NMR Study on the Reorientational Relaxation in Supercritical Alcohols

The single-molecular reorientational relaxation of supercritical deuterated methanol and ethanol is studied by the 2 H nuclear magnetic resonance spin-lattice relaxation measurement, and the results are compared with the corresponding study on supercritical water (Matubayasi, N.; Nakao, N.; Nakahara, M. J. Chem. Phys. 2001, 114, 4107). The relaxation times of the OD deuterons of both alcohols show significant increase when they are transferred from liquid to supercritical phases. The reorientational relaxation of the OD vector is thus largely enhanced in the supercritical phase, as is the case of water. The relative increase in the reorientational relaxation rates of alcohols with density is larger than that of water. It indicates that the reorientational motion of supercritical alcohols is more diffusive than that of water. The molecular dynamics simulation of the supercritical methanol is also performed, and the results are consistent with experimental ones. The reorientational relaxation of two deuterated solute molecules, benzene (C 6 D 6 ) and pyrazine (C 4 D 4 N 2 ), is studied in supercritical methanol in addition to the neat fluid. The density dependence of the 2 H spin-lattice relaxation rates of methanol, benzene, and pyrazine in supercritical methanol show a minimum. The density of the minimum relaxation rate decreases with increasing solute-solvent interaction, which can be explained in terms of the angular-momentum relaxation time.