Conflicting experimental results for the electrocatalytic reduction of CO2 to CH3OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine have been recently reported [ J. Am. Chem. Soc. 2014, 136, 14007−14010, J. Am. Chem. Soc. 2016, 138, 1017–1021]. In this connection, we have used computational chemistry to examine the issue of this molecule’s ability to act as a hydride donor to reduce CO2. We first determined that the most thermodynamically stable tautomer of this aqueous compound is its oxothione form, termed here PTE. It is argued that this species electrochemically undergoes concerted 2H+/2e– transfers to first form the kinetic product 5,8-dihydropteridine, followed by acid-catalyzed tautomerization to the thermodynamically more stable 7,8-dihydropteridine PTEH2. While the overall conversion of CO2 to CH3OH by three successive hydride and proton transfers from this most stable tautomer is computed to be exergonic by 5.1 kcal/mol, we predict high activation free energies (ΔG‡HT) of 29.0 and 29.7 kcal/mol for the homogeneous reductions of CO2 and its intermediary formic acid product by PTE/PTEH2, respectively. These high barriers imply that PTE/PTEH2 is unable, by this mechanism, to homogeneously reduce CO2 on a time scale of hours at room temperature.

Ferrite spinels are metal oxides used in a wide variety of applications, many of which are controlled by the diffusion of metal cations through the metal oxide lattice. In this work, we used density functional theory (DFT) to examine the diffusion of Fe, Co, and Ni cations through the Fe3O4, CoFe2O4, and NiFe2O4 ferrite spinels. We apply DFT and crystal field theory to uncover the principles that govern cation diffusion in ferrite spinels. We found that a migrating cation hops from its initial octahedral site to a neighboring octahedral vacancy via a tetrahedral metastable intermediate separated from octahedral sites by a trigonal planar transition state (TS). The cations hop with relative activation energies of Co ≈< Fe < Ni; the ordering of the diffusion barriers is controlled by the crystal field splitting of the diffusing cation. Specifically, the barriers depend on the orbital splitting and number of electrons which must be promoted into the higher energy t2g orbitals of the tetrahedral metastable intermediate as the cations move along the minimum energy pathway of hopping. Additionally, for each diffusing cation, the barriers are inversely proportional to the spinel lattice parameter, leading to relative barriers for cation diffusion of Fe3O4 < CoFe2O4 < NiFe2O4. This results from the shorter cation-O bonds at the TS for spinels with smaller lattices, which inherently possess shorter bond lengths, and consequently higher system energies at their more constricted TS geometries.

High-performance lithium-ion batteries require electrolytes that are stable over wide operating voltages. We used density functional theory to investigate the degradation of ethylene carbonate (EC) electrolytes activated by interactions with LiCoO2 cathode surfaces and PF5 species in the electrolyte. We report detailed mechanisms for the activation of EC ring-opening reactions by Lewis acids to form CO2, organics, or organofluorines. We find that Lewis acid–base complexation between EC and either PF5 or LiCoO2 weakens the C–O bonds of the EC ring and consequently lowers the barrier to and energy of EC ring-opening reactions. Our results predict that ring opening activated by the LiCoO2 cathode surface forms a cathode–electrolyte interphase primarily composed of an organic and organofluorine film. Simultaneous degradation of an EC molecule and PF6– forms PF5 and a surface organofluorine with an activation barrier of 1.28 eV and reaction energy of −0.26 eV. Ring opening of EC activated by the cathode to form short organic oligomers results from sequential ring-opening reactions at the surface with an activation barrier of 1.04 eV and an overall reaction enthalpy of −1.15 eV for the case of EC dimer formation. Complexation of EC with PF5 lowers the barrier to EC ring opening to form CO2 from 1.96 to 1.68 eV and the reaction energy from 0.02 eV to −1.38 eV relative to unactivated CO2 formation. We expect that EC electrolyte degradation at the cathode surface will be dominated by EC dimer formation reactions activated by PF5 because of their low reaction barriers relative to CO2 formation.Keywords: cathode electrolyte interphase; electrolyte degradation; ethylene carbonate oxidation; ethylene carbonate ring opening; Lewis acid activated

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in nitrogen (N2) to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH3, and NH3 desorption. We found activation barriers (Ea) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al–N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal–nitrogen bonds and, thus, more-facile NH3 liberation.

A screening method is developed to determine the viability of candidate redox materials to drive solar thermal water splitting (STWS) and the mechanism by which they operate using only the reduction enthalpy of the material. This method is applied to the doped-hercynite water splitting cycle, as well as FeAl2O4 and CoAl2O4, materials which have not been previously experimentally demonstrated for STWS. Density functional theory (DFT) calculations of reduction energies coupled with our screening method predict H2 production capacities for iron and cobalt aluminate spinels to be in the order FeAl2O4 > Co0.5Fe0.5Al2O4 > CoAl2O4 with relative H2 production capacity ratios of approximately 1.0 to 0.7 to 2 × 10−4, respectively. Experimental measurements for 1500/1350 °C redox temperatures validate the H2 production capacity predicted by the screening method by demonstrating H2 production ratios of 1.0 to 0.6 to 0. Un-doped hercynite (FeAl2O4) is shown to be a viable STWS material for the first time with a higher H2 production capacity than traditional doped-hercynite materials. Theory and experiments show that redox of the aluminate family of spinel materials operates via an O-vacancy mechanism rather than a stoichiometric one, which is more typical for ferrites. The screening approach is generally useful for predicting the ability of new complex materials to drive STWS and the mechanism by which they operate, thus, providing a method to identify promising new candidate STWS materials.

Electrochemical supercapacitors utilizing α-MnO2 offer the possibility of both high power density and high energy density. Unfortunately, the mechanism of electrochemical charge storage in α-MnO2 and the effect of operating conditions on the charge storage mechanism are generally not well-understood. Here, we present the first detailed charge storage mechanism of α-MnO2 and explain the capacity differences between α- and β-MnO2 using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the viability of a given electrode material. The high capacity of α-MnO2 results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial and substitutional cations (H+, Li+, Na+, and K+) incorporated into the material. Interstitial cations are found to induce charge-switching states by stabilizing Mn-O antibonding orbitals from the conduction band. Substitutional cations interact with O[2p] dangling bonds that are destabilized from the valence band by Mn vacancies to induce charge-switching states. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can be used to computationally screen new materials for improved charge storage via ion incorporation.

Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuPPh3 onto the Ag surface because the PPh3 ligand is large compared to the size of the exposed surface between adsorbed benzenes. Instead, the Au–P bond of MeAuPPh3 dissociates in solution (Ea = 38.5 kcal/mol) in the plasmon heated near-surface region followed by the adsorption of the MeAu fragment on Ag in the interstitial space of the benzene monolayer. The adsorbed benzene forces the Au precursor to react through the higher energy path of dissociation in solution rather than dissociatively adsorbing onto the bare surface. This requires a higher temperature if the reaction is to proceed at a reasonable rate and enables the control of deposition by the light induced SPR heating of the surface and nearby solution.Keywords: bottom-up nanofabrication; density functional theory; organogold chemistry; organometallic chemistry; preferential molecular adsorption; thermal decomposition;

Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied using density functional theory. Cathode degradation caused by reaction of HF with the hydroxylated (101̅4) LiCoO2 surface is dominated by formation of H2O and a LiF precipitate via a barrierless reaction that is exothermic by 1.53 eV. We present a detailed mechanism where HF reacts at the alumina coating to create a partially fluorinated alumina surface rather than forming AlF3 and H2O and thus alumina films reduce cathode degradation by scavenging HF and avoiding H2O formation. In contrast, we find that HF etches monolayer zinc oxide coatings, which thus fail to prevent capacity fading. However, thicker zinc oxide films mitigate capacity loss by reacting with HF to form a partially fluorinated zinc oxide surface. Metal oxide coatings that react with HF to form hydroxyl groups over H2O, like the alumina monolayer, will significantly reduce cathode degradation.Keywords: ald coatings; battery cycling; capacity fading; cathode thin films; LiCoO2 cathode

•Increased Lewis acid–base interactions correspond to lower HF formation barriers.•The barrier to HF generation from POF3 is 10.4 kcal mol−1 higher than from PF5.•An ethylene carbonate molecule acts as a catalyst to HF formation from PF5.We utilized density functional theory to examine HF generation in lithium-ion battery electrolytes from reactions between H2O and the decomposition products of three electrolyte additives: LiPF6, LiPOF4, and LiAsF6. Decomposition of these additives produces PF5, AsF5, and POF3 along with LiF precipitates. We found PF5 and AsF5 react with H2O in two sequential steps to form two HF molecules and POF3 and AsOF3, respectively. PF5 (or AsF5) complexes with H2O and undergoes ligand exchange to form HF and PF4OH (AsF4OH) with an activation barrier of 114.2 (30.5) kJ mol−1 and reaction enthalpy of 14.6 (−11.3) kJ mol−1. The ethylene carbonate (EC) electrolyte forms a Lewis acid–base complex with the PF4OH (AsF4OH) product, reducing the barrier to HF formation. Reactions of POF3 were examined and are not characterized by complexation of POF3 with H2O or EC, while PF5 and AsF5 complex favorably with H2O and EC. HF formation from POF3 occurs with a reaction enthalpy of −3.8 kJ mol−1 and a 157.7 kJ mol−1 barrier, 43.5 kJ mol−1 higher than forming HF from PF5. HF generation in electrolytes employing LiPOF4 should be significantly lower than those using LiPF6 or LiAsF6 and LiPOF4 should be further investigated as an alternative electrolyte additive.

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H+/1e–/1H+/1e– reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP+/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e– reduction. A HT–PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H+ such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.

We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH2). First, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+), followed by an e– transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate PyH+’s LUMO (E0calc ∼ −1.3 V vs SCE) to form the solution phase PyH0 via highly reducing electrons include the photoelectrochemical p-GaP system (ECBM ∼ −1.5 V vs SCE at pH 5) and the photochemical [Ru(phen)3]2+/ascorbate system. We predict that PyH0 undergoes further PT–ET steps to form the key closed-shell, dearomatized (PyH2) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT–ET–PT–ET mechanism for transforming Py into PyH2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C–H bonds in the photosynthetic CO2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO2, are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.

We utilized density functional theory (DFT) to systematically investigate the ability of B, C, and N interstitial and O substitutional surface and near-surface dopants in TiO2 to facilitate O2 reduction and adsorption. Periodic boundary condition calculations based on the PBE+U DFT functional show that dopants that create filled band gap states with energies higher than that of the near surface O2 πz* molecular orbital enable O2 adsorption and reduction. Sites that create unoccupied band gap states with energies below that of the O2 πz* orbital reduce TiO2’s reduction ability as these states result in photoexcited electrons with insufficient reduction potential to reduce O2. B dopants in interstitial and relaxed substitutional sites, whose gap states lie >1.5 eV above the valence band maximum (VBM) and hence above O2’s πz* level, facilitate the reduction of O2 to the peroxide state with adsorption energies on TiO2 of −1.22 to −2.77 eV. However, N dopants, whose gap states lie less than ∼1 eV above the VBM impede O2 adsorption and reduction; O2 on N-doped (101) anatase relaxes away from the surface. Interstitial and substitutional N dopants require two photoexcited electrons to enable O2 adsorption. C doping, which introduces gap states between those introduced by N and B, aids O2 adsorption as a peroxide for interstitial doping, although substitutional C does not facilitate O2 adsorption. Dopants for enhancing the photocatalytic reduction of O2 in order of predicted effectiveness are interstitial B, relaxed substitutional B, and interstitial C. In contrast, substitutional C and interstitial and substitutional N hinder O2 reduction despite increasing visible light absorption. Dopants within the surface layer likely deactivate quickly due to the high exothermicity of O2 reacting with them to form BO2, CO2, and NO2.

We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO2 by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO2 for reduction. We find that the LA (trichloroaluminum, AlCl3) alone catalyzes hydride transfer (HT) to CO2 while the LB (trimesitylenephosphine, PMes3) actually hinders HT; inclusion of the LB increases the HT barrier by ∼8 kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders HT by donating its lone pair to the LUMO of CO2, increasing the electron density on the C atom and thus lowering its hydride affinity. Although the LB hinders HT, it nonetheless plays a crucial role by stabilizing the active FLP·CO2 complex relative to the LA dimer, free CO2, and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO2 and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO2 reductions.

We utilize periodic density functional theory to study singly and triply N- and B-substituted graphene. We examine their doping mechanisms and effects on Pt atom adsorption and migration on graphene. We find a seemingly contradictory behavior between dopant type (n- vs p-type) and charge accumulation on the dopant atoms: the N atoms in both n-type singly N-doped graphene (NG) and p-type triply N-doped graphene (3NG) gain electron density while the B atoms in both singly (BG) and triply (3BG) B-doped graphene are p-type and lose electron density. This behavior arises from unequal charge sharing within C–B and C–N sp2 σ bonds and the requirement that the pz orbitals of N and B are singly occupied in order to maintain graphene’s aromaticity. NG’s N atom stabilizes Pt atom adsorption up to −0.39 eV (Eads = −1.86 eV) and by −0.13 eV even at distances 12.3 Å away from the N dopant. The Pt atom hopping energy barrier is lowered in graphene rings containing an NG N atom relative to undoped graphene, but the migration of a Pt atom over the N atom is unlikely due to a 1.0 eV barrier. 3NG’s most stable Pt adsorption site (Eads= −2.86 eV) is the vacant C site at the center of 3NG’s three N atoms and arises because of the formation of covalent bonds between Pt’s d orbitals and the N atoms’ three in-plane dangling sp2 orbitals. When a Pt atom adsorbs at a ring containing a pyridinic N, the strong N–Pt bonds trap the Pt atom, limiting its diffusion over the graphene sheet. The BG and 3BG structures bind Pt with a maximum adsorption energy of Eads= −2.16 eV and −5.30 eV, respectively. BG’s high-lying B–C bonding orbitals allow the Pt atom to form strong σ bonds directly to the graphene sheet, while 3BG’s B atoms donate electron density to the Pt atom creating an ionic bond between the negative Pt atom and the positive B atoms. These bonding mechanisms result in only short-range Pt stabilization and the B atoms having little influence on Pt atom migration outside B containing C rings; however, the depth and short-range nature of these energy wells funnel Pt atoms toward the B atoms and trap them there.

We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO2 reduction by Py commences with PyCOOH0 formation where: (a) protonated Py (PyH+) is reduced to PyH0, (b) PyH0 then reduces CO2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO2, and finally (c) proton transfer (PT) from PyH0 to CO2 produces PyCOOH0. The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH0 to CO2. However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6–16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pKa for PyH0 of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH0 formation, confirming PyH0’s pKa as irrelevant for predicting PT from PyH0 to CO2. Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO2, Py, and Py·CO2 of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO– stabilizes the anionic radicals CO2– and Py– to facilitate low barrier ET. As the reduction of CO2 proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO2 reduction by stabilizing the PCET TS and the PyCOOH0 product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH3OH through a series of one-electron reductions analogous to the PCET reduction of CO2 examined here, where the electrode only acts to reduce PyH+ to PyH0.

The roles of deposited Pt clusters and adsorbed O2 in the photoactivity of anatase TiO2 (101) surfaces have been studied using density functional theory. O2 only adsorbs to TiO2 surfaces when excess negative charge is available to form O–Ti bonds, which can be provided by a photoexcited electron or subsurface oxygen vacancy, in which cases the adsorption energies are −0.94 and −2.52 eV, respectively. When O2 adsorbs near a subsurface defect, it scavenges extra electron density and creates a hole that can annihilate excited electrons. In aqueous solutions, O2 interactions with the TiO2 surface are rare because water preferentially adsorbs at the surface. Pt clusters on TiO2 significantly enhance O2 adsorption providing many adsorption sites with adsorption energies up to −1.69 eV, stronger than the −0.52 eV adsorption energy of H2O on the Pt cluster. Consequently, Pt increases the rate of electron scavenging by O2 relative to that of undoped TiO2 leading to enhanced photocatalytic performance. Pt states completely bridge the band gap and act as electron–hole recombination centers, which are deleterious to the photoactivity of TiO2. The initial rise and subsequent fall in TiO2’s photoactivity with Pt loading results from the competition between enhanced electron scavenging due to increased O2 adsorption and increased electron–hole recombination.

Growth of Ptn (n ≤ 37) clusters on the defect-free TiO2 anatase (101) surface has been studied using ab initio pseudopotential calculations based on density functional theory. Several initial configurations for clusters of 1, 2, 7, 10, and 37 atoms were relaxed to determine the most stable structures. All final optimized structures are three dimensional, suggesting that formation of island-like particles is favored over planar monolayers, as verified experimentally using Pt atomic layer deposition and high-resolution transmission electron microscopy. Diffusion barriers of a single Pt adatom on TiO2 were calculated to understand the mobility of Pt atoms on the TiO2 surface. Activation barriers of 0.86 and 1.41 eV were calculated for diffusion along the [010] and [101̅] directions, respectively, indicating that Pt atoms are relatively mobile along the [010] direction at moderate temperatures. The energy barriers for a Pt atom to escape from an 11- and a 37-atom Pt cluster on (101) anatase are predicted to be 1.38 and 2.12 eV, suggesting that particle coarsening occurs by Ostwald ripening and that Ostwald ripening of deposited Pt particles is limited by atom detachment from particles as small as several tens of atoms.

The dynamic mechanisms involved in the dehydrogenation of ammonia borane are investigated using quasi-classical trajectory simulations. The effects of solvent and nuclear motion yield qualitatively different results compared to simulations where these considerations are neglected. Not only are rate-limiting barriers substantially different from the gas to solvent phase, trajectories leading from transition states branch to different products depending on the presence or lack of solvent. In addition, the formation of the diammoniate of diborane is shown to be noncompetitive in the gas phase due to the presence of a lower-barrier dehydrogenation pathway. The first comparative analysis of the pathways leading to the thermolysis of ammonia−borane is presented herein.Keywords: ab initio; ammonia borane; hydrogen evolution; molecular dynamics; solvent effects;

Reactions of amino acids on the Si(100)-2×1 surface have been investigated using the B3LYP hybrid density functional theory. Amino acids provide a diverse set of organic functional groups, several of which have not been studied previously for their reactivity on semiconductor surfaces. The amino acid common group is found to react through several low energy pathways, with O−H dissociation of the carboxyl group being the most kinetically favorable. Consequently, reactions of amino acids through the amine and carboxyl functionalities are not expected to be selective. The reactivity of several of the amino acids displays unique, and possibly useful, properties not exhibited by organic functionalities previously considered, while others react on Si(100)-2×1 analogously to their simpler organic analogues. Resonance effects significantly affect the reaction energetics of arginine, histidine, and tryptophan leading to reactivities qualitatively different from their analogous isolated functional groups. The most unique of these reactivities include pericyclic ene reactions of the imine functional group of arginine and the cyclic imine of the imidazole side-chain of histidine. Both of these reactions involve formation of Si−N dipolar (dative) bonds which are significantly stronger than any previously observed on Si(100)-2×1.

We use B3LYP hybrid density functional theory to investigate atomistic mechanisms for the atomic layer deposition (ALD) of tantalum nitride (TaN) grown using tert-butylimidotris(diethylamido)tantalum [(tBuN)(NEt2)3Ta, TBTDET], and ammonia (NH3) as precursors. Our calculations examine various possible mechanisms for TaN growth by ALD and metal organic chemical vapor deposition (MOCVD). In particular, we identify low barrier (10.6 and 27.6 kcal/mol) ligand exchange mechanisms with NH3 that lead to incorporation of NH3’s nitrogen into the film. Ligand exchange with NH3 is thermodynamically and kinetically favored over competing mechanisms that incorporate nitrogen from the metal precursor including: β-hydrogen elimination of isobutene or ethene; and NH3 catalyzed β-hydrogen elimination of isobutene or ethene. β-hydrogen elimination of isobutene or ethene is found to proceed through a barrier of 76.0 kcal/mol. However, our results indicate that ammonia or diethylamine produced by precursor reaction with surface amine groups can also catalyze β-hydrogen elimination of isobutene with a predicted barrier of 64.3 kcal/mol, thus making MOCVD reactions kinetically active above ∼600 °C. In addition to providing a fundamental understanding of the chemistry of TaN ALD from (tBuN)(NEt2)3Ta and NH3, the set of mechanisms analyzed provide new insights into the principles governing the ALD processes of other metal nitride films using imido or amido ligand transition metal complexes and ammonia as precursors.

Two-hydrogen transfer (simultaneous protic and hydridic hydrogen transfer) is examined as a potentially efficient mechanism for the selective reduction of CO2 to methanol. High-level ab initio CCSD(T) coupled-cluster theory simulations of ammonia−borane (AB), which contains both protic and hydridic hydrogen, show the effectiveness of this mechanism. AB demonstrates how simultaneous two-hydrogen transfer is kinetically efficient because (1) two-hydrogen transfer avoids high-energy single-electron-reduced intermediates, (2) the CO2’s HOMO is protonated while the LUMO is concurrently reduced by a hydride, and (3) complementary charge polarities around the six-membered-ring transition-state structures stabilize the transition states. This study suggests that an effective mechanism for the reduction of CO2 to methanol proceeds through three two-hydrogen-transfer steps and that suitable catalysts should be developed that exploit two-hydrogen transfer without the use of AB.