VIBRATIONAL ABSORPTION INTENSITIES IN CHEMICAL-ANALYSIS .8. DIMETHYL SULFIDE

The structure and electric dipole moment of dimethyl sulphide have been calculated with a variety of basis sets. It is shown that the force fields computed with the 3-21G*, 3-21G** and 6-311G** bases are very similar after scaling. The vibrational assignments are discussed. The force field is compared with that of trimethyl phosphine. There is an excellent accord for the CH stretching modes, but the ab initio calculations indicate that the X-C/X-C (X = S, P) interaction terms have opposite signs. This appears to be incompatible with the experimental data. Effective atomic charges and atomic polar tensors (APT) are computed with the above bases and also with bond centred sp functions added to the 3-21G* basis. It is shown that the APT are much more sensitive to the basis set than are the effective atomic charges. The effects of the lone pair electrons on the bond electrical properties are much less than with oxy-ethers, though the overall perturbation of the sulphur atom on the intensities is greater. A number of inadequacies of the SCF calculations are identified.