Recent theoretical studies using stochastic classical and quantum dynamics models developed in this lab indicate that NMR relaxation involving a methyl attached to a large- or intermediate-sized molecule may show distinctly quantum mechanical behavior. The relevant correlation times depend on the parameter tau-omega, which is the correlation time for the collision induced randomization of the internal angular momentum of the methyl group. Molecular dynamics simulations of a methyl group in water have been carried out in order to determine plausible values for tau-omega and assess the potential importance of quantum effects. The simulations indicate that the dynamics are non-Markovian. To the extent that a Markovian model can be used, however, the results show that extended diffusion, rather than Fokker-Planck-Langevin, dynamics are appropriate for this system. Using the tau-omega determined from the simulation in water and a larger tau-omega deduced from experimental data for a methyl in a hydrocarbon environment, it is shown that quantum effects can be significant for these types of systems at physiological temperatures.
