Liquid state dynamics of CH2Cl2. Evaluation of models

Abstract

Absorption data in the range up to THz frequencies for pure liquid CH₂Cl₂ and in isotropic solutions in cholesteryl linoleate are used to evaluate critically some recent modelling of the liquid state N-body problem, and to detect some prescience of cholesteric behaviour in monofringent solutions. Adjustable parameters are avoided as far as is necessary to produce the fundamental theoretical absorption contour. The models used can all be expressed as various early approximants of the Mori continued faction expansion of the Liouville equation, and therefore describe the absorption data accurately only at low frequencies, or equivalently at long times when the orientational autocorrelation function decays exponentially. In contrast to the confused and often contradictory evaluations drawn from the available literature on Raman, infrared and n.m.r. spectroscopy, together with depolarised Rayleigh wing and incoherent neutron scattering studies on liquid CH₂Cl₂ it is shown that the zero to THz absorption profile discriminates clearly between models such as rotational diffusion, extended rotational diffusion, planar itinerant libration and Brownian motion of coupled dipoles. A scheme is suggested for a self-consistent evaluation of the available data from all sources within the context of the Mori continued fraction. This aims at a satisfactory evaluation of such quantities as the mean square torque, and derivatives, so that some statistical assessment may be made of the potential part of the total N-particle hamiltonian. At present, features of observed spectra are reflected in model correlation times which are often physically meaningless and directly contradictory.