1-allyl-3-methylimidazolium chloride ([Amim]Cl) ionic liquid. The reaction catalyzed by the ion-pair is calculated to be exothermic by 36.5 kcal/mol and the barrier is reduced to 29.3 kcal/mol, which are further corrected to 28.0 and 29.1 kcal/mol by considering the solvent effect of [Amim]Cl ionic liquid. Calculated results verified the experimental finding that imidazolium-based ionic liquids can promote the reaction of chitosan with epoxy compounds.
Co-reporter:Yingying Wang, Peng Liu, Dongju Zhang, Chengbu Liu
International Journal of Hydrogen Energy 2016 Volume 41(Issue 18) pp:7342-7351
Publication Date(Web):18 May 2016
DOI:10.1016/j.ijhydene.2016.03.116
•The possible pathways of HCOOH decomposition on PdAg(111) surfaces are investigated.•HCOOH preferentially decomposes to CO2 regardless of without or with the presence of water.•PdAg catalysts show high catalytic activity and tolerance to CO poisoning toward HCOOH decomposition.A deep knowledge about the mechanism of formic acid (HCOOH) decomposition on Pd-based materials is of fundamental importance to structural designs of efficient catalysts used in direct formic acid fuel cells (DFAFCs). This work presents a theoretical study of the mechanism of HCOOH decomposition on the PdAg(111) surface with the absence and presence of water molecules. The calculated results show that HCOOH preferentially decomposes to CO2 regardless of without or with the presence of water. The energy barrier difference of the rate-determining steps for the formations of CO2 and CO on PdAg(111) surface is found to be much larger than that on monometallic Pd(111) surface. The theoretical results indicate that bimetal PdAg(1111) surface can suppress formation of CO, which rationalize well the experimental observation that PdAg bimetal catalysts exhibit improved tolerance toward CO poisoning for HCOOH decomposition.
Co-reporter:Likai Du, Cuihuan Geng, Dongju Zhang, Zhenggang Lan, and Chengbu Liu
The Journal of Physical Chemistry B 2016 Volume 120(Issue 27) pp:6721-6729
Publication Date(Web):June 8, 2016
DOI:10.1021/acs.jpcb.6b04218
A fundamental understanding of the structural heterogeneity and optical properties of ionic liquids is crucial for their potential applications in catalysis, optical measurement, and solar cells. Herein, a synergistic approach combining molecular dynamics simulations, excited-state calculations, and statistical analysis was used to explore the explicit correlation between the structural and optical properties of one imidazolium amino acid-based ionic liquid, 1-butyl-3-methylimidazolium glycine. The estimated absorption spectrum successfully rationalizes the unusual and non-negligible absorption band beyond 300 nm for the neat imidazolium-based ionic liquid. The absorption behavior of imidazolium-based ionic liquids is shown to be sensitive to the details of their locally heterogeneous environments. We quantitatively highlight the imidazolium moiety and its various molecular aggregations, rather than the monomeric imidazolium moiety, that are responsible for the absorption characteristics. These results would improve our understanding of the preliminary interplay between structural heterogeneity and optical properties for neat imidazolium-based ionic liquids.
The calculated results in the literature for the decomposition of formic acid into CO on Pt(111) cannot rationalize the well-known facile CO poisoning of Pt-based catalysts. The present work reexamines formic acid decomposition on Pt(111) by considering both the monomer and dimer pathways both in the absence and in the presence of water molecules. Upon a thorough search, we locate some new adsorption configurations of formic acid for the subsequent C–H or O–H cleavage, which were ignored in previously published literature. From the calculated minimum energy pathways (MEPs), we propose that formic acid decomposition on Pt(111) is initiated by C–H bond cleavage rather than O–H bond cleavage. Monomeric formic acid preferentially decomposes to CO2 regardless of the absence or presence of water, while the dimeric form favors the formation of CO in the gas phase, but competitively reacts to form CO and CO2 in the presence of water molecules. The surrounding water molecules and the second formic acid molecule in the dimer play substantial roles in the process of formic acid decomposition, and can be regarded as promoters, assisting formic acid decomposition on Pt(111). In all situations, that is, regardless of the presence of a monomer or dimer, or the presence or absence of water, the calculated barrier differences in the rate-determining steps of CO2 and CO formation are much smaller than those reported in previously published studies. These results improve our understanding of the mechanism of formic acid oxidation catalyzed by Pt-based catalysts, and rationalize the performance durability problem of Pt-based catalysts used in direct formic acid fuel cells.
Co-reporter:Yuanyuan Qi, Jun Gao, Dongju Zhang and Chengbu Liu
RSC Advances 2015 vol. 5(Issue 27) pp:21170-21177
Publication Date(Web):09 Feb 2015
DOI:10.1039/C5RA01925G
Pt-based catalysts are known as the best electrocatalysts for formic acid (HCOOH) oxidation. Maximizing their use efficiency and reducing the CO poisoning effect are highly desirable, however, very challenging. Aiming at these interesting issues, this work presents a theoretical study of the catalytic decomposition of HCOOH on an ideal single-atom model catalyst of PtAg nanostructures, which consists of isolated Pt atoms anchored to an Ag(111) surface and is referred to as PtAg(111). The barrier of the rate-determining step of HCOOH decomposition to CO2 on PtAg(111) is calculated to be 0.38 eV, which is not significantly different from that on a pure Pt(111) surface, 0.35 eV. On the other hand, the barrier of HCOOH decomposing to CO on PtAg(111) is found to be higher than that on pure Pt(111), 0.83 vs. 0.67 eV. These results indicate that the single-atom PtAg(111) (Pt-decorated Ag(111) surface) presents promising catalytic performance for HCOOH oxidation, which promotes HCOOH dehydrogenation to CO2 as good as pure Pt(111) and inhibits HCOOH dehydration to undesirable CO that poisons the catalyst. The present results rationalize the experimental observation that Pt–Ag alloy electrocatalysts show improved catalytic performance toward HCOOH oxidation, and provide a clue for the rational design of Pt-based single-atom catalysts.
Co-reporter:Jingjing Li, Jinghua Li, Dongju Zhang, and Chengbu Liu
The Journal of Physical Chemistry B 2015 Volume 119(Issue 42) pp:13398-13406
Publication Date(Web):October 4, 2015
DOI:10.1021/acs.jpcb.5b07773
While the catalytic conversion of glucose to 5-hydroxymethyl furfural (HMF) catalyzed by SO3H-functioned ionic liquids (ILs) has been achieved successfully, the relevant molecular mechanism is still not understood well. Choosing 1-butyl-3-methylimidazolium chloride [C4SO3HmimCl] as a representative of SO3H-functioned IL, this work presents a density functional theory (DFT) study on the catalytic mechanism for conversion of glucose into HMF. It is found that the conversion may proceed via two potential pathways and that throughout most of elementary steps, the cation of the IL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bonding, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol, respectively, which are compatible with the observed catalytic performance of the IL under mild conditions (100 °C). The present results provide help for rationalizing the effective conversion of glucose to HMF catalyzed by SO3H-functionalized ILs and for designing IL catalysts used in biomass conversion chemistry.
While the catalytic transformation of cellulose to glucose by functionalized ionic liquids (ILs) has been achieved successfully under mild conditions, insight into the fundamental molecular mechanism is still lacking. The present work presents the first attempt to address the fundamental reaction chemistry of the catalytic transformation. An enzyme-like catalytic mechanism of ILs, in which glycosidic bond hydrolysis proceeds through a retaining mechanism and/or an inverting mechanism, is proposed. DFT calculations show that both mechanisms involve moderate barriers (<30 kcal mol−1), which is consistent with the catalytic performance of the ILs under mild conditions (<100 °C). The “biomimetic” mechanism model proposed herein is expected to be viable for understanding the unique catalytic activity of ILs under mild conditions.
Co-reporter:Hao Xu, Zhe Han, Dongju Zhang, Chengbu Liu
Journal of Molecular Catalysis A: Chemical 2015 Volume 398() pp:297-303
Publication Date(Web):March 2015
DOI:10.1016/j.molcata.2014.12.018
•Elucidating the dual role of IL as catalyst and extractant in ODS by DFT method.•Offering useful information for understanding the detailed mechanism of the ODS.•Both the cation and anion of the IL play important roles in the oxidation process.•The cation plays a more important role in the extraction and separation process.While the oxidative desulfurization (ODS) of aromatic sulfur compounds, such as dibenzothiophene (DBT), in ionic liquids (ILs) has attracted great attention, the role of ILs is still not well elucidated at the molecular level. Focusing on a model system, i.e., the ODS of DBT using H2O2 as oxidant in 1-methylimidazolium tetrafluoroborate ([HMIm]BF4) IL, this work presents a theoretical elucidation of the dual role of IL as a catalyst and an extractant by performing density functional theory calculations. It is found that as a catalyst, both the cation and anion of the IL play important roles in the oxidation process of DBT to dibenzothiophene sulfone (DBTO2): they act respectively, as the donor of one intermolecular hydrogen bond and the acceptor of another intermolecular hydrogen bond to promote the OO and OH cleavages of H2O2 and to stabilize transition state structures. With the assistance of the IL, the barriers of two elementary steps involved in the transformation from DBT to DBTO2 are reduced to 13.1 and 23.8 kcal mol−1 from 32.1 and 37.8 kcal mol−1, respectively. On the other hand, as an extractant both the cation and anion of IL show stronger interactions with DBTO2 than with DBT, which is attributed to the larger polarity of the former than the latter. Furthermore, calculated interaction energies of the cation–molecule complexes are larger than those of the anion–molecule complexes, indicating that the cation plays a more important role in the extraction and separation process of aromatic sulfur compound from fuels.
While selective C–H and C–F activations of fluoroaromatic imines and ketones with transition metal complexes supported by PMe3 have been successfully achieved in recent publications, insight into the molecular mechanism and energetics of those reactions is still lacking. Focusing on three typical substrates, 2,6-difluorobenzophenone imine (A) and 2,6-difluorobenzophenone (B), and 2,4′-difluorobenzophenone (C), the present work theoretically studied their C–H and C–F cyclometalation reactions promoted by the activator Co(PMe3)4 or CoMe(PMe3)4. It is found that reaction A + Co(PMe3)4 favors the C–F activation, reaction A + CoMe(PMe3)4 prefers the C–H activation, whereas both the C–H and C–F activation pathways may be viable for reactions B + CoMe(PMe3)4 and C + CoMe(PMe3)4. The experimentally observed C–H and C–F cyclometalation products have been rationalized by analyzing the thermodynamic and kinetic properties of two activation pathways. From calculated results combined with the experimental observations, we believe that three factors, i.e. the oxidation state of the metal center in the activators, the anchoring group of substrates, and substituted fluoroatom counts of the aromatic ring in substrates, affect the selectivity of C–H and C–F activations of fluoroaromatic ketones and imines. Calculated results are enlightening about the rational design of activators and substrates of fluoroaromatic imines and ketones to obtain the exclusive C–H or C–F bond activation product.
Co-reporter:Yuxia Liu;Guang Chen;Hongliang Wang;Siwei Bi
European Journal of Inorganic Chemistry 2014 Volume 2014( Issue 15) pp:2502-2511
Publication Date(Web):
DOI:10.1002/ejic.201400123
Abstract
With the help of density functional theory (DFT) calculations, we theoretically investigated and rationalized the controlled metathesis reactions of methylruthenium complex trans-[Ru(CH3)2(dmpe)2] [dmpe = 1,2-bis(dimethylphosphanyl)ethane] (A) with terminal acetylenes. On the basis of the mechanistic study, two important issues related to the generation of mono- and bis-acetylido Ru complexes have been addressed. One issue is why the formation of bis-acetylido complex C, trans-[Ru(C≡CPh)2] (dmpe)2, is more difficult than the formation of mono-acetylido complex B, trans-[Ru(C≡CPh)(CH3)(dmpe)2]. A strong dπ*(C≡C) interaction between Ru and the alkynyl in B is believed to weaken π back-donation of Ru to the phenylacetylene C–H σ* bond, and then makes the phenylacetylene C–H bond difficult to break. The second issue is what role methanol plays in promoting the ligand metathesis between the methyl group in B and one alkynyl moiety in 1,4-diethynylbenzene to give bis-acetylido species D, trans-[(PhC≡C)Ru(dmpe)2(C≡CC6H4C≡CH)]. The relatively strong proton-donating ability of methanol was found to be the major reason. The present study is indicative of the controllable synthesis of mono- and bis-acetylido, and even di- and trinuclear acetylide-bridged ruthenium(II) complexes.
Co-reporter:Shunwei Chen, Zhe Han, Dongju Zhang and Jinhua Zhan
RSC Advances 2014 vol. 4(Issue 94) pp:52415-52422
Publication Date(Web):10 Oct 2014
DOI:10.1039/C4RA06011C
Dioxins are a group of persistent organic pollutants which cause extreme harm to animals and human beings. There is great significance in developing fast and effective methods for enriching (capturing) and detecting dioxins. In this work, molecular dynamics (MD) simulations and quantum chemistry (QM) calculations have been used to study the inclusion complexation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic dioxin, with cucurbit[n]urils (CBn, n = 6, 7, and 8), a group of well-known host complexes applied in the study of host–guest interactions. The inclusion of TCDD with all three CBn hosts is found to be an energetically favorable process without remarkable energy barriers. In general, the host and guest form stable 1:1 complexes (TCDD–CBn), as indicated by calculated large complexation energies and small deformation energies of the host and guest. Moreover, the 1:2 host–guest complex (2TCDD–CB8) can be formed for CB8 due to its relatively larger cavity. The characteristic infrared (IR) and Raman peaks of TCDD are recognizable in the corresponding spectra of TCDD–CBn complexes. Based on the theoretical results, CBn are believed to be capable of including TCDD, and the TCDD in the inclusion complexes can be detected using both IR and Raman techniques. The results shown in this work are expected to be informative to the relevant experimental researchers.
•The reactivities of Ge doped BNNT towards CO and NO are theoretically investigated.•Toxic CO and NO molecules present strong chemisorption on the Ge doped BNNT.•The doping of the Ge atom improves the electronic property of the BNNT.•Ge doped BNNT is expected to be a potential sensor for detecting toxic CO and NO.To explore a novel sensor to detect toxic pollutant in the atmosphere, we investigate reactivities of the germanium doped (Ge-doped) (8, 0) single-walled boron nitride nanotubes (BNNTs) towards carbon monoxide (CO) and nitric oxide (NO) by performing density functional theory (DFT) calculations. CO and NO are found to present strong chemisorption on the Ge-doped BNNT with substituted boron and nitrogen defect site. Calculated data for the electronic density of states and the electronic charge densities further indicate that the doping of Ge atom improves the electronic transport property of the BNNT, induces magnetism of the BNNT, and increases its adsorption sensitivity towards CO and NO. Doping BNNTs with Ge is expected to be an available strategy for improving the properties of BNNTs, and Ge-doped BNNT is expected to be a potential resource for detecting the presence of CO and NO.
Co-reporter:Yingying Wang, Yuanyuan Qi, Dongju Zhang, and Chengbu Liu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 4) pp:2067-2076
Publication Date(Web):January 8, 2014
DOI:10.1021/jp410742p
While the high performance for electrooxidation of formic acid (HCOOH) has been recognized, Pd-based catalysts still suffer from CO poisoning, even though they are much more tolerant than Pt-based catalysts. Existing theoretical studies on the decomposition of HCOOH on Pd(111) surface cannot rationalize the catalyst poisoning effect. By performing density functional theory calculations, the present work reexamined the decomposition of HCOOH on Pd(111) along with the dual-path mechanism consisting of indirect and direct pathways. Two new adsorption configurations of HCOOH on Pd(111) are presented, from which the formation of CO is found to be either the same or more favorable in comparison with the formation of CO2. The present results are in distinct contrast to previous calculations where the barrier for the formation of CO2 was much lower than that for the formation of CO. Furthermore, this work also discussed the formation of CO through the reduction of CO2 and the effects of coadsorbed HCOOH and H2O molecules on the reactivity. From calculated results, it seems that the newly formed CO2 on Pd(111) can return to the surface to interact with adsorbed H atoms, partly contributing to the formation of CO. Coadsorbed HCOOH and H2O molecules are found to importantly affect the initial adsorption configuration and the decomposition mechanism of HCOOH on Pd(111). These results provide new insight into the reactivity of HCOOH on the Pd(111) surface and rationalize CO poisoning of Pd-based catalysts.
Co-reporter:Yanfang Zhang, Dongju Zhang, Jun Gao, Jinhua Zhan, and Chengbu Liu
The Journal of Physical Chemistry A 2014 Volume 118(Issue 2) pp:449-456
Publication Date(Web):December 23, 2013
DOI:10.1021/jp410077g
Although chlorophenols (CPs) are considered to be the most important and direct precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), our understanding of the formation mechanism of PCDD/Fs is exclusively limited to an invariable idea that chlorophenoxy radicals (CPRs) are only necessary intermediates. The present work presents a systematic theoretical study that aims at providing new insight into the homogeneous formation of PCDD/Fs from CPs. Two different types of radicals from CPRs, i.e., substituted phenyl radicals and phenoxyl diradicals, are proposed to serve as potential sources contributing to the formation of PCDD/Fs. The thermodynamic and kinetic properties of reactions of 2-chlorophenol (2-CP), as a representative of CP congeners, with atomic H to produce various potential radicals forming PCDD/Fs are studied by performing density functional theory calculations and direct kinetics studies. The newly proposed radicals, especially substituted phenoxyl diradicals (the most direct intermediates of PCDD/Fs), can be formed via reactions of 2-CP with atomic H with small barriers and large reaction energies. They should be expected to be responsible for the homogeneous formation of PCDD/Fs under high temperature. Several typical PCDD/F products are predicated through direct self- and cross-couplings of the newly proposed radicals. The radical coupling patterns proposed in the present work expand our understanding of the formation mechanism of PCDD/Fs from CP precursors.
Co-reporter:Bingchuan Yang, Xiaochen Tan, Ruiying Guo, Shunwei Chen, Zeyuan Zhang, Xianglong Chu, Caixia Xie, Dongju Zhang, and Chen Ma
The Journal of Organic Chemistry 2014 Volume 79(Issue 17) pp:8040-8048
Publication Date(Web):August 7, 2014
DOI:10.1021/jo5011729
A series of 1,4-thiazepin-5(4H)-one derivatives were synthesized via a transition metal-free one-pot Smiles rearrangement process at room temperature. Regioselective seven-membered heterocycles were constructed in good to excellent yields. To gain an in-depth understanding of the S–N type Smiles rearrangement mechanism, a theoretical study was also performed by quantum chemistry calculations.
Chlorophenols are known as precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). The widely accepted formation mechanism of PCDD/Fs always assumes chlorophenoxy radicals as key and important intermediates. Based on the results of density functional theory calculations, the present work reports new insight into the formation mechanism of PCDD/Fs from chlorophenol precursors. Using 2-chlorophenol as a model compound of chlorophenols, we find that apart from the chlorinated phenoxy radical, the chlorinated phenyl radical and the chlorinated α-ketocarbene also have great potential for PCDD/F formation, which has scarcely been considered in previous literature. The calculations on the self- and cross-coupling reactions of the chlorophenoxy radical, the chlorinated phenyl radical and the chlorinated α-ketocarbene show that the formations of 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF are both thermodynamically and kinetically favorable. In particular, some pathways involving the chlorinated phenyl radicals and the chlorinated α-ketocarbene are even energetically more favorable than those involving the chlorophenoxy radical. The calculated results may improve our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors and be informative to environmental scientists.
The reactions of (2,6-difluorophenyl)phenylmethanone (2,6-F2C6H3–C(O)–C6H5) (1) and (2,6-difluorophenyl)phenylmethanimine (2,6-F2C6H3–C(NH)–C6H5) (3) with Fe(PMe3)4 afforded different selective C–F/C–H bond activation products. The reaction of 1 with Fe(PMe3)4 gave rise to bis-chelate iron(II) complex [C6H5–C(O)–3-FC6H3)Fe(PMe3)]2 (2) via C–F bond activation. The reaction of 3 with Fe(PMe3)4 delivered chelate hydrido iron(II) complex 2,6-F2C6H3–C(NH)–C6H4)Fe(H)(PMe3)3 (4) through C–H bond activation. The DFT calculations show the detailed elementary steps of the mechanism of formation of hydrido complex 4 and indicate 4 is the kinetically preferred product. Complex 4 reacted with HCl, CH3Br and CH3I delivered the chelate iron halides (2,6-F2C6H3–C(NH)–C6H4)Fe(PMe3)3X (X = Cl (5); Br (6); I (7)). A ligand (PMe3) replacement by CO of 4 was observed giving (2,6-F2C6H3–C(NH)–C6H4)Fe(H)(CO)(PMe3)2 (8). The chelate ligand exchange occurred through the reaction of 4 with salicylaldehydes. The reaction of 4 with Me3SiCCH afforded (2,6-F2C6H3–C(N)–C6H5)Fe(CC–SiMe3)(PMe3)3 (11). A reaction mechanism from 4 to 11 was discussed with the support of IR monitoring. The molecular structures of complexes 2, 4, 6, 7, 10 and 11 were determined by X-ray diffraction.
With the aid of density functional theory (DFT) calculations, we have performed a detailed mechanism study for the catalytic cycloisomerization reactions of propargylic 3-indoleacetate to better understand the observed divergent reactivities of two catalysts, PtCl2 and (PPh3)AuSbF6, which result in [3 + 2]- and [2 + 2]-cycloaddition products, respectively. The calculated results confirm that the lactone intermediates are common and necessary species for the formation of the two products and that PtCl2 and (PH3)AuSbF6 respectively favor the formation of the [3 + 2]- and [2 + 2]-products. The intrinsic reasons for the divergent reactivities of the two catalysts have been analyzed in detail. We believe that the essentially different metal–ligand interactions in PtCl2 and (PPh3)AuSbF6 are mainly responsible for their divergent regioselectivities, while the solvent effects have little impact on the catalyst activity. Starting from lactone intermediates, PtCl2 induces the intramolecular nucleophilic addition to give the [3 + 2] cycloisomerization product due to the strong π-electron-donating ability of chlorine ligands, while (PPh3)AuSbF6 results in the intramolecular nucleophilic addition reaction to form the [2 + 2] cycloisomerization product because of the strong σ-electron-donating ability of phosphine ligands.
•Alkylmethylimidazolium ionic liquids are non-inert solvents of cellulose.•The side reaction mechanism of [bmim]+ with glucose was studied theoretically.•The reaction involves the deprotonation of [bmim]+ and its nucleophilic addition.•The deprotonation process of [bmim]+ by TEA promotes the side reaction.To understand the non-inert nature of ionic liquids in cellulose chemistry, density functional theory calculations were carried out to investigate the reaction of 1-butyl-3-methylimidazolium cation ([bmim]+) with model compounds α- and β-glucoses (the basic units of cellulose and its degradation products) in the absence and presence of base, triethylamine (TEA).The calculated energy barriers for the reactions of [bmim]+ with α- and β-glucoses without TEA were 67.97 and 66.19 kcal mol−1, respectively. With the assistance of TEA, the barriers were reduced to 48.17 and 46.96 kcal mol−1, due to the enhanced electrophilic abilities of H6 and nucleophilic abilities of C2 induced through deprotonation of the C2 atom, respectively. The present results rationalize the experimental finding well and provide a clear explanation on why imidazolium-based ionic liquids are considered as non-inert solvents in cellulose chemistry.Graphical abstract
Computational and Theoretical Chemistry 2013 Volume 1014() pp:24-28
Publication Date(Web):15 June 2013
DOI:10.1016/j.comptc.2013.03.019
•Alkylmethylimidazolium ILs are not inert solvents under mildly basic conditions.•The reaction of [bmim]+ with benzaldehyde was studied theoretically.•The reaction involves the deprotonation of [bmim]+ and its nucleophilic addition.•The deprotonation process is crucially responsible for the reaction under study.Alkylmethylimidazolium ionic liquids were reported to be non-innocent solvents in Baylis–Hillman reactions under mildly basic conditions. Here we present a theoretical explanation on why imidazolium-based ionic liquids are reactive solvents in such a kind of reactions. By performing density functional theory calculations, we have investigated the reactions of a representative imidazolium cation, 1-butyl-3-methylimidazolium cation ([bmim]+) with benzaldehyde (one of the reactants in Baylis–Hillman reactions) in the absence and presence of base, 1,4-diazabicyclo[2.2.2]octane (DABCO), to understand the side-reaction mechanism involved in Baylis–Hillman reactions. In the absence of DABCO, the barrier of the reaction is calculated to be as high as 54.53 kcal mol−1, implying the high stability of [bmim]+. However, with the assistance of DABCO, it is found that C2 atom of [bmim]+ is very easily deprotonated, which significantly reduces the positive charge on C2 and hence remarkably enhances the nucleophilic ability of C2 atom. The overall energy barrier for the base-catalyzed reaction is remarkably reduced to 24.81 kcal mol−1, indicating the increasing reactivity of [bmim]+ towards benzaldehyde under mildly basic conditions.Graphical abstract
Journal of Inclusion Phenomena and Macrocyclic Chemistry 2013 Volume 76( Issue 3-4) pp:301-309
Publication Date(Web):2013 August
DOI:10.1007/s10847-012-0199-4
The effective enrichment and identification of lowly concentrated polychlorinated biphenyls (PCBs) in the environment is attracting much research attention due to human health concerns raised from their emissions. Cyclodextrins (CDs) are known to be capable to form inclusion complexes with a variety of organic molecules. The purpose of this study is to provide theoretical evidences whether CDs can form energetically stable inclusion complexes with PCBs through a host–guest interaction, and if so, whether infrared and Raman techniques are suitable for the detection of CD-modified PCBs. Focusing on a representative PCB molecule, 3,3′,4,4′,5-pentachlorobiphenyl (PCB126), we studied its molecular inclusion by β-CD (BCD) by performing molecular dynamics simulations and density functional theory calculations. Calculated results show that PCB126 and BCD preferentially form the stable 1:1 inclusion complex. The calculated IR spectra of the 1:1 inclusion complexes mainly present the spectra features of BCD and give only a slight indication for bands of the guest molecule. In contrast, the characteristic vibration modes of the guest molecule are remarkably prominent in the Raman spectra of the inclusion complexes. Based on the present results, we propose that BCD can potentially serve as a candidate for including PCB126 to form the stable 1:1 host–guest complex, and that Raman spectroscopy technology is expected to be suitable for the identification of the CD-modified PCBs, whereas IR spectroscopy is not feasible for such an application.
Co-reporter:Tao Liu, Chunmei Du, Zhangyu Yu, Lingli Han, and Dongju Zhang
The Journal of Physical Chemistry B 2013 Volume 117(Issue 7) pp:2081-2087
Publication Date(Web):February 5, 2013
DOI:10.1021/jp311868a
Protonated adrenaline (PAd) can be oxidized to protonated adrenaline quinone (PAdquinone) through a one-step, two-electron redox reaction. The electron-transfer property of PAd and its supramolecular complex with glycine has been investigated by cyclic voltammetry (CV) experiment and theoretical calculations. From CV curves, the conditional formal redox potential E°′ of PAd/PAdquinone couple at the pH value of 0.29 is determined to be 0.540 V. The calculated E°′ using the G3MP2//B3LYP method and the B3LYP method with 6-31G(d,p), 6-31+G(d,p), 6-311G(d,p), and B3LYP/6-311+G(d,p) basis sets are in reasonable agreement with the experimental value. PAd can form supramolecular complex (PAd–Gly) with glycine (Gly) through hydrogen bond (H-bond), and the calculated E°′ values of PAd–Gly/PAdquinone–Gly redox couple are larger than those of PAd/PAdquinone couple. The theoretical results are in good agreement with the experimental finding that the formation of H-bonds weaken the electron-donating ability of PAd.
Although imidazolium-based ionic liquids (ILs) combined with oxygen-containing anions were proposed as the potential solvents for the selective separation of acetylene (C2H2) and ethylene (C2H4), the detailed mechanism at the molecular level is still not well understood. The present work focuses on a most effective IL for removing C2H2 from a C2H4 stream, 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), aiming at understanding the first steps of the adsorption process of the molecules at the IL surface. We present a combined quantum mechanical (QM) calculation and molecular dynamics (MD) simulation study on the structure and property of the IL as well as its interaction with C2H2 and C2H4 molecules. The calculated results indicate that C2H2 presents a stronger interaction with the IL than C2H4 and the anion of the IL is mainly responsible for the stronger interaction. QM calculations show a stronger hydrogen-binding linkage between an acidic proton of C2H2/C2H4 and the basic oxygen atom in [OAc]− anion, in contrast to the relative weaker association via the C–H···π interaction between C2H2/C2H4 and the cation. From MD simulations, it is observed that in the interfacial region, the butyl chain of cations and methyl of anions point into the vapor phase. The coming molecules on the IL surface may be initially wrapped by the extensive butyl chain and then devolved to the interface or caught into the bulk by the anion of IL. The introduction of guest molecules significantly influences the anion distribution and orientation on the interface, but the cations are not disturbed because of their larger volume and relatively weaker interaction with the changes in the guest molecules. The theoretical results provide insight into the molecular mechanism of the observed selective separation of C2H2 form a C2H4 stream by ILs.Keywords: 1-butyl-3-methylimidazolium acetate; acetylene; ethylene; ionic liquids; molecular dynamics; quantum chemistry;
Chemical Physics Letters 2012 Volume 543() pp:61-67
Publication Date(Web):10 August 2012
DOI:10.1016/j.cplett.2012.06.067
The potential energy surfaces for the OH + divinyl sulfoxide reaction in the presence of O2/NO are theoretically characterized at the CCSD(T)/6-311+G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE level of theory. Various possible pathways including the direct hydrogen abstraction channels and the addition–elimination channels are considered. The calculations show that the exclusive feasible entrance channel is the formation of adduct CH2(OH)CHS(O)CHCH2 (IM1) in the initial reaction pathways. In the atmosphere, the newly formed adduct IM1 can further react with O2/NO to form the dominant products HCHO + C(O)HS(O)CHCH2 (P9). The calculated results confirm the experimental studies.Graphical abstractHighlights► The OH radicals adds predominantly to the terminal carbon atom. ► The main products from OH + DVSO + O2/NO are HCHO and C(O)HS(O)CHCH2. ► Divinyl sulfone CH2CHS(O)2CHCH2 is expected to be minor product.
Co-reporter:Xueying Zhu, Jianqiang Liu, Dongju Zhang, Chengbu Liu
Computational and Theoretical Chemistry 2012 Volume 996() pp:21-27
Publication Date(Web):15 September 2012
DOI:10.1016/j.comptc.2012.07.008
By performing density functional theory calculations, we systematically studied the Diels–Alder (D–A) reaction between cyclopentadiene and methacrylate catalyzed by alanine methyl ester nitrate ([AME][NO3]), an amino acid-based ionic liquid (AAIL). The uncatalyzed reaction was first calculated in both gas phase and dichloromethane, and then the catalytic effect of [AME][NO3] AAIL on the D–A reaction was mimicked by using one, two, and up to three ion pairs as catalysts. The calculated results show that [AME][NO3] plays a role of Lewis acid to promote the reaction and the catalytic active center is the NH3 group in [AME]+ cation, which forms the NH⋯O H-bond with the carbonyl oxygen atom in methacrylate to effectively polarize the CC double bond in methacrylate. As a result, the energy barrier of reaction is remarkably reduced, and the asynchronicity of reaction is increased. The calculated energy barrier for the reaction with the presence of two ion pairs is lower than those with the presences of one and three ion pairs, implying that the optimal molar ratio among two reactants and the reaction medium/catalyst [AME][NO3] should be 1:1:2. The present results rationalize the early experimental findings, and provide a useful reference for the rational design of usual D–A reactions in AAILs.Graphical abstractHighlights► The [AME][NO3]-catalyzed Diels–Alder reaction between cyclopentadiene and methacrylate is studied theoretically. ► [AME][NO3] plays a role of Lewis acid to promote the reaction. ► The catalytic active center is the NH3 group in [AME]+ cation. ► The optimal molar ratio among two reactants and the catalyst may be 1:1:2.
Co-reporter:Xiangting Sun, Rongxiu Zhu, Jun Gao, Dongju Zhang, and Dacheng Feng
The Journal of Physical Chemistry A 2012 Volume 116(Issue 26) pp:7082-7088
Publication Date(Web):May 29, 2012
DOI:10.1021/jp2124873
The asymmetric direct aldol reactions of aliphatic ketones (acetone, butanone, and cyclohexanone) with 4-nitrobenzaldehyde catalyzed by a chiral primary–tertiary diamine catalyst (trans-N,N-dimethyl diaminocyclohexane) have been investigated by performing density functional theory calculations to rationalize the experimentally observed stereoselectivities. Focused on the crucial C–C bond-forming steps, we located several low-lying transition states and predicted their relative stabilities. The calculated results demonstrate that the catalytic direct aldol reactions of acetone favors the (S)-enantiomer and that butanone prefers the branched syn-selective product, while cyclohexanone yields predominantly the opposite anti-selective product. The theoretical results are in good agreement with the experimental findings and provide a reasonable explanation for the high enantioselectivity and diastereoselectivity, as well as regioselectivity, of the aldol reactions under consideration.
To understand the mechanism of allene formation through the rearrangement of cyclopropenes catalyzed by PtCl2, we have performed a detailed density functional theory calculation study on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)cyclopropene. Three reaction pathways proposed in the original study have been examined; however the calculated results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we present a novel mechanism of PtCl2, which is fixed appropriately on the cyclopropene/allene to form the linear Cl–Pt–Cl disposition, a vital configuration for catalyzing the rearrangement of cyclopropene. The newly proposed mechanism involves an SN2-type C–C bond activation of the cyclopropene by PtCl2 fixed on a cyclopropene molecule via the d−π interaction between the metal center and the substrate to form the product precursor PtCl2-allene with the metal center coordinated to the external C═C bond in the allene framework. Once formed, the PtCl2-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociating into the allene product and the PtCl2 catalyst due to its high stability. During the catalytic cycle, an allene-PtCl2-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytically active center PtCl2-allene with an energy demand of 24.4 kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, to understand the experimental finding that the H-substituted cyclopropenes do not provide any allenes, we have also performed calculations on the H-substituted cyclopropene system and found that the highest barrier to be overcome during the catalytic cycle amounts to 35.2 kcal/mol. This high energy barrier can be attributed to the fact that the C–H bond activation is more difficult than the C–Si bond activation. The theoretical results not only rationalize well the experimental observations but provide new insight into the mechanism of the important rearrangement reaction.
Co-reporter:Wenhui Zhong, Rongyue Wang, Dongju Zhang, and Chengbu Liu
The Journal of Physical Chemistry C 2012 Volume 116(Issue 45) pp:24143-24150
Publication Date(Web):October 27, 2012
DOI:10.1021/jp307923x
By performing density functional theory calculations, we have studied the dual-path mechanism of formic acid (HCOOH) oxidation on the PtAu(111) surface in the continuum water solution phase. The direct pathway involving the dehydrogenation of HCOOH to form CO2 occurs with a barrier of 15.5 kcal/mol, which is in contrast to the much higher barrier of 99.2 kcal/mol in the indirect pathway involving the dehydration of HCOOH to form a CO intermediate. In comparison, the calculated barriers on the Pt(111) surface in direct and indirect pathways are 5.8 and 32.9 kcal/mol, respectively. The theoretical results emphasize that a bimetallic PtAu(111) surface significantly increases the barrier difference between the two pathways to 83.7 kcal/mol from 27.1 kcal/mol on the Pt(111) surface and thus can hinder remarkably the indirect pathway. The theoretical results rationalize well the experimental finding that bimetallic PtAu catalysts show higher catalytic activity toward HCOOH oxidation than pure Pt catalysts.
Journal of Molecular Modeling 2012 Volume 18( Issue 7) pp:3051-3060
Publication Date(Web):2012 July
DOI:10.1007/s00894-011-1318-7
The density functional theory (DFT) calculations are carried out to study the mechanism details and the ensemble effect of methanol dehydrogenation over Pt3 and PtAu2 clusters, which present the smallest models of pure Pt clusters and bimetallic PtAu clusters. The energy diagrams are drawn out along both the initial O-H and C-H bond scission pathways via the four sequential dehydrogenation processes, respectively, i.e., CH3OH → CH2OH → CH2O → CHO → CO and CH3OH → CH3O → CH2O → CHO → CO, respectively. It is revealed that the reaction kinetics over PtAu2 is significantly different from that over Pt3. For the Pt3-mediated reaction, the C-H bond scission pathway, where an ensemble composed of two Pt atoms is required to complete methanol dehydrogenation, is energetically more favorable than the O-H bond scission pathway, and the maximum barrier along this pathway is calculated to be 12.99 kcal mol-1. In contrast, PtAu2 cluster facilitates the reaction starting from the O-H bond scission, where the Pt atom acts as the active center throughout each elementary step of methanol dehydrogenation, and the initial O-H bond scission with a barrier of 21.42 kcal mol-1 is the bottom-neck step of methanol decomposition. Importantly, it is shown that the complete dehydrogenation product of methanol, CO, can more easily dissociate from PtAu2 cluster than from Pt3 cluster. The calculated results over the model clusters provide assistance to some extent for understanding the improved catalytic activity of bimetal PtAu catalysts toward methanol oxidation in comparison with pure Pt catalysts.
While N,N′-dialkylimidazolium ionic liquids (ILs) have been well-established as effective solvents for dissolution and processing of cellulose, the detailed mechanism at the molecular level still remains unclear. In this work, we present a combined quantum chemistry and molecular dynamics simulation study on how the ILs dissolve cellulose. On the basis of calculations on 1-butyl-3-methylimidazolium chloride, one of the most effective ILs dissolving cellulose, we further studied the molecular behavior of cellulose models (i.e. cellulose oligomers with degrees of polymerization n = 2, 4, and 6) in the IL, including the structural features and hydrogen bonding patterns. The collected data indicate that both chloride anions and imidazolium cations of the IL interact with the oligomer via hydrogen bonds. However, the anions occupy the first coordination shell of the oligomer, and the strength and number of hydrogen bonds and the interaction energy between anions and the oligomer are much larger than those between cations and the oligomer. It is observed that the intramolecular hydrogen bond in the oligomer is broken under the combined effect of anions and cations. The present results emphasize that the chloride anions play a critically important role and the imidazolium cations also present a remarkable contribution in the cellulose dissolution. This point of view is different from previous one that only underlines the importance of the chloride anions in the cellulose dissolution. The present results improve our understanding for the cellulose dissolution in imidazolium chloride ILs.
Journal of Molecular Modeling 2012 Volume 18( Issue 5) pp:1809-1818
Publication Date(Web):2012 May
DOI:10.1007/s00894-011-1210-5
The detailed mechanisms of catalytic CO oxidation over Au2- and AuAg- dimers, which represent the simplest models for monometal Au and bimetallic Au-Ag nanoparticles, have been studied by performing density functional theory calculations. It is found that both Au2- and AuAg- dimers catalyze the reaction according to the similar mono-center Eley–Rideal mechanism. The catalytic reaction is of the multi-channel and multi-step characteristic, which can proceed along four possible pathways via two or three elementary steps. In AuAg-, the Au site is more active than the Ag site, and the calculated energy barrier values for the rate-determining step of the Au-site catalytic reaction are remarkably smaller than those for both the Ag-site catalytic reaction and the Au2- catalytic reaction. The better catalytic activity of bimetallic AuAg- dimer is attributed to the synergistic effect between Au and Ag atom. The present results provide valuable information for understanding the higher catalytic activity of Au-Ag nanoparticles and nanoalloys for low-temperature CO oxidation than either pure metallic catalyst.
Co-reporter:Wenxiao Pan, Wenhui Zhong, Dongju Zhang, and Chengbu Liu
The Journal of Physical Chemistry A 2012 Volume 116(Issue 1) pp:430-436
Publication Date(Web):December 6, 2011
DOI:10.1021/jp208571d
Silica is the main component of combustion-generated fly ash and is expected to have an important impact on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal waste incinerators. In this work, we theoretically studied the reactions of 2-chlorinated phenol (2-CP) over the clusters (SiO2)3 and (SiO2)3O2H4, which mimic the dehydrated and hydroxylated silica structures, respectively. The dehydrated cluster is much more active toward the attack of 2-CP to form highly stable 2-chlorophenolate than the hydroxylated silica cluster. The further dissociation of chlorophenolates to form CP radicals (CPRs) is calculated to be very difficult. The calculated energy barrier of the reaction of 2-CP over the dehydrated (SiO2)3 cluster and IR data are in good agreement with early experimental observations. On the basis of the calculated results, we propose that the formation of PCDD/Fs from CPs over silica surfaces may not involve CPRs, but be relevant to the further conversion of chlorophenolates over silica surfaces. This mechanism is very different from the corresponding reactions mediated by transition metal oxides. The results presented here may be helpful to understand the chemisorption mechanism of CPs on silica surfaces in real waste combustion.
Density functional theory (DFT) calculations have been carried out to study the detailed mechanism of CO inserting into the N–H bond rather than the common Fe–N bond of the iron(II) amido complex (dmpe)2Fe(H)(NH2) (dmpe = 1,2-bis(dimethylphosphino)ethane). Three mechanisms proposed in previous literature have been computationally examined, and all of them are found to involve high barriers and thus cannot explain the observed N–H insertion product. Alternatively, on the basis of the calculated results, a novel reactant-assisted (self-promoted) mechanism is presented, which provides the most efficient access to the insertion reaction via the assistance of a second reactant molecule. In detail, the reaction starts from direct attack of CO at the amide nitrogen atom of (dmpe)2Fe(H)(NH2), followed by a second reactant-assisted H abstraction/donation processes to afford the trans product of CO inserting into the N–H bond of the amido complex. The present theoretical results provide a new insight into the mechanism of the unusual insertion reaction and rationalize the experimental findings well.
Co-reporter:Dr. Yuxia Liu; Dongju Zhang;Dr. Jun Gao ; Chengbu Liu
Chemistry - A European Journal 2012 Volume 18( Issue 48) pp:15537-15545
Publication Date(Web):
DOI:10.1002/chem.201200093
Abstract
The aromatic CC bond cleavage by a tungsten complex reported recently by Sattler and Parkin15 offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained CC bond is activated at the molecular level.16, 17 In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic CC bond in quinoxaline (QoxH) by the W-based complex [W(PMe3)4(η2-CH2PMe2)H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol−1 and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe3 ligand on the tungsten center. In our new mechanism, it is proposed that the CC cleavage occurs prior to the second CH bond addition, in contrast to Sattler and Parkin’s mechanism in which the CC bond is broken after the second CH bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol−1 after the first PMe3 ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable CC bond cleavage.
Co-reporter:Yongmei Zhao, Yanmiao Wu, Jiong Jia, Dongju Zhang, and Chen Ma
The Journal of Organic Chemistry 2012 Volume 77(Issue 19) pp:8501-8506
Publication Date(Web):September 5, 2012
DOI:10.1021/jo3014287
Benzo[1,4]thiazin-3(4H)-one derivatives are conveniently prepared in one pot via a Smiles rearrangement (SR) tandem reaction. In order to understand the reaction, we present here a theoretical study on the S–N type SR mechanism.
The Journal of Physical Chemistry C 2012 Volume 116(Issue 4) pp:2994-3000
Publication Date(Web):January 5, 2012
DOI:10.1021/jp210304z
By performing density functional theory calculations, we have studied the CO pathway and non-CO pathway of methanol oxidation on the PtAu(111) bimetallic surface. CO is shown to possess larger adsorption energy on the PtAu(111) surface than that on the pure Pt(111) surface, and the non-CO pathway on the bimetallic surface is found to be energetically more favorable than the CO pathway. These calculated results propose that the improved electrocatalytic activity of PtAu bimetallic catalysts for methanol oxidation should be attributed to the alternation in the major reaction pathway from the CO pathway on the pure Pt surface to the non-CO pathway on the PtAu bimetallic surface rather than the easier removal of CO on PtAu catalysts than on pure Pt catalysts.
Co-reporter:Xueying Zhu, Peng Cui, Dongju Zhang, and Chengbu Liu
The Journal of Physical Chemistry A 2011 Volume 115(Issue 29) pp:8255-8263
Publication Date(Web):June 14, 2011
DOI:10.1021/jp201246j
By performing density functional theory calculations, we have studied the synthesis mechanism, electronic structure, and catalytic reactivity of a pyridinium-based ionic liquid, 1-ethylpyridinium trifluoroacetate ([epy]+[CF3COO]−). It is found that the synthesis of the pyridinium salt follows a SN2 mechanism. The electronic structural analyses show that multiple H bonds are generally involved in the pyridinium-based ionic liquid, which may play a decisive role for stabilizing the ionic liquid. The cation–anion interaction mainly involves electron transfer between the lone pair of the oxygen atom in the anion and the antibonding orbital of the C*–H bond (C* denotes the carbon atom at the ortho-position of nitrogen atom in the cation). This present work has also given clearly the catalytic mechanism of [epy]+[CF3COO]− toward to the Diels–Alder (D-A) reaction of acrylonitrile with 2-methyl-1,3-butadiene. Both the cation and anion are shown to play important roles in promoting the D-A reaction. The cation [epy]+, as a Lewis acid, associates the C≡N group by C≡N···H H bond to increase the polarity of the C═C double bond in acrylonitrile, while the anion CF3COO– links with the methyl group in 2-methyl-1,3-butadiene by C–H···O H bond, which weakens the electron-donating capability of methyl and thereby lowers the energy barrier of the D-A reaction. The present results are expected to provide valuable information for the design and application of pyridinium-based ionic liquids.
Journal of Molecular Modeling 2011 Volume 17( Issue 5) pp:1069-1073
Publication Date(Web):2011 May
DOI:10.1007/s00894-010-0815-4
The question whether Au can alloy with Pt at the nano-scale size is still controversial. By performing density functional theory calculations for several small Au/Pt bimetallic clusters AumPtn (m + n = 4–6, 13), we find that, in all the most stable geometries, Pt atoms prefer to assemble together to form the core while Au atoms like to surround the Pt atoms to form the shell, and that evenly mixed clusters are structurally unstable. The unique geometric characteristics can be explained by analyzing the different electronic properties of Pt–Pt, Au–Pt and Au–Au bonds, and is expected also to apply to larger Au/Pt bimetallic clusters.
Journal of Molecular Modeling 2011 Volume 17( Issue 8) pp:1997-2004
Publication Date(Web):2011 August
DOI:10.1007/s00894-010-0879-1
To better understand the property of the binary systems composing of imidazolium salt, [emim]+Aˉ (A=Clˉ, Brˉ, BF4ˉ, and PF6ˉ) and methanol, we have investigated in detail the interactions of methanol molecule with anions Aˉ, cation [emim]+, and ion pair [emim]+Aˉ of several ionic liquids (ILs) based on 1-ethyl-3-methylimidazolium cation by performing density functional theory calculations. It is found that H-bonds are universally involved in these systems, which may play an important role for the miscibility of methanol with imidazolium-based ILs. The interaction mechanisms of methanol molecule with anion and cation are found to be different in nature: the former mainly involves LPX-\( \sigma_{{O - H}}^{*} \) interaction, while the latter relates with the decisive orbital overlap of the type of LPO-\( \sigma_{{C - H}}^{*} \). Based on the present calculations, we have provided some reasonable interpretations for properties of the binary mixtures of ILs and alcohol and revealed valuable information for the interaction details between ILs and alcohols, which is expected to be useful for the design of more efficient ILs to form superior solvent system with alcohol.
Co-reporter:Jinxin Guo, Dongju Zhang, Chonggang Duan, Chengbu Liu
Carbohydrate Research 2010 Volume 345(Issue 15) pp:2201-2205
Publication Date(Web):13 October 2010
DOI:10.1016/j.carres.2010.07.036
The interactions of the cellulose molecule with several anions, including acetate , alkyl phosphate, tetrafluoroborate and hexafluorophosphate anions which are most commonly involved in the imidazolium ionic liquids (ILs), have been studied by performing density functional theory calculations. Based on calculated geometries, energies, IR characteristics, and electronic properties of the cellulose–anion complexes, it is found that the strength of interactions of anions with cellulose follows the order: acetate anion > alkyl phosphate anion > tetrafluoroborate anion > hexafluorophosphate anion, which is consistent with the experimentally observed solubility trend of cellulose in the corresponding imidazolium-based ILs. The present study may provide basic aids to some extent for understanding the dissolution behavior of cellulose in the imidazolium-based ILs.
The Journal of Physical Chemistry C 2010 Volume 114(Issue 33) pp:14076-14082
Publication Date(Web):August 3, 2010
DOI:10.1021/jp101470c
By performing density functional theory calculations, we show the mechanism details of CO oxidation catalyzed by several PtmAun (m + n = 4) clusters. It is found that in all situations, the reaction prefers to proceed via the single-center pathway to the two-center pathway according to a two-step mechanism: the initial activation of O2 molecule to form a peroxide-like intermediate followed by the rupture of the peroxide bond to complete the reaction. In Pt−Au bimetallic clusters, Pt sites are the catalytically active centers, whereas Au sites are “formally spectators” for CO oxidation. The calculated barriers for the reactions mediated by bimetallic clusters Pt3Au, Pt2Au2, PtAu3, are comparable with that catalyzed by monometallic Pt4 cluster, implying that the catalytic activity of Pt centers in the bimetallic clusters seems not to be dependent on its surroundings. On the basis of the present results, we propose an ideal configuration of Pt−Au bimetallic catalysts, where each active Pt atom is suitably spaced (stabilized) by Au atoms. Such catalysts are “less expensive and more efficient” compared to the corresponding pure Pt catalysts for CO oxidation at room temperature.
Co-reporter:Yuxia Liu, Dongju Zhang, Jianhua Zhou and Chengbu Liu
The Journal of Physical Chemistry A 2010 Volume 114(Issue 20) pp:6164-6170
Publication Date(Web):April 30, 2010
DOI:10.1021/jp102542r
The Au(I)-catalyzed cycloisomerization reactions of cycloalkyl-substituted 1,5-enynes (A) have been investigated by performing density functional theory (DFT) calculations. Theoretical calculations suggest that the reaction proceeds via a stepwise mechanism by the initial formation of a Au(I)-carbene intermediate (B), followed by a 1,2-alkyl shift or C−H insertion reaction to form the ring-expanded three-cyclic product (C) or ring-closed four-cyclic product (D) depending on the size of cycloalkyl substitutions. It is found that the formation of intermediate B is the rate-determining step, and the formation of products C or D is controlled by the size (n) of cycloalkyl substitutions in 1,5-enynes. In the situations with n = 1 and 2, the calculated relative free energies and the barriers consistently indicate that the 1,5-enynes prefer to evolve into product C to product D. In contrast, for the situation of n = 4, the barrier forming product C is found to be higher than that forming product D, supporting the experimental observation that a range of the 1,5-enynes with n = 4 isomerize into product D, although it is thermodynamically less favorable than product C. The present theoretical results provide insight into the mechanism details of the catalytic rearrangement of 1,5-enynes and rationalize the early experimental observations well.
Co-reporter:Peng Liu, Dongju Zhang, and Jinhua Zhan
The Journal of Physical Chemistry A 2010 Volume 114(Issue 50) pp:13122-13128
Publication Date(Web):November 19, 2010
DOI:10.1021/jp109306v
The effective enrichment and identification of lowly concentrated polychlorinated biphenyls (PCBs) in the environment is attracting enormous research attention due to human health concerns. Cyclodextrins (CDs) are known to be capable of forming inclusion complexes with a variety of organic molecules. The purpose of this study is to provide theoretical evidence of whether CDs as host molecules can include the guest molecules PCBs to form stable host−guest inclusion complexes, and if so, whether the general infrared and Raman techniques are suitable for the direction of CD-modified PCBs. Focusing on a representative PCB molecule, 2,2′,5,5′-tetrachlorobiphenyl (PCB52), we carried out density functional theory calculations and molecular dynamics (MD) simulations on its complexes with α-, β-, and γ-CDs with different host−guest stoichiometry ratios, including 1:1, 1:2, 2:1, and 2:2. On the basis of both the optimized geometries and calculated energy changes raised from encapsulating the guest molecule into the cavities of CDs, the CDs are believed to be suitable hosts for accommodating PCB52 guest molecules. The stability of inclusion complexes depends on both the type of CD and host−guest stoichiometry ratio. MD simulations give a clear picture of the scene on how the PCB52 molecule enters the cavity of β-CD. The vibrational analyses on the 1:1 complexes of CDs provide information for the spectral characterization of the inclusion complexes: Raman spectroscopy can deliver the characteristic bands of PCB52, whereas IR spectroscopy cannot uniquely assign them, implying that Raman spectroscopy is a useful technique for the identification of CD-modified PCBs. The present theoretical results are expected to provide guidance for the relevant experimental research.
Co-reporter:Hui Sun, Baofu Qiao, Dongju Zhang and Chengbu Liu
The Journal of Physical Chemistry A 2010 Volume 114(Issue 11) pp:3990-3996
Publication Date(Web):January 15, 2010
DOI:10.1021/jp910361v
Density functional theory (DFT) calculations combined with molecular dynamic (MD) simulations have been performed to show in detail the structure characteristic of 1-butylpyridinium tetrafluoroborate ([BPy+][BF4−]), a representative of pyridinium-based ionic liquids (ILs). It is found that the relative stability for ion pair configurations is synergically determined by the electrostatic attractions and the H-bond interactions between the ions of opposite charge. [BPy+][BF4−] IL possesses strong long-range ordered structure with cations and anions alternately arranging. The spatial distributions of anions and cations around the given cations are clearly shown, and T-shaped orientation is indicated to play a key role in the interaction between two pyridine rings. DFT calculations and MD simulations uniformly suggest that the H-bonds of the fluorine atoms with the hydrogen atoms on the pyridine rings are stronger than those of the fluorine atoms with the butyl chain hydrogens. The present results can offer useful information for understanding the physicochemical properties of [BPy+][BF4−] IL and further designing new pyridinium-based ILs.
Co-reporter:Rong-Xiu Zhu, Dong-Ju Zhang, Jin-Xin Guo, Jing-Lin Mu, Chong-Gang Duan and Cheng-Bu Liu
The Journal of Physical Chemistry A 2010 Volume 114(Issue 13) pp:4689-4696
Publication Date(Web):March 15, 2010
DOI:10.1021/jp100291c
A computational study with the B3LYP density functional theory was carried out to study the reaction mechanism for the cycloisomerization of allenes catalyzed by Au(I) and Au(III) complexes. The catalytic performance of Au complexes in different oxidation states as well as the effects of the counterion on the catalytic activities has been studied in detail. Our calculations show that the catalytic reaction is initiated by coordination of the Au(I) or Au(III) catalyst to the distal double bond of allene and activation of allene toward facile nucleophilic attack, then 3-pyrroline obtained via two-step proton shift, followed by demetalation. On the basis of our calculations, H shifts are key steps of the catalytic cycle, which are significantly affected by the gold oxidation state, counterion, ligands, and assistant catalyst. AuCl is found to be more reactive than AuCl3; however, the Au(III)-catalyzed path does not involve an oxidation state change from Au(III) to Au(I). Our calculated results rationalize the experimental findings well and overthrow the previous conjecture about Au(I) serving as the catalytically active species for Au(III)-catalyzed cycloisomerization.
The Journal of Physical Chemistry A 2010 Volume 114(Issue 49) pp:12893-12899
Publication Date(Web):November 16, 2010
DOI:10.1021/jp105292s
By carrying out density functional theory calculations, we have performed a detailed mechanism study for the cycloisomerization reaction of 4-phenyl-hexa-1,5-enyne catalyzed by homogeneous gold to better understand the observed different catalytic activity of several catalysts, including (PPh3)AuBF4, (PPh3)AuCl, AuCl3, and AuCl. In all situations, the reaction is found to involve two major steps: the initial nucleophilic addition of the alkynyl onto the alkene group and the subsequent 1,2-H migration. It is found that the potential energy surface profiles of systems are very different when different catalysts are used. For (PMe3)AuBF4- and (PMe3)AuCl-mediated systems, the nucleophilic addition is the rate-determining step, and the calculated free energy barriers are 15.2 and 41.9 kcal/mol, respectively. In contrast, for AuCl3- and AuCl-mediated systems, the reactions are controlled by the dissociations of catalysts from the product-like intermediates, and the calculated dissociation energies are 18.1 and 21.7 kcal/mol, respectively, which are larger than both the corresponding free energy barriers of the nucleophilic addition and the H-migration processes (8.5 and 7.3 kcal/mol for the AuCl3-mediated reaction, and 16.9 and 11.3 kcal/mol for the AuCl-mediated reaction). These results can rationalize the early experimental observations that the reactant conversion rates are 100, 0, and 50% when using (PPh3)AuBF4, (PPh3)AuCl, and AuCl3 as catalysts, respectively. The present study indicates that both the ligand and counterion of homogeneous Au catalysts importantly influence their catalytic activities, whereas the oxidation state of Au is not a crucial factor controlling the reactivity.
Co-reporter:Yingying Wang, Dongju Zhang, Zhangyu Yu and Chengbu Liu
The Journal of Physical Chemistry C 2010 Volume 114(Issue 6) pp:2711-2716
Publication Date(Web):January 26, 2010
DOI:10.1021/jp9103596
Density functional theory calculations have been performed to elucidate the mechanism of N2O formation over the Au(111) surface during NO reduction. It is shown that the dissociation of NO into an N atom and an O atom involves a barrier as high as 3.9 eV, implying that the formation of N2O does not occur via the direct dissociation mechanism of NO. Alternatively, we find that the reaction may occur via a dimer mechanism; i.e., two NO molecules initially associate into a dimeric (NO)2, which then dissociates into a N2O molecule and a N atom. We have scanned the potential energy surface forming N2O along different pathways, which involve a trapezoid OadNNOad dimer, an inverted trapezoid ONadNadO dimer, a zigzag ONadNOad dimer, or a rhombus ONadOadN dimer. The trapezoid dimer, OadNNOad, is found to be a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. The present results rationalize the early experimental findings well and enrich our understanding of the reduction of NO on the Au surface.
Co-reporter:Zhe Han, Dongju Zhang, Youmin Sun, Chengbu Liu
Chemical Physics Letters 2009 Volume 474(1–3) pp:62-66
Publication Date(Web):25 May 2009
DOI:10.1016/j.cplett.2009.04.044
Abstract
By performing DFT calculations, the reaction of 4-chlorophenol with OH is reexamined to reconcile an experimental finding with a recent theoretical result. The calculated results show that abstracting hydrogen atom in the hydroxyl of 4-chlorophenol by the OH is the most plausible process for forming 4-chlorocaechol intermediate, while adding OH to the aromatic ring is the dominant pathway for forming the hydroquinone. The 4-chlorophenoxyl radical plays a crucial role during the hydroxyl-initiated 4-CP degradation due to its energetic stability and the low barrier involved in the reaction, which supports the experimental finding but differs from the recent theoretical result.
Co-reporter:Fang Wang, Dongju Zhang, Xiaohong Xu and Yi Ding
The Journal of Physical Chemistry C 2009 Volume 113(Issue 42) pp:18032-18039
Publication Date(Web):September 24, 2009
DOI:10.1021/jp903392w
By carrying out density functional theory calculations, we studied the CO oxidation promoted by cationic, neutral, and anionic Au trimers, which represent the prototypes of Au-cluster-based catalysts with different charge states. The reaction is explored along three possible pathways: one involves the reaction of the initial complexes between Au trimers and O2 with CO; another is related to O2 interacting with the complexes between Au trimers and CO; and the third refers to a self-promoting mechanism; that is, the second CO oxidation is promoted by a preadsorbed CO molecule. The theoretical results show that all three species may promote the reaction, as indicated by calculated low energy barriers and high exothermicities, supporting the fact that cationic, neutral, and anionic Au species were all observed to present catalytic activity toward CO oxidation. Along the reaction coordinates for all of the reactions, Au−carbonate species are not found to be the necessary intermediates, although they are calculated to be energetically very stable. In contrast, by performing atom-centered density matrix propagation molecular dynamics simulations, the formation of such highly stable species is attributed to the effective collision between Au−oxides and CO2 with the carbon atom of CO2 directly attacking the O atom in the oxides. The present results enrich our understanding of the catalytic oxidation of CO by Au-cluster-based catalysts.
Co-reporter:Dongju Zhang, Hui Sun, Jianqiang Liu and Chengbu Liu
The Journal of Physical Chemistry C 2009 Volume 113(Issue 1) pp:21-25
Publication Date(Web):2017-2-22
DOI:10.1021/jp808819x
On the basis of the stoichiometry of TiO2 and the general bonding principles of Ti and O atoms, we show new defect-free and fullerene-like forms of TiO2 nanostructures. The proposed heterofullerenes of TiO2 possess the frameworks of carbon fullerenes and can be considered as arising from Ti2O4 units, the basic building blocks for TiO2 nanostructures, which are of structural stability and appropriate growth activity. By performing density functional theory calculations, we have presented theoretical evidence for the structural and thermal stabilities of the heterofullerenes. These stoichiometric and structurally defect-free nanocages display unique frequency modes at ∼827−854 cm−1 and a larger HOMO−LUMO energy gap than bulk TiO2 materials. The present study provides a practical guide for the experimentalists who are devoting their attention to novel TiO2 polymorphs. TiO2 heterofullerenes, if synthesized, may find novel applications in nanotechnology.
Journal of Molecular Structure: THEOCHEM 2009 Volume 895(1–3) pp:77-81
Publication Date(Web):15 February 2009
DOI:10.1016/j.theochem.2008.10.027
The fragment ions Cu+(CH3N) and Cu+(C2H5N) have been detected in the recent experiment from the products of the ion-molecule reactions of the Cu+ ion with methylamine (MA, CH3NH2) and dimethylamine (DMA, (CH3)2NH) using the time-of-flight mass spectrum technology. Their forming mechanisms and the structural characteristics, however, are still not clear. Here, we show a detailed DFT study for the potential energy surfaces relevant for the reactions of the Cu+ ion with MA and DMA to address the concerned issues. We find that the ion-molecule reactions are driven by a large energy gain upon the association of the Cu+ ion with MA or DMA. A general dehydrogenation mechanism of MA and DMA promoted by the Cu+ ion has been shown, and the preponderant structures contributing to the recorded mass spectra for the product ions Cu+(CH3N) and Cu+(C2H5N) have been formulized as Cu+–CH2NH and Cu+–CH2NHCH2. The present study represents a prototype of the reaction of the Cu+ ion with amine, and the conclusion drawn out from this work is expected to provide a consistent view on the reactivity of this kind of reactions.
Chemical Physics Letters 2008 Volume 467(1–3) pp:131-135
Publication Date(Web):15 December 2008
DOI:10.1016/j.cplett.2008.11.002
Abstract
The adsorption of formaldehyde (HCOH) molecule on the pristine and silicon-doped (Si-doped) single-walled (8, 0) boron nitride nanotubes (BNNTs) is investigated using density functional theory (DFT) calculations. Compared with the weak physisorption on the pristine BNNT, the HCOH molecule presents strong chemisorption on both silicon-substituted boron defect site and silicon-substituted nitrogen defect site of the BNNT, as indicated by the calculated geometrical structures and electronic properties for these systems. It is suggested that the Si-doped BNNT presents high sensitivity to toxic HCOH. Based on calculated results, the Si-doped BNNT is expected to be a potential novel sensor for detecting the presence of HCOH.
The Journal of Physical Chemistry C 2008 Volume 112(Issue 43) pp:16729-16732
Publication Date(Web):October 4, 2008
DOI:10.1021/jp807264n
We show by performing density functional theory calculations that the dimer of titanium dioxide (TiO2) molecule, Ti2O4, is qualified for serving as a basic building block of TiO2 nanostructures owing to its structural stability and appropriate growth activity. In particular, the two thinnest titanium dioxide nanowires, as the prototypes of TiO2 nanostructures, have been assembled and proved to be geometrically graceful and dynamically stable. Calculated results show that the size and shape of TiO2 nanowires have important effects on their structural stabilities and energy gaps, proposing that tailoring the size and shape of TiO2 nanowires may be an effective way to modulate the band gap and finally improve their optical properties.
Co-reporter:Xiaofeng Wei, Dongju Zhang, Chengbu Liu
Journal of Molecular Structure: THEOCHEM 2008 Volume 859(1–3) pp:1-6
Publication Date(Web):30 June 2008
DOI:10.1016/j.theochem.2008.02.020
The reactivity of silica toward to water is an important issue in environmental science and materials science for many years. In this paper, we study the hydrolysis stabilities of silica molecular chains and molecular rings based on two-membered silica ring by performing density functional theory calculations. It is found that the hydrolysis for the linear molecular chains firstly takes place in the middle parts and carries on gradually to the ends. In contrast with the early conjecture that the molecular rings might have higher hydrolysis stability than the corresponding linear chains, we find that these fully coordinated molecular rings are less stable than the corresponding molecular chains with the presence of water molecule.
Co-reporter:Dongju Zhang, Rongxiu Zhu and Chengbu Liu
Journal of Materials Chemistry A 2006 vol. 16(Issue 25) pp:2429-2433
Publication Date(Web):25 Apr 2006
DOI:10.1039/B517480E
Single-walled boron nanotubes (BNTs), which has been synthesized successfully recently, were imagined as duals (hexagonal pyramidal structures) of carbon nanotubes (CNTs) in the literature. In this work, we call attention to the fact that BNTs are not limited to hexagonal pyramidal structures constructed from the so-called Aufbau principle, and alternatively, we propose that the thinnest BNT may be a geometrical analog of the corresponding CNT. As shown by our density functional theory calculations, both the tubular open-end cluster models and the infinitely long tube possess high structural, dynamic, and thermal stability, which should be of interest for attempts at its synthesis. Compared to the energetically most stable isomers of the corresponding clusters, the thinnest BNT might be a metastable structure, and from an electronic view of point, it was predicted to have metallic conductivity like hexagonal pyramidal BNTs predicted previously, in contrast to semiconducting crystalline rhombohedral α- and β-boron.
Co-reporter:Dongju Zhang Dr.;Chengbu Liu Dr.;Siwei Bi;Shiling Yuan
Chemistry - A European Journal 2003 Volume 9(Issue 2) pp:
Publication Date(Web):16 JAN 2003
DOI:10.1002/chem.200390051
The reactions of Sc+(3D) with methane, ethane, and propane in the gas phase were studied theoretically by density functional theory. The potential energy surfaces corresponding to [Sc, Cn, H2n+2]+ (n=1–3) were examined in detail at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(d,p) level of theory. The performance of this theoretical method was calibrated with respect to the available thermochemical data. Calculations indicated that the reactions of Sc+ with alkanes are multichannel processes which involve two general mechanisms: an addition–elimination mechanism, which is in good agreement with the general mechanism proposed from earlier experiments, and a concerted mechanism, which is presented for the first time in this work. The addition–elimination reactions are favorable at low energy, and the concerted reactions could be alternative pathways at high energy. In most cases, the energetic bottleneck in the addition–elimination mechanism is the initial CC or CH activation. The loss of CH4 and/or C2H6 from Sc++CnH2n+2 (n=2, 3) can proceed along both the initial CC activation branch and the CH activation branch. The loss of H2 from Sc++CnH2n+2 (n=2, 3) can proceed not only by 1,2-H2 and/or 1,3-H2 elimination, but also by 1,1-H2 elimination. The reactivity of Sc+ with alkanes is compared with those reported earlier for the reactions of the late first-row transition-metal ions with alkanes.
Co-reporter:Jingjing Li, Zhe Han, Dongju Zhang, Jun Gao, Chengbu Liu
Applied Catalysis A: General (October 2014) Volume 487() pp:
Publication Date(Web):1 October 2014
DOI:10.1016/j.apcata.2014.08.033
•The hydrodesulfurization of thiophene by a W-based complex is theoretically studied.•The catalyzed hydrodesulfurization process involves five sub-processes.•The theoretical results rationalize the earlier experimental observations.•W-based complex may be a potential hydrodesulfurization catalyst.While the hydrodesulfurization (HDS) of thiophenes has been achieved successfully by tungsten-based complexes, the relevant molecular mechanism is still not well understood. By performing density functional theory calculations, the present work for the first time provides a detailed mechanism study of the entire HDS process of thiophene by a representative tungsten complex W(PMe3)4(η2-CH2PMe2)H. In detail, the HDS process consists of four sub-processes: (i) binding of thiophene to the metal center to give the metallathiacycle species W(PMe3)4(η2-SC4H4), (ii) formation of the tungsten butadiene–thiolate intermediate, (η5-C4H5S)W(PMe3)2(η2-CH2PMe2), (iii) hydrogenation of the butadiene–thiolate intermediate, and (iv) liberation of the desulfur product but-1-ene. The overall barriers of the four sub-processes were calculated to be 25.5, 26.7, 31.5, and 43.3 kcal/mol, respectively, which qualitatively rationalizes the experimental observations that the tungsten butadiene–thiolate intermediate was observed under the mild temperature (60 °C), whereas the desulfur product was obtained upon thermolysis at elevated temperature (100 °C). The regeneration of the tungsten complex is also discussed to evaluate its potential possibility serving as a HDS catalyst. The present theoretical results are expected to shed light on practical heterogeneous HDS mechanism of tungsten-based complexes.Download high-res image (124KB)Download full-size image
Catalysis Communications (5 December 2012) Volume 29() pp:82-86
Publication Date(Web):5 December 2012
DOI:10.1016/j.catcom.2012.09.002
Density functional theory (DFT) calculations show a new concerted mechanism of formic acid (HCOOH) oxidation on Pt (111), which involves the simultaneous formation of CO2 and CO via the HCOOH dimer in an elementary step. The newly proposed mechanism rationalizes the easy CO poisoning of Pt-based catalysts and improves our understanding for the mechanism of catalytic HCOOH oxidation.Download full-size imageHighlights► A new dimer model of formic acid oxidation (HCOOH) on Pt (111) surface is presented. ► The mechanism of simultaneous formation of CO2 and CO is obtained. ► The easy CO poisoning of Pt-based catalysts is rationalized.
Journal of Organometallic Chemistry (1 March 2017) Volume 831() pp:
Publication Date(Web):1 March 2017
DOI:10.1016/j.jorganchem.2017.01.001
•Catalytic borylations of aryl acetylene and aryl ethylene are designed and calculated theoretically.•Aryl acetylene favors the diborylation product by inserting the CC bond into the RhB bond.•Aryl ethylene prefers the borylation product by breaking its terminal hydrogen-vinyl bond.•The borylation of aryl ethylene is more favorable than ones of aryl acetylene and aryl cyanide.As a new type of catalytic borylations, the Rh-catalytic borylation reaction of aryl cyanides has been reported experimentally (Tobisu et al. J. Am. Soc. Chem. 2012, 134, 115). Here, to aid experimental designs of the novel type of borylation reactions, we theoretically predict Rh-catalytic borylations of two alternative substrates of aryl cyanides, i.e. aryl acetylene and aryl ethylene. Both borylation and diborylation pathways have been calculated in details for each substrate. It is found that aryl ethylene prefers to the borylation product via the β-H elimination mechanism, in contrast, aryl acetylene favors the diborylation product by inserting the CC bond into the RhB bond. The theoretical results are expected to be helpful for the preparation of organoboron compounds.DFT calculation results showed diborylation of aryl acetylene and borylation of aryl ethylene are thermodynamically and kinetically favorable.
With the aid of density functional theory (DFT) calculations, we have performed a detailed mechanism study for the catalytic cycloisomerization reactions of propargylic 3-indoleacetate to better understand the observed divergent reactivities of two catalysts, PtCl2 and (PPh3)AuSbF6, which result in [3 + 2]- and [2 + 2]-cycloaddition products, respectively. The calculated results confirm that the lactone intermediates are common and necessary species for the formation of the two products and that PtCl2 and (PH3)AuSbF6 respectively favor the formation of the [3 + 2]- and [2 + 2]-products. The intrinsic reasons for the divergent reactivities of the two catalysts have been analyzed in detail. We believe that the essentially different metal–ligand interactions in PtCl2 and (PPh3)AuSbF6 are mainly responsible for their divergent regioselectivities, while the solvent effects have little impact on the catalyst activity. Starting from lactone intermediates, PtCl2 induces the intramolecular nucleophilic addition to give the [3 + 2] cycloisomerization product due to the strong π-electron-donating ability of chlorine ligands, while (PPh3)AuSbF6 results in the intramolecular nucleophilic addition reaction to form the [2 + 2] cycloisomerization product because of the strong σ-electron-donating ability of phosphine ligands.
The reactions of (2,6-difluorophenyl)phenylmethanone (2,6-F2C6H3–C(O)–C6H5) (1) and (2,6-difluorophenyl)phenylmethanimine (2,6-F2C6H3–C(NH)–C6H5) (3) with Fe(PMe3)4 afforded different selective C–F/C–H bond activation products. The reaction of 1 with Fe(PMe3)4 gave rise to bis-chelate iron(II) complex [C6H5–C(O)–3-FC6H3)Fe(PMe3)]2 (2) via C–F bond activation. The reaction of 3 with Fe(PMe3)4 delivered chelate hydrido iron(II) complex 2,6-F2C6H3–C(NH)–C6H4)Fe(H)(PMe3)3 (4) through C–H bond activation. The DFT calculations show the detailed elementary steps of the mechanism of formation of hydrido complex 4 and indicate 4 is the kinetically preferred product. Complex 4 reacted with HCl, CH3Br and CH3I delivered the chelate iron halides (2,6-F2C6H3–C(NH)–C6H4)Fe(PMe3)3X (X = Cl (5); Br (6); I (7)). A ligand (PMe3) replacement by CO of 4 was observed giving (2,6-F2C6H3–C(NH)–C6H4)Fe(H)(CO)(PMe3)2 (8). The chelate ligand exchange occurred through the reaction of 4 with salicylaldehydes. The reaction of 4 with Me3SiCCH afforded (2,6-F2C6H3–C(N)–C6H5)Fe(CC–SiMe3)(PMe3)3 (11). A reaction mechanism from 4 to 11 was discussed with the support of IR monitoring. The molecular structures of complexes 2, 4, 6, 7, 10 and 11 were determined by X-ray diffraction.
The calculated results in the literature for the decomposition of formic acid into CO on Pt(111) cannot rationalize the well-known facile CO poisoning of Pt-based catalysts. The present work reexamines formic acid decomposition on Pt(111) by considering both the monomer and dimer pathways both in the absence and in the presence of water molecules. Upon a thorough search, we locate some new adsorption configurations of formic acid for the subsequent C–H or O–H cleavage, which were ignored in previously published literature. From the calculated minimum energy pathways (MEPs), we propose that formic acid decomposition on Pt(111) is initiated by C–H bond cleavage rather than O–H bond cleavage. Monomeric formic acid preferentially decomposes to CO2 regardless of the absence or presence of water, while the dimeric form favors the formation of CO in the gas phase, but competitively reacts to form CO and CO2 in the presence of water molecules. The surrounding water molecules and the second formic acid molecule in the dimer play substantial roles in the process of formic acid decomposition, and can be regarded as promoters, assisting formic acid decomposition on Pt(111). In all situations, that is, regardless of the presence of a monomer or dimer, or the presence or absence of water, the calculated barrier differences in the rate-determining steps of CO2 and CO formation are much smaller than those reported in previously published studies. These results improve our understanding of the mechanism of formic acid oxidation catalyzed by Pt-based catalysts, and rationalize the performance durability problem of Pt-based catalysts used in direct formic acid fuel cells.
While selective C–H and C–F activations of fluoroaromatic imines and ketones with transition metal complexes supported by PMe3 have been successfully achieved in recent publications, insight into the molecular mechanism and energetics of those reactions is still lacking. Focusing on three typical substrates, 2,6-difluorobenzophenone imine (A) and 2,6-difluorobenzophenone (B), and 2,4′-difluorobenzophenone (C), the present work theoretically studied their C–H and C–F cyclometalation reactions promoted by the activator Co(PMe3)4 or CoMe(PMe3)4. It is found that reaction A + Co(PMe3)4 favors the C–F activation, reaction A + CoMe(PMe3)4 prefers the C–H activation, whereas both the C–H and C–F activation pathways may be viable for reactions B + CoMe(PMe3)4 and C + CoMe(PMe3)4. The experimentally observed C–H and C–F cyclometalation products have been rationalized by analyzing the thermodynamic and kinetic properties of two activation pathways. From calculated results combined with the experimental observations, we believe that three factors, i.e. the oxidation state of the metal center in the activators, the anchoring group of substrates, and substituted fluoroatom counts of the aromatic ring in substrates, affect the selectivity of C–H and C–F activations of fluoroaromatic ketones and imines. Calculated results are enlightening about the rational design of activators and substrates of fluoroaromatic imines and ketones to obtain the exclusive C–H or C–F bond activation product.
