Dimers, trimers and oligomers of sulfur oxides: an ab initio and density functional study

Ab initio (HF/3-21G^*), DFT (B3LYP with basis sets 6-31G^*, 6-311+G^* and 6-311+G(2d)) and, in some cases, MP2/6-31G^* calculations, were done on cyclic dimers, trimers, etc. and on acyclic oligomers (with OH and H on the ends) of sulfur monoxide and sulfur dioxide. The four cyclic (SO)_n molecules were (S–O)₂ (1,3,2,4-dioxadithietane, 1a), (S–O)₃ (1,3,5,2,4,6-trioxatrithiane, 2a), (S(=O))₄ (tetrathietane 1,2,3,4-tetraoxide, 1b), and (S(=O))₆ (hexathiane 1,2,3,4,5,6-hexaoxide, 2b). The four cyclic (SO₂)_n molecules were the dioxide of 1a (1,3,2,4-dioxadithietane 2,4-dioxide, 1c), the trioxide of 2a (1,3,5,2,4,6-trioxatrithiane 2,4,6-trioxide, 2c), the tetraoxide of 1b (tetrathietane 1,1,2,2,3,3,4,4-octaoxide, 1d) and the hexaoxide of 2b (hexathiane 1,1,2,2,3,3,4,4,5,5,6,6-dodecaoxide, 2d). The 16 acyclic molecules (oxides of disulfane, trisulfane, etc. and oxides of oxadisulfane, dioxatrisulfane, etc.) were (–S–O–)_n, (–S(=O)–)_n, (–S(=O)O–)_n, and (–S(=O)₂–)_n, with n from 2 to 5 and HO, H at the ends. Most of these species are relative minima on the B3LYP/6-31G^* potential energy surface. In energy content, the SO dimer, etc. lie below, and the SO₂ dimer, etc. above, their SO_x components, at all the electron-correlated levels.     

Benzooxirene. Ab initio calculations

Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (“ketocarbene”), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G^*//MP2(FULL)/6-1G^* with MP2(FULL)/ 6-31G^* zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and −17.8 kJ mol⁻¹. Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G^* geometry is C_2v but the MP2(Fermi contact)/6-31G^* geometry is C_s with a 6-/3-ring coplanarity deviation of about 6.9 °, although in the MP2(FULL)/6-31G^* geometry this is reduced to about 3.1 °.     

Molecular structure and conformational stability of allylisocyanate—post-Hartree–Fock and density functional theory studies

The molecular structure and conformational stability of allylisocyanate (CH₂CHCH₂NCO) molecule was studied using the ab initio and DFT methods. The geometries of possible conformers, C-gauche (δ=120°, θ=0°) (δ=C=C–C–N and θ=C–C–N=C) and C-cis N-trans (δ=0° and θ=180°) were optimized employing HF/6-31G^*, MP2/6-31G^* levels of theory of ab initio and BLYP, B3LYP, BPW91 and B3PW91 methods of DFT implementing the atomic basis set 6-311+G(d,p). The structural and physical parameters of the above conformers were discussed with the experimental and theoretical values of the related molecules, methylisocyanate and 3-fluoropropene. It has been found that the N=C=O bond angle is not linear as the experimental result for both the conformers and the theoretical bond angle is 173°. The rotational potential energy surfaces have been performed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The Fourier decomposition potentials were analysed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The HF/6-31G^* level of theory predicted that the C-gauche conformer is more stable than the C-cis N-trans conformer by 0.41 kJ/mol, but the MP2 and DFT methods predicted the C-cis N-trans conformer is found to be more stable than the C-gauche conformer. The calculated chemical hardness value at the HF/6-31G^* level of theory predicted the C-cis N-trans form is more stable than C-gauche form, whereas the chemical hardness value at the MP2/6-31G^* level of theory favours the slight preference towards the C-gauge conformer.     

Vibrational spectra and density functional study of propylgermane

Raman and infrared spectra of propylgermane, CH₃CH₂CH₂GeH₃, and its Ge-deuterated analog, CH₃CH₂CH₂GeD₃, were investigated in their gaseous, liquid and solid states. The normal coordinate treatment was carried out by density functional theory (DFT) calculation, using B3LYP/6-31G^* and 6-311++G^** basis sets, and the corresponding fundamental vibrations were assigned. The trans (T) and gauche (G) forms around the central C–C bond coexisted in the gaseous and liquid states and only the T form existed in the solid state. From the temperature dependent measurements of the Raman spectra in the liquid state, the enthalpy difference was found to be ΔH(T−G)=−0.36±0.02 kcalmol⁻¹ with the T form being more stable. The energy differences between the isomers obtained by DFT calculations were ΔE(T−G)=−0.46 kcalmol⁻¹ and ΔE(T−G)=−0.87 kcalmol⁻¹ by the 6-31G^* basis set and 6-311++G^** basis set, respectively.     

Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation

The use of B3LYP/6–31G^* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G^* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G^* are found to do as well as MP2/6–31G^* geometries in the calculation of the enthalpies of formation.     

Conformational stability of 1-bromo-2-fluoroethane from temperature dependent FT-IR spectra of rare gas solutions

Variable temperature (−55 to −135°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-bromo-2-fluoroethane, BrCH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. From these data, the enthalpy difference has been determined to be 108±9 cm⁻¹ (1.296±0.113 kJ/mol) and 112±8 cm⁻¹ (1.346±0.098 kJ/mol) from the krypton and xenon solutions, respectively, with the trans conformer the more stable rotamer. Complete vibrational assignments are presented for both conformers which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G^* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G^* and/or MP2/6-31G^* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate. Structural parameters and conformational stability have also been obtained from MP2/6-311+G^** calculations. Combining the ab initio predicted structural parameters with the microwave rotational constants, r_o parameters have been obtained for the gauche conformer.     

乙烯醇锂的从头算研究

用限制的HF/3-21G和HF/6-31G^*优化乙烯醇锂的几种可能构型,比较了它们的稳定性.用限制的HF/3-1G,从乙醛开始,探讨了气相反应生成乙烯醇锂的机理,并在MP2水平上用6-31G^*基组计算了反应热.     

Ab initio molecular orbital study on three feasible mechanisms for substitution of the vinylic carbon in F2C=C(OMs)BMe3−

A substitution on 2,2-difluorovinylic carbon was investigated by using ab initio molecular orbital calculations. Three feasible mechanisms, which are the S_N1-like, the S_N2-type and the addition-elimination mechanisms, were ex- amined for a model borate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borate. Four TSs were obtained depending on the position of Li⁺ around the vinylborate although activation energies in the gas phase are rather high (ca. 30–40 kcal mol⁻¹) in comparison with that expected from the experimental conditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the acti- vation energy of one S_N2-type mechanism very much (4. l kcal mol⁻¹ at the B3LYP/6-31+G^*//RHF/6-31+G^/s* level of theory) while those for the other mechanisms do not change very much. Therefore, the S_N2-type mechanism is applicable to the substitution reaction observed for the vinylborate.     

Electronic structure of N-sulfonylimines

The electronic structure of N-sulfonylimines has been studied in detail using ab initio MO and density functional methods. The S–N rotational barriers in HS(O)₂N=CH₂ at G2MP2 and CBS-Q levels have been found to be 3.25 and 3.43 kcal/mol respectively. Complete optimization at HF/6-31+G^*, MP2(full)/6-31+G^* and B3LYP/6-31+G^* levels have shown that synperiplanar arrangement of S–O with respect to C=N is more stable. NBO analysis has been carried out to quantitatively estimate these delocalisations and charge polarization in RS(O)₂N=CH₂ (R=H, Me, Cl, F). The Lewis basic character in N-sulfonylimines is less compared to N-alkylimines due to anomeric interactions that reduce the lone pair electron density on nitrogen in 1.     

Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Geometry optimizations at the UHF/6-31G^* and UMP2/6-31G^* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS^+·₁ and TS^+·₂, were obtained which have C₁ and C₂ symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS^+·₁ is the true transition state connecting N^+· and Q^+·, while TS^+·₂ is a second order saddle point.     

Ab initio calculations of minimum-energy pathways of the nucleophilic addition of the H anion, LiH molecule and Li/H ion pair to acetylene and methylacetylene

Ab initio calculations were performed for special points of the minimal energy pathways (MEP) of the nucleophilic addition reactions of the isolated H⁻ anion, LiH molecule and Li⁺/H⁻ ion pair to acetylene (A) and methylacetylene (MA) molecules, proceeding in accordance (M) and against (aM) the Markovnikov's rule. All structural parameters were optimized using the restricted Hartree–Fock (RHF) method. For the addition of H⁻, the 6-31++G^* basis set was used and for the reactions of LiH and Li⁺/H⁻ the 6-31G^* basis set with the subsequent recalculation of single point energies, taking into account of electron correlation energy by means of the second-order Möller–Plesset perturbation theory at the MP2/6-31++G^** level. The results of calculations demonstrate, that the energy characteristics of both M- and aM-additions with H⁻ do not differ sufficiently (0.1–1.2 kcal/mol for the activation energies (ΔE_a) and the reaction heats (ΔQ)). The substitution of the H atom by the CH₃ group in A molecule results in practically the same values of ΔQ and ΔE_a. On the contrary, for the LiH molecule and Li⁺/H⁻ ionic pair, the M-addition is favorable (charge control). It is found that the presence of electrophile decreases the activation energy by 3–5 kcal/mol as compared with the addition of the isolated hydride ion H⁻.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

Theoretical study of some 1,3-substituted (1,3-diketonato)borondifluorides. Potential energy curve relevant to the barrier crossing reaction

Ab initio Hartree-Fock calculations utilising STO-3G, 3-21G^* and 6-31G^* basis sets have been performed on three neutral and highly polar molecules, (diformylmethine)borondifluoride, (acetylacetonato)borondifluoride and (dibenzylmethine) borondifluoride. The calculated and experimental structures are well correlated when using the HF/3-21G^* basis set, except for the structure parameters involving the boron atom. The HF/6-31G^* basis set does not improve the accuracy in structure calculations. The conformational analysis is in agreement with the experimentally observed C_2v symmetrical structures, where the boron atom is tetrahedrally coordinated. The calculations support a one-dimensional ground state barrier crossing reaction for (dibenzylmethine)borondifluoride, where the phenyl torsion is the most likely reaction coordinate. Both HF/6-31G^* calculations and the second-order Møller-Plesset correction with the 3-21G^* basis set suggest an activation energy of the ground state reaction of about 30 kJ mol⁻¹. The ground state barrier crossing reaction kinetics is evaluated by the Kramers theory. The calculated ground state parameters relevant to the barrier crossing reaction are compared with the experimentally observed excited state values.     

Non-empirical molecular orbital studies of the possibility of ferromagnetic ordering in isoelectron-substituted ferrocarbon modifications

The vertical singlet-triplet (ST) splitting in the C₃H₆ molecule and C₂BH⁻₆ and C₂NH⁺₆ ions has been investigated by means of ab initio calculations. The molecular geometry was either taken as that corresponding to the UHF/6-31G^* or UHF/6-31+G^* energy minimum of the triplet configuration (for C₃H₆, C₂BH⁻₆, C₂NH⁺₆), or was extracted from the ferrocarbon crystal (for C₃H₆). Polarized split-valence basis sets (6-31G^*, 6-31+G^* and 6-311G^**) and fourth-order Møller-Plesset perturbation theory at the MP4SDTQ level of approximation provides more reliable ST splitting values, than the MP4SDQ level, the latter being the most accurate method used in our previous work. These calculations prove the presence of ferromagnetic ordering within the same quasi-graphite plane of ferrocarbon crystal. The presence of such ordering has been shown within the same quasi-graphite plane upon substitution of sp³ carbon atoms by ions B⁻, N⁺ and so on.     

Theoretical studies of electronic absorption and emission spectra of Pt(saloph)

The performance of ab initio calculations for the ground and excited states of the Pt(saloph) complex is examined in detail. The S₀–S_i and T₁–T_i absorption spectra are calculated, and the transition between the ground S₀ state and the excited S₁ state involves the HOMO-2, HOMO-1, HOMO and LUMO. Moreover, calculations show that the emissive singlet is of mixed MLCT/LLCT characteristic. On the other hand, the molecular geometry of the complex is nearly planar in the ground state while the geometry is obviously nonplanar in the excited state of S₁(π, π^*) in the gas phase.     

Ab initio studies on the electronic structures of certain 10π-electron six-membered ring compounds

By using ab initio methods of all-electron or effective core potential calculations, the electronic structures and the possible aromaticity of some 10π-electron systems, C₆H₆⁴⁻ (1), N₆⁴⁻ (2), P₆⁴⁻ (3), S₆²⁻ (4), Te₆²⁻ (5) and S₃N₃⁻ (6), have been studied at the SCF levels using 4-31G//4-31G and 6-31G^*//6-31G^* basis sets. The bonding characteristics of these systems are analysed in terms of the canonical molecular orbital and the Foster-Boys localized molecular orbital results. The application of the second-order Jahn-Teller theorem to the stability of these diamagnetical planar species is presented.     

Predissociation of chlorobenzene, beyond the pseudo-diatomic model

Dissociation of chlorobenzene via the lowest singlet excited state has been investigated by means of pump–probe femtosecond spectroscopy and spin–orbit corrected ab initio quantum chemistry. We have found that the so far accepted model with a ¹ππ^* → ³π/nσ^* reaction mechanism has to be amended. We suggest that the mechanism goes via a transition from ¹ππ^* to a πσ^* state that is to 90% a singlet. Further, three nuclear degrees of freedom required to describe the dissociation have been defined.     

Simulated T ← S spectra of benzaldehydes as a function of the energy gap between the ππ and nπ levels

The T_1,2 ← S₀ spectra of benzaldehydes have been studied as a function of the energy separation between the vibrationless levels. It is shown that the spectra are very complicated in the region of ΔE[T₂⁰(nπ^*)-T₁⁰(ππ^*)] = 250–400 cm⁻¹, reflecting effective vibronic interferences between T₂⁰(0-0) and each of the ν₃₆-ν₃₃ out-of-plane vibrational levels of T₁⁰(ππ^*). The simulated spectra correspond to the observed spectra. In the case of T₁⁰ = ³nπ^* and T₂⁰ = ³ππ^* the spectral change is not so drastic as in the reverse case loc. cit. because the optical intensity generally concentrates in the longest wavelength band, i.e., the origin band of the T₁(nπ^*) ← S₀ transition. The simulation spectra are useful for interpretation of the absorption spectra in similar electronic structure systems of substituted benzaldehydes.