Liquid-phase force field and dipole moment derivatives with respect to internal coordinates of benzene

This paper presents an analysis of the infrared vibrational intensities found for C6H6, C6D6 and C6H5D in the liquid phase, motivated in part by the quite marked intensity differences between the fundamentals of C6H6 and C6D6 in the liquid, and between corresponding vibrations in the liquid and gas phases. The analysis is carried out under the harmonic approximation and results from a determination of the force field for liquid C6H6, C6D6 and C6H5D. The force constants for the liquid-phase are presented and compared to those in the literature for the gas-phase. Previously reported experimental intensities are used along with the eigenvectors of the force field analysis to determine the dipole moment derivatives with respect to symmetry and internal coordinates. The dipole moment derivatives with respect to internal coordinates obtained are ∂μ/∂s = 0.38 ± 0.02 Debye Å−1, ∂μ/∂t = 0.24 ± 0.01, ∂μ/∂β = 0.26 ± 0.01, and ∂μ/∂γ = 0.64 ± 0.03 Debye Å−1. There is very little difference between the dipole moment derivatives with respect to internal coordinates obtained from non-linear least squares fitting of the two D6h isotopomers and those obtained from non-linear least squares fitting of the three isotopomers. The results show that there is significant intensity sharing in the CH stretch region of C6H5D between the fundamental and combination bands.