The theory of stochastic quantum dynamics of internal rotation has been extended into the non-Markovian regime. The results were extrapolated to h-O and compared with angular momentum correlation functions calculated from a molecular dynamics (MD) simulation. With an appropriate choice of the adjustable parameter (the correlation time of the intermolecular torques), the quantum mechanical analogue of the FPL theory was found to agree well with functions derived from the MD simulations. The extended diffusion analogue, on the other hand, did not agree with the simulations. And neither model in the Markovian limit agreed with the simulations. The relative magnitudes of various angular momentum correlation functions also indicate that the reorientation of the methyl group is not Caussian random. Thus, the numerous dynamical models that assume Caussian random processes may not be applicable to methyl group motions. Reorientational correlation functions were calculated using the parameter which produced results in agreement with the MD simulations. Except for negligible short time transients, the reorientational correlation functions of methyl groups with internal barriers to rotation greater than about 3kT could all be described as activated rate processes:
