The pure rotational spectrum of solvated HCl: Solute-bath interaction strength and dynamics

A combination of pulsed THz transmission and FTIR spectroscopy was employed to measure the normalized frequency dependent absorption coefficient of HC l in spherical, dipolar, and linear solvents (CCl4, CHCl3, and alkanes, res pectively) in the 0-350 cm(-1) portion of the far-infrared spectral region. The analysis applied to the measured spectra describes the interaction bet ween the quantum mechanical rigid rotator motion of HCl and the solvent thr ough explicit consideration of the anisotropic potential between HCl and th e bath. Nominally, the theory requires two adjustable parameters to fit the solvated HCl absorbance spectra. However, a compilation of experimental re sults for HCl dissolved in various solvents of high symmetry reveals a quad ratic dependence of one parameter, the mean square field of the bath, on so lvent polarizability, It is shown that dipole-induced dipole (DID) interact ions account for the observed quadratic form. This observation introduces a constraint that reduces the number of adjustable parameters so that unique values for the second fitting parameter, the exponential decay rate of the anisotropic potential time correlation function, may be extracted from the measured absorbance curves. The analysis of HCl-alkane solution spectra re veals a more subtle aspect of this dependence. Only a very weak polarizabil ity dependence was found for solvents of large aspect ratio such as the alk anes. This difference indicates that the molecular polarizability density, not simply the molecular polarizability, dictates the strength of the solve nt mean square field. Last, a simple scheme for classifying nonpolar solven ts based on DID interactions between the solute and the bath is established .