(181c) Molecular Modes from NMR Relaxation in Fluids: Going Beyond the Bpp Theory

Nuclear Magnetic Resonance (NMR) relaxation is a phenomenon by which nuclei with a non-zero spin return to equilibrium in the presence of an applied static magnetic field after it has been excited by an applied radio-frequency magnetic field. It is characterized by two time constants, T1 and T2, corresponding to relaxation in the longitudinal and transverse directions, respectively. T1 and T2 are dependent on the equilibrium structure and dynamics of the system, providing an avenue to probe matter non-destructively. For close to 70 years, the interpretation of T1 and T2 in liquids has relied on the Bloembergen-Purcell-Pound(BPP) theory, which made rather severe assumptions about the fluid structure and dynamics, such as treating molecules as hard spheres and the magnetic dipoles as freely rotating, features that ultimately justify a mono-exponential decay of magnetic dipole-dipole autocorrelations.

Analysis of magnetic dipole-dipole autocorrelations from molecular simulations shows that even for simple alkanes and water, the mono-exponential assumption is not met. We show that the T1 relaxation in the slow-motion regime of crude oils, hydrocarbon viscosity standards, and glycerol all significantly depart from the BPP theory. The NMR relaxation autocorrelation exponents are distinctly different and reveal the existence of a distribution of correlation times within the autocorrelation. This distribution appears to be multimodal on the logarithm of the autocorrelation times. The modes correspond to the molecular structure and environment studied, hence termed molecular modes.

I will present our findings on NMR relaxation from simulations in bulk fluids, fluids under confinement, and water in the presence of paramagnetic ions. I will show that the assumptions underlying BPP theory are invalid, a finding that has far reaching implications for how one interprets NMR relaxation phenomena in condensed media. I will also show the potential for using molecular modes distribution to enhance the interpretation of NMR relaxation.