(310h) Deciphering the Molecular Modes Underlying NMR Relaxation Response in Fluids

Nuclear magnetic resonance (NMR) relaxation is a powerful technique for probing matter non-destructively, with applications spanning medicine to material science to petrophysics. At high NMR frequencies, quantum effects are usually negligible, and a semi-classical statistical mechanical description is sufficient to model relaxation. Traditional theories of the NMR autocorrelation function for intramolecular dipole pairs assume single-exponential decay, while the actual autocorrelation of realistic systems display a rich, multi-exponential behavior resulting in anomalous NMR relaxation dispersion (i.e., frequency dependence). In this context, we have developed an approach based on “molecular modes of relaxation” to model and interpret the multi-exponential autocorrelation using simple, physical models within a rigorous statistical mechanical development. Our approach recasts the problem of evaluating the autocorrelation in terms of averaging over a diffusion propagator whose evolution is described by a Fokker-Planck equation. We have also shown that NMR relaxation of fluids can be probed within classical and semi-classical Molecular Dynamics (MD) simulations trajectories, which supports the validation of our theory and expands the range of practical applications. These molecular modes can be used to model and predict NMR dipole-dipole relaxation dispersion of fluids on the molecular level. It is worth noticing that both our statistical mechanical description of the distribution of molecular modes and our molecular dynamics simulations results assume no adjustable parameters.