MO calculations, ab initio

In the RISM-SCF theory, the statistical solvent distribution around the solute is determined by the electronic structure of the solute, whereas the electronic strucmre of the solute is influenced by the surrounding solvent distribution. Therefore, the ab initio MO calculation and the RISM equation must be solved in a self-consistent manner. It is noted that SCF (self-consistent field) applies not only to the electronic structure calculation but to the whole system, e.g., a self-consistent treatment of electronic structure and solvent distribution. The MO part of the method can be readily extended to the more sophisticated levels beyond Hartree-Fock (HF), such as configuration interaction (Cl) and coupled cluster (CC). 

The molecular and liquid properties of water have been subjects of intensive research in the field of molecular science. Most theoretical approaches, including molecular simulation and integral equation methods, have relied on the effective potential, which was determined empirically or semiempirically with the aid of ab initio MO calculations for isolated molecules. The potential parameters so determined from the ab initio MO in vacuum should have been readjusted so as to reproduce experimental observables in solutions. An obvious problem in such a way of determining molecular parameters is that it requires the reevaluation of the parameters whenever the thermodynamic conditions such as temperature and pressure are changed, because the effective potentials are state properties. 

Ab initio MO calculations using 4-3IG orbitals indicate that the eclipsed conformation of acetaldehyde is more stable than the staggered conformation. 

The other possible osmium(VIII) oxyfluoride OsOF6 has so far eluded synthesis and recent ab initio MO calculations indicate it is unlikely to exist. 

In the later 1970s there were, however, some papers which asserted the importance of including d orbitals in the basis set in ab initio MO calculations. For instance, calculations... 

Arising from studies of the photochemistry of benzenesulfonyl systems, extensive ab initio MO calculations have been made for various sulfonyl radicals and related species253. The STO-3G basis set, which includes d-type polarization functions on second-row atoms, was used. The inclusion of d orbitals on sulfur was found to be very... 

In this review, we consider the application of quantum-chemical calculations to a range of sulfur-rich compounds. The empirical, semiempirical and ab initio MO calculations published before 1980 are considered rather inadequate. Therefore, theoretical calculations before 1980 will not be mentioned in detail here. 

The lower members of the polysulfane series with n=2-6 have been prepared as pure compounds, while all members with n values up to 35 have been detected by H-NMR spectroscopy in so-called crude sulfane [31]. The gas-phase structures of the first three members of the polysulfane series are well established from either microwave spectra (H2S2 [32], H2S3 [33]) or high-level ab initio MO calculations (H2S4 [34]). Systematic ab initio studies of the structures, vibrational frequencies and heats of formation of H2S with i up to 6 have also been reported [4, 16]. 

The optical properties of solid Sg have been studied by ab initio MO calculations of the energy band structure [70] but no experimental data for solid Sg are known. 

The barriers for the rotation around the SS bond have been estimated from microwave data [40] and ab initio MO calculations [41, 42] as 34-36 kj mor (cis-barrier) and 24-25 kJ mol (trans-barrier), respectively. They are explained as the result of three effects First, the repulsion of the two 3p lone pairs at the sulfur atoms is lowest for r=90°. Second, the 3p lone pair electrons are partly delocalized into the antibonding a orbitals of the SH bonds originating from the neighboring sulfur atom. This hyperconjugation or 7i bond is at its maximum for r=90° but disappears for r=0° and 180°. Third,... 

In the literature tetrathiosulfuranes have been discussed as possible intermediates in the thermal decomposition of sulfanes and other polysulfur compounds. High-level ab initio MO calculations have in fact revealed that such species are local minima on the potential energy hypersurface [34]. However, recent results show that both the Gibbs reaction energies as well as the activation enthalpies for reactions of the type... 

The electronic structure and the UV-Vis spectrum of 83 have been studied by high-level ab initio MO calculations [139]. 

The molecular structures of the isolated polysulfide monoanions 8 with n=2-9 have been studied by density functional calculations and those of the smaller ions also by ab initio MO calculations. Compared to the neutral 8 molecules the extra electron occupies an antibonding orbital resulting in longer 88 bonds. The species 83 is bent and of C2V symmetry (a=115°) [140, 141]. 84 was calculated to be a planar ion of C2V symmetry (similar to the neutral molecule 84) but the planar C2h structure is only slightly less stable [140, 141]. The most stable isomer of 85 is a chain of Q symmetry sim-... 

Ab initio MO calculations at a high level of theory [28] support the open-chain structure for the ground state of S2O while density functional calculations predicted a cyclic ground-state structure [29]. 

According to ab initio MO calculations [50] protonation of S2O produces the planar chain-like molecule ds-SSOH+ as the isomer of lowest energy. The tmns-isomei is by only 2 kj mol less stable and S-protonated as well as 0-protonated cyclic isomers are next highest in energy. 

A molecule of composition S4O has not been observed yet. Ab initio MO calculations indicate that a cyclic and an open-chain structure have almost the same energy but no details have been released yet [58]. 

The oxygen atoms of the homo cyclic oxides occupy axial positions in solid S7O and SsO. In fact, recent ab initio MO calculations at the G3X(MP2) level of theory have demonstrated that these isomers are, by 7 and 9 kj mol respectively, more stable than the alternative structures with the oxygen atoms in equatorial positions [89]. In the case of SsO, however, the energy difference is only 1 kJ mol and the conformation with the oxygen atom in the equatorial position is more stable. This result agrees with the observation that SsO exists as two isomeric forms in the solid state depending on the crystallization conditions (see above). 

In principle, oxides of composition S 0 could also exist as heterocycles in which the oxygen atom is part of the ring which is then larger by one atom. Ab initio MO calculations by Wong at the G3X(MP2) level have shown, however, that these isomers are less stable than the homocyclic molecules, by the following relative energies at 0 K (in kJ mol ) [89] ... 

Valence Isomerism. Restricted Rotation and Permutational Isomer i sm. - The variable temperature spectra of bicyclic tetraphosphines (29) has been analysed in terms of valence isomerism involving the corresponding two coordinate open-chain isomers.62 A report on ab initio MO calculations of the inversion barriers for a series of methyl phosphines includes a discussion of the electronic consequences of steric effects.63 Inversion barriers of 1,2-diphosphinobenzenes are in the range 100 - 110 kJ mol". 66 The ring inversion barrier for a dibenzophosphorin has also been measured.63... 

The stability of an unsubstituted carbene is quite low in water. Highly correlated ab initio MO calculations have been used to study the energetics and mechanism governing the reaction between the radical CH2 and H20 in the gas phase and in solution, and it was found that methylene reacts in a barrierless fashion to produce the ylide-like intermediate methyleneoxonium, H2C-OH2, which in turn undergoes a 1,2-hydrogen shift to produce CH3OH.128 The presence of substituents appears to stabilize carbenes toward water.129... 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical... 

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