The benzene ground state potential surface. II. Harmonic force field for the planar vibrations

Abstract

A complete harmonic force field in terms of nonredundant coordinates has been generated from experimental frequencies for D6h, D3h, and D2h isotopically labeled benzenes and degenerate mode Coriolis constants predicting broken symmetry labeled benzene frequencies to ±0.1% and Coriolis constants to ±0.01 units, on the average. Exact solutions have been obtained for the six E1u force constants from D6h symmetry frequency data with the inclusion of 13C6H6 information. Some modes (e.g., the e2g mode ν8, in Wilson notation) are significantly altered from previous experimental force field predictions, rationalizing unclearly understood vibronic features of phosphorescence and two-photon spectra. A conundrum regarding the e1u Coriolis constant for ν18 (Wilson notation) is identified: no harmonic force field is capable of predicting the reported experimental magnitudes for this constant for both C6H6 and C6D6. The Pulay et al. scaled ab initio force field is in qualitative agreement with the experimental field for every symmetry class, with significant quantitative discrepancies involving CH stretching constants.