INFRARED ABSORPTION INTENSITIES OF LIQUID $C_{6}H_{6}$ AND $C_{6}D_{6}$.

Abstract

The intensities of the absorption bands of liquid $C_{6}H_{6}$ and $C_{6}D_{6}$ between 5000 and $400 cm^{-1}$ have been measured at $25^{\circ}C$, fully corrected for reflection and dielectric effects. Intensities of all of the bands have been determined by fitting classical damped harmonic oscillator bands to the imaginary molar polarizability spectrum. The intensities of the overtone and combination bands will be compared with those calculated for the isolated molecule by $Maslen et al^{1}$. The intensities of the fundamentals will be compared with those of the gas. To understand the differences observed, the dipole moment derivatives with respect to symmetry coordinates have been calculated from the intensities of both molecules through eigenvectors obtained from normal coordinate calculations. For the gas, the eigenvectors were calculated from the benchmark vibrational potential surface of $Goodman et al^{2}$. For the liquid, this force field was adjusted to fit the liquid-phase wavenumbers. The different intensities observed for corresponding bands of $C_{6}D_{6}$ and $C_{6}D_{6}$ are well explained by the different forms of the vibrations in the two isotopomers. In particular, $\nu_{13}$ of $C_{6}H_{6}$ is 4 to 5 times stronger than $\nu_{13}$ of $C_{6}D_{6}$ in both phases, largely due to the smaller contribution from the D-C-C in-plane bend in $C_{6}D_{6}$. For both molecules the eigenvectors are very similar for corresponding vibrations in the liquid and gas phases, and the significantly different intensities observed in the two phases are due to differences in the dipole moment derivatives with respect to intramolecular displacement. With respect to symmetry coordinates, the ratios of the dipole moment derivatives in the liquid to those in the gas are 0.74, 0.87, 1.15 and 1.02, for C-H, C-C, H-C-C in plane and H-C-C out-of-plane displacements, respectively. For $\nu_{13}$ of $C_{6}D_{6}$ the intensity in the liquid is twice as great as that in the gas, a much larger difference than for the other vibrations. This is due to unusually effective cancellation of opposing dipoles in $\nu_{13}$ of the gaseous molecule.