The <i>a</i>2<i>g</i> mode in ground state benzene

Abstract

Harmonic ground state frequencies for the a2g mode, ν3, in C6H6, C6D6, and 13C6H6 have been determined by application of the Teller–Redlich product rule to b2u mode frequencies obtained from two-photon fluorescence excitation experiments for the D6h symmetry molecules and a′2 frequencies from 1,3,5-C6H3D3 determined at rotational contour resolution. The a2g harmonic frequency values are 1366.6 cm−1 for C6H6, 1063.0 cm−1 for C6D6, and 1355.4 cm−1 for 13C6H6. The anharmonic correction to ν3 in C6H6, estimated from the difference in product rule and infrared frequencies is −17 cm−1. The b2u mode frequencies, ν14 and ν15, respectively, have also been significantly modified from those obtained from lower resolution spectra to 1309.4 and 1149.7 cm−1 in C6H6, 1286.3 and 827.9 cm−1 (harmonic value) in C6D6, and 1269.8 and 1138.1 cm−1 in 13C6H6. The anharmonic correction for ν15 in C6D6 is estimated as −4 cm−1. The B2u←A1g electronic origin of 13C6H6, has been determined for the first time as 38 105.5±0.3 cm−1. The B2u←A1g electronic origin of 1,3,5-C6H3D3 has been redetermined to be 38 187.5±0.3 cm−1, 3.5 cm−1 higher than previously reported.