Force field, dipole moment derivatives, and vibronic constants of benzene from a combination of experimental and a b i n i t i o quantum chemical information

Abstract

The quadratic and the most important cubic force constants of benzene have been determined from ab initio Hartree–Fock calculations with a double-zeta basis set. Some constants have also been recalculated using other basis sets, including a polarized one. A few empirical scale factors, applied to the ab initio force field, allow the reproduction of a large number of observed vibrational frequencies, isotope shifts, and Coriolis constants within the uncertainties of experiment and the harmonic model. It is shown that the simultaneous utilization of ab initio and spectroscopical information is sufficient for the conclusive resolution of the uncertainties and alternatives in previous empirical force fields. The resulting scale factors can be used directly to obtain force fields for other aromatic hydrocarbons from ab initio calculations. Reproduction of the observed infrared intensities is only moderately successful, even with the polarized basis set. The calculated vibronic coupling constants show qualitative agreement but important deviations from previous simpler calculations. The predicted vibrational patterns confirm Lindholm’s assignment of the photoelectron spectrum of benzene.