Infrared intensities of liquids XXIV: optical constants of liquid benzene-h 6 at 25 °C extended to 11.5 cm −1 and molar polarizabilities and integrated intensities of benzene-h 6 between 6200 and 11.5 cm −1

Abstract

We have previously reported accurate absolute infrared intensities of liquid benzene-h 6 between 6225 and 500 cm −1, of liquid benzene-d 6 between 5000 and 450 cm −1, and of liquid benzene-d 1 between 6200 and 500 cm −1. The absolute intensities of benzene-h 6 were reported as spectra and tables of the real refractive index, n, the imaginary refractive index, k, and the molar absorption coefficient, E m, and also as tables of the average areas under, and average peak heights of, bands in the imaginary refractive index and molar absorption coefficient spectra. For benzene-d 6 and benzene-d 1, the absolute intensities were reported as spectra and tables of the real and imaginary refractive indices, and as spectra of the molar absorption coefficient and the imaginary molar polarizability ( α″ m) under the Lorentz local field. Further, the contributions from the different bands to the imaginary molar polarizability spectrum below 5000 cm −1 were separated by fitting the spectrum with Classical Damped Harmonic Oscillator (CDHO) bands. The integrated intensities C j were then obtained from the parameters of the CDHO bands as the areas under the corresponding v ̃ α″ m bands. The integrated intensities gave the transition dipole moments and also, for the fundamentals that are infrared-active in the gas phase, the dipole moment derivatives with respect to the normal coordinates under the double harmonic approximation. The present paper completes the experimental study of benzene-h 6 by presenting the results of the following work. The imaginary refractive index spectrum of C 6H 6 was extended to 11.5 cm −1 from its previous lower limit of 500 cm −1. The real refractive index spectrum between 8000 and 11.5 cm −1 was obtained by Kramers–Kronig transformation of the imaginary refractive index spectrum. It agrees excellently with the real refractive index spectrum directly measured below 100 cm −1, which suggests that the accuracy of the k spectrum is very good, particularly at low wavenumbers. The imaginary molar polarizability spectrum of C 6H 6 between 6200 and 11.5 cm −1 was calculated from the n and k spectra and was fitted with 226 CDHO bands and 1 Gaussian band. No baseline was required in the fit and the baseline absorption was described accurately by the bands used. The integrated intensity, C j,of each band used to fit the spectrum was calculated analytically from the band's parameters. The transition dipole moments were then calculated for the clearly assigned transitions, and the dipole moment derivatives with respect to the normal coordinates were calculated under the double harmonic approximation for the fundamentals that are infrared-active in the gas phase. The values obtained are related to literature values for the liquid and the gas, and are very close to those published previously and compared with the intensities of C 6D 6 and C 6H 5D.