Nuclear magnetic resonance and molecular dynamics study of methanol up to the supercritical region

Self-diffusion coefficients and chemical shifts of the methanol OH proton referred to the CH3 proton have been measured in a wide temperature range from 289 to 580 K. The reduced densities rho(r) (rho(r) = rho/rho(C), rho(C) = 272 kg m(-3)) of methanol studied are 0.183, 0.256, 0.372, 0.622 and 1.008 in the supercritical condition. Molecular dynamics (MD) simulations have been performed in the same temperature and density range. The observed self-diffusion coefficients are in a good agreement with the Chapman-Enskog kinetic theory in the supercritical region. They are consistent with the MD simulation results over the whole range studied. MD calculations show that hydrogen-bonded clusters of methanol are chain like both at room temperature and in the supercritical state. The formation energy and entropy of hydrogen bonding were obtained from the temperature dependence of the OH chemical shifts for supercritical methanol. The thermodynamic model which takes account of cluster distribution provides the degree of hydrogen bonding in methanol. The number of hydrogen-bonded clusters decreases with increasing temperature and decreasing density. in the supercritical region, the calculated results for the cluster size distribution from the thermodynamic model are in a good agreement with the MD calculation. (C) 1998 American Institute of Physics. [S0021-9606(98)50846-X].