FAR-INFRARED SPECTROSCOPY OF LIQUID BENZENE - LONG-RANGED AND SHORT RANGED DYNAMICS IN THE NEAT LIQUID AND IN SOLUTION

Detailed analysis of the interaction-induced far-infrared spectrum of benzene (in both the neat liquid and dilute carbon tetrachloride solution) has been used to investigate both short time and long time behaviour of the intermolecular dynamics. For neat liquid benzene, at long times, the spectral density is found to be dependent upon both the reorientational and the relative translational diffusive motions of the molecules. Collective interactions (i.e. three body contributions) strongly influence the intensity (via cancellation effects) and the shape (or broadening) of the I(nu) profile. At short times, in the power absorption, alpha (nu), representation, the point quadrupole approximation is totally inadequate and the molecular details of the short ranged potential must be taken into account. This means that molecular 'packing' is important but only over a short time range of 0-0.2 psec. Our results confirm the molecular dynamics simulations in which a distributed quadrupole was needed to reproduce the observed alpha (nu) profile. For a very dilute solution of benzene in carbon tetrachloride the shape of the I(nu) profile is influenced by the pair interactions between solute and solvent molecules in such a way that the appearance of the spectrum is dominated by rotational diffusion of the benzene molecules and the relative translation of the pair. On the other hand, the alpha (nu) profile is found to have the same shape as that found in the pure liquid. This indicates that the short ranged dynamics are dominated by the benzene librational motion. We emphasise the need to combine far-infrared and microwave data (over four decades of frequency) in order to achieve a meaningful analysis of the intermolecular dynamics over a wide time range.