THE QUALITY OF AB-INITIO QUANTUM-MECHANICAL PREDICTION OF VIBRATIONALTRANSITION DIPOLE DIRECTIONS AND INFRARED INTENSITIES

This is a preliminary report of our research aimed at the joint use of up-to-date experimental (polarized infrared spectra of oriented sampl es) and theoretical (ab initio quantum chemical) methods of determinin g the vibrational electric transition moment directions to help in the assignment of infrared absorption spectra of polyatomic molecules. Th e systems studied include ethylene, p-nitrobromobenzene, thiosemicarba zide and N,N-dimethyl-S-methylmonothiobarbamate. Our results show that this bilateral approach works well for molecules with orthogonal (C-2 v or higher) symmetry. Special emphasis was given to testing the capab ilities 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 infr ared intensities: agreement between the calculated and measured band p olarization values within tight limits is quite rare, while completely wrong results have also been obtained, especially in the presence of unaccounted hydrogen bonding. (C) 1997 Elsevier Science B.V.