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This is the second of two articles concerned with the translational and rotational motion in molecular liquids. In this paper, orientational single-particle correlation functions are considered. By applying the approximations developed in Paper I of this series, Gordon's J diffusion model theory is derived. The analysis does not require one to adopt the unrealistic views of the liquid state that are usually attributed to that theory. The range of validity of the J diffusion model approximation is ascertained. Further, a microscopic expression is derived for the relaxation time which Gordon introduced phenomenologically. The expression leads naturally to a simple method for calculating τJ. These predictions are tested against results obtained from recent computer simulations of liquid N2. The comparison shows that the theory is qualitatively accurate.
David Chandler (Wed,) studied this question.