The quality of ab initio quantum mechanical prediction of vibrational transition dipole directions and infrared intensities

Abstract

This is a preliminary report of our research aimed at the joint use of up-to-date experimental (polarized infrared spectra of oriented samples) and theoretical (ab initio quantum chemical) methods of determining the vibrational electric transition moment directions to help in the assignment of infrared absorption spectra of polyatomic molecules. The systems studied include ethylene, p-nitrobromobenzene, thiosemicarbazide and N, N-dimethyl- S-methylmonothiobarbamate. Our results show that this bilateral approach works well for molecules with orthogonal ( C 2 ν or higher) symmetry. Special emphasis was given to testing the capabilities of the method for systems with lower symmetry ( C 2h, C s, etc.) where it proved to be no more successful than the prediction of infrared intensities: agreement between the calculated and measured band polarization values within tight limits is quite rare, while completely wrong results have also been obtained, especially in the presence of unaccounted hydrogen bonding.