Highlights

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The bowl-shaped B₃₀, B₃₆ and planar circle B₄₂ clusters have a disk aromaticity.

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A stable B₄₀ fullerene having two hexagons and four heptagons is identified.

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MOs of the B₄₀ fullerene closely mimic those of the buckyball B₈₀.

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The B₄₂ triple ring tube is more stable than the planar disk.

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The B₄₂ tube has diatropic ring current making it a tubular aromatic species.

Highlights

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The heat of formation of silole is predicted to be 44 ± 2 kcal/mol.

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IE, EA, PA, DPE, binding and dissociation energies of silole are theoretically determined.

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Both homolytic and heterolytic dissociations of silole take place at Si.

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Both protonation and H-atom addition preferentially take place at the C_α-site.

Abstract

Integration of the (RO)B3LYP/6-311++G(2df,2p) with the PM6 method into a two-layer ONIOM is found to produce reasonably accurate BDE(O–H)s of phenolic compounds. The chosen ONIOM model contains only two atoms of the breaking bond as the core zone and is able to provide reliable evaluation for BDE(O–H) for phenols and tocopherol. Deviation of calculated values from experiment is ±(1–2) kcal/mol. BDE(O–H) of several curcuminoids and flavanoids extracted from ginger and tea are computed using the proposed model. The BDE(O–H) values of enol curcumin and epigallocatechin gallate are predicted to be 83.3 ± 2.0 and 76.0 ± 2.0 kcal/mol, respectively.

Abstract

Absolute rate constants for the gas-phase reaction C₂H + SO₂ are experimentally determined over the temperature range 295–800 K. C₂H radicals are generated by pulsed 193-nm photolysis of C₂H₂ in the presence of SO₂ and He or N₂ buffer gas. The temperature dependence of the rate constants is established as k_SO2$k_{{\text{SO}}_2}$ (T) = (0.86 ± 0.08) T^3.80±0.13 exp[−(1222 ± 79)/T] cm³ s⁻¹. The rate constants are moderately high at these temperatures and show negative temperature dependence. CCSD(T)/6-311++G(3df,2p) calculations of the potential energy surface confirm experimental findings and show that the reaction products are HCCO + SO.

Abstract

A ruthenium complex bearing cooperative PNP ligands showed unprecedented activities in homogenous catalysis of the reversible dehydrogenation of ammonia borane at both N-functionality and ethylene backbone. Quantum chemical calculations on a simple model of Ru-complex are carried out to probe the H₂ release mechanism. The energy barrier for H₂ formation from NH₃BH₃ is found to be strongly reduced with the presence of Ru-catalyst, and one additional H₂ molecule can be produced. The Ru-complex can also be used as a catalyst for H₂ release from other potential materials for chemical hydrogen storage such as hydrazine and ammonia alane.

Abstract

Ammonia borane and lithium amidoborane proved to have potential as efficient hydrogen storage materials. Although a combination of both compounds, LiNH₂BH₃–NH₃BH₃ has been shown to have even better hydrogen release characteristics, the inherent process was not clear. We determined the reaction mechanism of H₂ release from LiNH₂BH₃–NH₃BH₃ using computational quantum chemistry methods (MP2 and CCSD(T) methods and aug-cc-pVnZ basis sets). The energy barrier for H₂ release becomes lower than in pure LiNH₂BH₃ or NH₃BH₃ in agreement with experiment. Relevant transition structures are analyzed in detail and the initial formation of a cyclic adduct is likely responsible for the strong reduction of energy barriers.

Abstract

Small zinc-doped tin clusters Sn_nZn, n = 1–12, are studied using DFT and CCSD(T) methods. The isomers are located using a stochastic search algorithm. The growth mechanism can be formulated as follows: (i) small clusters Sn_nZn with n ⩽ 8 are formed by capping Zn on a surface of Sn_n, (ii) a competition between exohedral and endohedral structures appears at n = 9 and 10, from which the endohedral structures become predominant, (iii) for n = 11 and 12, the clusters are formed by encapsulating Zn into the empty cages Sn_n. Icosahedral Sn₁₂Zn (I_h) and Sn₁₀Zn (D_4d) are magic clusters with large HOMO–LUMO gaps, high binding energies and embedding energies. While both Sn₁₂Zn and Sn₁₀Zn clusters can be considered to be spherically aromatic with 8 valence π-electrons that satisfy the electron count rule of 2(N + 1)², the enhanced stability of Sn₁₂Zn (I_h) can further be rationalized in terms of its closed crystal field splitting shell.

Abstract

Abstract

The catalytic effect of LiH, 2LiH, BeH₂ and 2BeH₂ on the hydrogen release from hydrazine is studied using quantum chemical calculations. Potential energy surfaces of the relevant reactions are constructed using MP2 and CCSD(T) methods and the aug-cc-pVnZ basis functions and extrapolated to the complete basis sets. No extra energy is needed for hydrogen release, and regeneration of the catalyst in the case of 2LiH. A small amount of energy is needed for the regeneration of 2BeH₂. The results suggest the capacities of small hydride molecules to act as efficient catalysts for H₂ production.

Abstract

Substitution of C–H groups in 1,3,5-trimethylenebenzene (1,3,5-TMB) triradical by one, two or three N, P and As atoms is studied using CASSCF/CASPT2 computations with a ANO-RCC triple-ζ basis set. The computations showed that consistently with the previous results, the unpaired electrons in 1,3,5-TMB are highly delocalized, but our computations show that they become localized on the P and As atoms, with a more pronounced localization effect of As. All studied compounds have a quartet ground state, but the results showed in this Letter indicate that the quartet–doublet energy gap is reduced upon substitution. Localization of the unpaired electrons on the ring reduces the average distances between them, which induces a destabilization of the quartet states compared to the doublet. Therefore the position of the unpaired electrons and the doublet–quartet gap can be tuned by chemical methods, which opens the way to fine-tune the properties of the building blocks of organic magnets.

Abstract

CO adsorption on Au_nY (n = 1–9) is investigated using density functional theory, and compared with pure gold clusters Au_n+1. CO is confirmed to prefer on-top binding to the least coordinated Au atom for the pure gold clusters, whereas for Y-doped gold clusters, the most favorable sites are dependent on the cluster size; the Au on-top sites are favored from n = 6. CO behaves as an electron acceptor, and the C–O vibrational frequency of the metal–CO complexes correlates with the degree of charge transfer to CO. Reactivity with CO is decreased after one gold atom is replaced by one yttrium for most sizes.

Abstract

Lithium amidoboranes show potential as hydrogen storage materials. The mechanism for production of LiNH₂BH₃ from LiH and NH₃BH₃ is studied using quantum chemical methods (MP2. CCSD(T)). The reaction is exothermic with a low energy barrier. Pathways of H₂ production from LiNH₂BH₃, NaNH₂NH₃ and (LiNH₂BH₃)₂ show that a H-transfer from B to Li or Na constitutes the rate-determining step. Substitution of one borane hydrogen by a more electron-donating group (OH and NH₂) tends to lower the energy barriers for H₂ release. Calculated results point out that LiNH₂BH₂OH and LiNH₂BH₂NH₂ can be considered as potentially promising materials for hydrogen storage.

Abstract

Structure and stability of the Ge₁₂M^x clusters with M = Li, Na, Be, Mg, B, Al, and x from −1 to +1, each containing 50 valence electrons are investigated using DFT calculations. The global minima turn to be the high symmetry icosahedral structures, with large HOMO–LUMO gaps and high detachment or ionization energies. In particular, Li is found for the first time to be located at the center of an icosahedron Ge₁₂Li⁻. The high thermodynamic stability of the icosahedra arises from a combination of their closed crystal field shells, spherical aromaticity and electrostatic attraction force.

Abstract

The electronic structure and molecular properties of a series of small mixed lithium boron clusters B_nLi (n = 1–8) in both neutral and anionic states are investigated using quantum chemical methods. The lowest-energy structures are identified. Standard heats of formation, adiabatic electron affinities (EAs) and vertical detachment energies (VDEs) are predicted using coupled-cluster theory CCSD(T)/6-311+G(d) calculations. Addition of Li to B_n to form B_nLi clusters marginally distorts the geometries of the parent B_n. The chemical bond in B_nLi has a highly ionic character in which the positive charges are located on Li. MO analysis confirms a similar degree of aromaticity between B_n  Li and ${\text{B}}_n^{-}$. The relative stabilities of the clusters are evaluated using the average binding energy (E_b), second order difference in energy (Δ²E), resonance energy (RE) and normalized resonance energy (NRE). Both REs and NREs quantities of the Li-doped boron clusters are consistently larger than those of the pure B_n clusters. The B₃Li species exhibits a remarkably high stability within the B_nLi series.

Abstract

Abstract

Producing H₂ from water is a very challenging task. Using quantum chemical calculations with the MP2 and CCSD(T) methods and the aug-cc-pVnZ basis sets (extrapolated to CBS), we investigated the possibilities of B₂H₆, AlH₃, Al₂H₆ and AlH₃BH₃ to act as catalysts in the reactions that split water. Hydrogen production from H₂O is greatly accelerated in the presence of alane, dialane or borane–alane in such a way that reaction H₂O + Al₂H₆ is a nearly spontaneous process. The main catalytic effect of AlH₃ arises from the occurrence of a strong dihydrogen bond of the type Al–H^δ−–^δ+H–O within a cyclic transition structure.

Abstract

The Cu₆Sc⁺ cation was detected experimentally as a stable bimetallic cluster [N. Veldeman, T. Höltzl, S. Neukermans, T. Veszprémi, M.T. Nguyen, Phys. Rev. A. 76 (2007) 011201]. DFT calculations using the BP86 functional with various basis sets yield a D_3h tricapped tetrahedron shaped structure as the most stable isomer. This cluster has a S²P⁶ electron configuration with a closed electronic shell, and its electrons tend to localize around Sc thereby fully occupying electron shell orbitals. Stability, high symmetry, and the NICS show that this cluster is aromatic. The neutral Cu₅Sc cluster has the same electron configuration as Cu₆Sc⁺. In agreement with qualitative predictions, DFT computations show that the C_4v octahedron-shaped Cu₅Sc cluster features a closed electronic structure and an aromatic character.

Abstract

Electronic structure of Al₂Na was reinvestigated using MP2, CCSD(T) and MRMP2/CASSCF(7,7) calculations with aug-cc-pVnZ (n = D, T and Q) basis sets. In contrast to a recent report, the triangular Al₂Na has a ²B₁ ground state with a ⁴B₁–²B₁ gap of 2.5 kcal/mol, rather than a ²A₁ state (13.7 kcal/mol higher). In addition, Al₂Na⁺ exhibits a linear ¹Σ⁺ ground state with a singlet–triplet gap of −5.2 kcal/mol rather than a cyclic ¹A₁ state (6.0 kcal/mol above the linear). Al₂Na⁻ has a triangular ³B₁ ground state and a singlet–triplet gap of 6.9 kcal/mol. We obtained IE_a(Al₂Na) = 5.3 ± 0.2 eV and EA_a(Al₂Na) = 1.3 ± 0.2 eV.

Abstract

Quantum chemical calculations (G3B3 and B3LYP/6-311++G(d,p)) and tandem mass spectrometric experiments demonstrate the higher stability of 1,3-cyclohexadienylidenemethanone radical cation compared to ionized benzaldehyde. Characterized by a heat of formation of 833 kJ mol⁻¹ (at 298 K), this ketene C₇H₆O⁺ isomer is found 43 kJ mol⁻¹ more stable. It has been generated by ion/molecule reaction between ionized benzaldehyde and neutral methanol, a new example of proton transport catalysis tautomerization. The greater stability of the ketene ion is due to the low IE of the neutral (7.49 eV).

Abstract

Abstract

MS/MS/MS experiments were carried out using a large-scale tandem mass spectrometer featuring a sector-quadrupole-sector geometry to probe the ${\mathrm{C}}_7{\mathrm{H}}_8^{\text{<glyph name="rad"></glyph>}+}$ and ${\mathrm{C}}_5{\mathrm{H}}_6^{\text{<glyph name="rad"></glyph>}+}$ radical cations. These ions (m/z 92 and 66) were produced from metastable ionized precursors or charge exchange reactions in the quadrupole cell; subsequent fragmentations and double ionization were performed after mass-selection and collisional activation with oxygen. Charge stripping spectra show significant variations in the relative intensities of the same doubly charged species generated in different ways. These variations are likely to be associated with the inherent differences in internal energy content.

On the basis of quantum chemical calculations (CCSD(T), CASSCF and B3LYP), we have demonstrated in this work that the CP group, when connected to a valence-deficient center, in particular in a triplet state or a radical, its π-electrons tend to move into other parts of the system, thus transforming the formal RCP system into a P-centered RCP species. The CP group does not behave as a phosphaethynyl substituent, but rather as a P-centered phosphinidene or a P-radical. Copyright © 2006 John Wiley & Sons, Ltd.

High-level ab initio (CCSD(T), CBS-QB3 and CASSCF, CASPT2, MR-ACPF, MR-ACPF-2) and density functional theory (B3LYP) calculations were carried out to study the dimerization of phosphaacetylene or phosphaethyne (HCP). Seventeen low energy closed-shell and five open-shell phosphaacetylene dimers were found on the potential energy surface. Two head-to-head, one head-to-tail and three other dimerization reaction pathways were determined, all with high activation barriers, suggesting that closed-shell minima are usually kinetically stable. An open-shell head-to-head reaction pathway has also been found with moderate initial barrier (95.0 kJ mol⁻¹) leading to 1,2- and 1,3-diphosphacyclobutadiene, suggesting that polymerization of HCP and oligomerization of its derivatives have open-shell mechanism. Formation of 1,2-diphosphacyclobutadiene is both thermodynamically and kinetically favored over 1,3-diphosphacyclobutadiene. A head-to-head reaction involving LiBr as a catalyst was also studied. It has been pointed out that LiBr catalyze the closed-shell mechanism. All the four possible reaction channels of this reaction yield 1,4-diphosphatriafulvene with a fairly low activation Gibbs-free energy (44.8 kJ mol⁻¹), suggesting that this compound could be synthesized. This finding fully supports the experimental results.

Quantum chemical calculations using molecular orbital (HF) and density functional (B3LYP) methods, in conjunction with the 6-311 + + G(d,p) basis set, have been applied to investigate the electronic structure of a series of diamino-meta-quinonoid molecules, each containing a six-membered ring coupled with two exocyclic CO bonds situated in a meta position, along with two amino substituents (NH₂ and NHR). The chemical bonding phenomena in these zwitterions and isomers where one or two hydrogens are transferred from N to O are analyzed with the aid of the atoms-in-molecules (AIM) approach. The relative energies between zwitterionic and quinonoid isomers in both neutral and ionized states also have been evaluated. Substituents exert a strong effect that in many cases changes the energy ordering. Copyright © 2005 John Wiley & Sons, Ltd.

Abstract

Quantum chemical calculations were applied to investigate the electronic structure of the parent 1,3,5-tridehydrobenzene triradical (C₆H₃, TDB) and its anion $({\mathrm{C}}_6{\mathrm{H}}_3^{-})$, cation $({\mathrm{C}}_6{\mathrm{H}}_3^{+})$ and protonated form $({\mathrm{C}}_6{\mathrm{H}}_4^{+})$. Our results obtained using the state-averaged complete active space self-consistent-field (CASSCF) followed by second-order multi-state multi-configuration perturbation theory, MS-CASPT2, and MRMP2 in conjunction with the large ANO-L and 6-311++G(3df,2p) basis set, confirm and reveal the followings: (i) TDB has a doublet ²A₁ ground state with a ⁴B₂–²A₁ energy gap of 29 kcal/mol, (ii) the ground state of the ${\mathrm{C}}_6{\mathrm{H}}_3^{-}$ anion in the triplet ³B₂ being 4 kcal/mol below the ¹A₁ state. (iii) the electron affinity (EA), ionization energy (IE) and proton affinity (PA) are computed to be: EA = 1.6 eV, IE = 7.2 eV, PA = 227 kcal/mol using UB3LYP/6-311++G(3df,2p) + ZPE; standard heat of formation ΔH_{f(298 K, 1 atm)}(TDB) = 179 ± 2 kcal/mol was calculated with CBS-QB3 method. An atoms-in-molecules (AIM) analysis of the structure reveals that the topology of the electron density is similar in all compounds: hydrogens connect to a six-membered ring, except for the case of the ²A₂ state of ${\mathrm{C}}_6{\mathrm{H}}_4^{+}$ (MBZ⁺) which is bicyclic with fused five- and three-membered rings. Properties of the chemical bonds were characterized with Electron Localization Function (ELF) analysis, as well as Wiberg indices, Laplacian and spin density maps. We found that the radicals form separate monosynaptic basins on the ELF space, however its pair character remains high. In the ²A₁ state of TDB, the radical center is mainly localized on the C1 atom, while in the ²B₂ state it is equally distributed between the C3 and C5 atoms and, due to the symmetry, in the ⁴B₂ state the C1, C2 and C3 atoms have the same radical character. There is no C3–C5 bond in the ²A₁ state of TDB, but the interaction between these atoms is strong. The ground state of cation ${\mathrm{C}}_6{\mathrm{H}}_3^{+}$ (DHP), ¹A₁, is not a diradical and has a doubly aromatic character. Aromaticity of the different compounds was studied within the ELF framework and the standard deviation of the bond lengths and bond orders. The Jahn–Teller distorted ²A₁ and ²B₂(C_2v) states of TDB were found to exhibit an aromaticity comparable to that of benzene. Overall protonation of the TDB reinforces the stability of the low-spin doublet states, the classical Hund’s rule is not obeyed. In a view, these species could better be regarded as radicals than triradicals.

Abstract

1,3,5-Trimethylenebenzene (TMB, C₉H₉) is a prototypical hydrocarbon triradical. CASPT2/ANO-L calculations confirm that TMB has a quartet ground state with a ${}^2{\mathrm{B}}_1''–''{}^4{\mathrm{A}}_1^{″}$ gap of 12.4 ± 2.0 kcal/mol; both ²B₁ and ²A₂ states are quasi-degenerate. ${\mathrm{C}}_9{\mathrm{H}}_9^{-}$ has nearly degenerate ³B₂, ³A₁ and ¹A₁ states and ${\mathrm{C}}_9{\mathrm{H}}_9^{+}$ nearly degenerate ³A₁, ¹A₁ and ³B₂ states. Exocyclic protonation is favoured over ring-protonation; both processes induce a low-spin doublet ground state. Ionization energy, electron affinity, proton affinity are: IE_a(TMB) = 7.5 ± 0.2 eV, EA(TMB) = 22 ± 2 kcal/mol, PA(C₉H₉) = 235 ± 2 kcal/mol.