The Fokker–Planck–Langevin model for rotational Brownian motion. II. Comparison with the extended rotational diffusion model and with observed infrared and Raman band shapes of linear and spherical molecules in fluids

Abstract

A detailed comparison of two extensions of the Debye model for molecular reorientation in liquids, Gordon’s extended J-diffusion model and the rotational Fokker–Planck–Langevin model, is presented. It is shown that the two models, although they represent very different physical pictures of rotational dynamics in fluids, lead to remarkably similar reorientational correlation functions, memory functions, correlation times and spectral densities. Only for values of the angular momentum correlation time appropriate to low density fluids do the two models give significantly different results, but the validity of the FPL model is questionable in this region. The results of an infrared study of the ν3 band of N2O in N2O/O2 along the coexistence curve from 105–153 K, and in N2O/N2 along the 80 bar isobaric line from 100–300 K are presented. The reorientational correlation functions from these measurements, and from earlier infrared studies of CO/N2 and Raman studies of CF4 and N2O liquids are compared with correlation functions computed with the EDJ and FPL models. Neither the EDJ nor the FPL model gave a substantially better description of the spectral band shape data.