Ab initio MO theory

The theoretical methods used commonly can be divided into three main categories, semi-empirical MO theory, DFT and ab-initio MO theory. Although it is no longer applied often, Hiickel molecular orbital (HMO) theory will be employed to introduce some of the principles used by the more modem techniques. 

Essentially, the RISM and extended RISM theories can provide infonnation equivalent to that obtained from simulation techniques, namely, thermodynamic properties, microscopic liquid structure, and so on. But it is noteworthy that the computational cost is dramatically reduced by this analytical treatment, which can be combined with the computationally expensive ab initio MO theory. Another aspect of such treatment is the transparent logic that enables phenomena to be understood in terms of statistical mechanics. Many applications have been based on the RISM and extended RISM theories [10,11]. 

Before 1980, force field and semiempircal methods (such as CNDO, MNDO, AMI, etc.) [1] were used exclusively to study sulfur-containing compounds due to the lack of computer resources and due to inefficient quantum-chemical programs. Unfortunately, these computational methods are rather hmit-ed in their reliability. The majority of the theoretical studies under this review utilized ab initio MO methods [2]. Not only ab initio MO theory is more reliable, but also it has the desirable feature of not relying on experimental parameters. As a consequence, ab initio MO methods are apphcable to any systems of interest, particularly for novel species and transition states. 

Wong et al. [9] calculated the wavelengths of the lowest-energy electronic transitions of S2, S3, and of the three most stable isomers of S4 from ab initio MO theory as follows (wavelengths in nm oscillator strengths in parentheses) ... 

The trisulfane molecule exists as two conformers which have been termed as cis- and trans-HzSi. While the trans-form is a helical molecule of C2 symmetry with the motif ++ (or — for the enantiomer), the cfs-form is of Q symmetry with the motif +- (identical to -+). Both forms have been detected by rotational spectroscopy [17, 45, 46]. The motif gives the order of the signs of the torsion angles at the SS bonds. The geometrical parameters [17] are presented in Table 4. The trans-isomer is by only 1 kj mol more stable than the cfs-form but the barrier to internal rotation from tmns to cis is 35 kJ mor [46]. The dipole moments were calculated by ab initio MO theory at the QCISD/TZ+P level as 0.68 D (trans) and 2.02 D (cis) [46]. For geometrical parameters of cis- and trans-trisulfane calculated at the MP2/6-311++G> > level, see [34]. 

The Onsager s reaction field theory [3] has been incorporated into MO calculations by Tapia and Goscinski [6], The model has been applied to different problems using either semiempirical [51] or ab-initio MO theory [52], or correlated ab-initio techniques [52],... 

Maxwell, D. S., Tirado-Rives, J., and Jorgensen, W. L. (1995) A comprehensive study of the rotational energy profiles of organic systems by ab initio MO theory, forming a basis for peptide torsional parameters. J. Comput. Chem. 16, 984-1010. 

A direct dynamics technique has been employed312 to study the thermal isomerization of (5VV)-tra .v-cyclopropane-1,2-ch and cyclopropane-1,2,3-d in an attempt to examine the possibility of stereochemical control in the isomerization. The stereochemistry of the ring expansion of various 2-isopropenylcyclopropane-l-carbonyl compounds (276) to the corresponding methyldihydrooxepins (277) has been studied,313 and the rearrangement of cyclopropylsilylene to silacyclobutene has been examined314 by ab initio MO theory. 

Extensions of RISM-SCF/MCSCF and Other Combinations of Ab Initio MO Theory with IET... 

Application of ab initio MO theory usually begins at the monoconfigurational level, with the Hartree-Fock-Roothaan or LCAO-SCF methodology [4,5]. In this scheme the wave function for a closed-shell molecule containing N electrons is approximated as an antisymmetrized product (determinant) of spin-orbitals, ... 

The most recent view on the 7t-accepting abilities of phosphines in transition metal-phosphine complexes is by Marynick 247) who has used approximate and ab initio MO theory to demonstrate that n accepting into o orbitals is important for PF3. Calculations on the model complex [Cr(NH3)5(PF3)] compared to [Cr(NH3)s(PH3)] showed... 

Interactions of single transition metal atoms (e.g., Cr, Fe, Co, and Ni) with PF3 (and CO) have been studied by an ab initio MO theory and the... 

The route from a-iminothioaldehydes to 2,3-dihydrothiazoles has been studied by ab initio MO theory and it was shown that the most energy requiring step is the trans —> air-isomerization of the methylimino group <1995J(P2)1077>. 

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