Nuclear magnetic resonance and molecular dynamics simulation study on the reorientational relaxation of solutes in supercritical methanol

The effect of solvent density on the single-molecular reorientational relaxation of deuterated methanol and benzene in supercritical methanol are investigated both by the nuclear magnetic resonance (NMR) experiment and the molecular dynamics (MD) simulation. In the NMR experiment, the rank-2 reorientational relaxation time is determined from the spin-lattice relaxation time of deuterons. The relaxation time of the O-D vector of methanol is an increasing function of density above 0.3 ρc, where ρc stands for the critical density, while that of the C-D vector of benzene shows a shallow minimum around ρc. To clarify the origin of the difference, MD simulations of both neat methanol and benzene in methanol are performed. The MD simulation revealed that the reorientational relaxation of the O-D vector of methanol is diffusive above 0.3 ρc, whereas the effect of the inertial motion is strong on the reorientational relaxation of benzene.