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

The structure and electric dipole moment of dimethyl sulphide have bee n 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 s imilar after scaling. The vibrational assignments are discussed. The f orce field is compared with that of trimethyl phosphine. There is an e xcellent accord for the CH stretching modes, but the ab initio calcula tions indicate that the X-C/X-C (X = S, P) interaction terms have oppo site 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 t he 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 SC F calculations are identified.