PRESSURE AND TEMPERATURE EFFECTS ON THE HYDROGEN-BOND STRUCTURES OF LIQUID AND SUPERCRITICAL-FLUID METHANOL

The proton spin-lattice relaxation times and proton chemical shifts fo r the hydroxyl and methyl protons in methanol were measured at liquid and supercritical densities using capillary high-pressure NMR spectros copy. The pressure range for the proton nuclear relaxation measurement s was between 50 and 3500 bar over a temperature range of 298-573 K. T he proton chemical shifts of methanol were investigated for a pressure range of 50-3500 bar and a temperature range of 298-773 K. Attempts w ere made to separate the contributions of the dipolar and spin-rotatio n interactions to the spin-relaxation processes at each thermodynamic condition over methanol densities ranging from liquid to supercritical fluid. An average number of hydrogen bonds per molecule in methanol a nd the apparent activation energy of the methyl group internal rotatio n have been extracted from the experimental relaxation data. The extra cted quantities show a moderate pressure dependence in addition to tem perature effects, which suggest that molecular packing effects on hydr ogen-bonded methanol are important at higher pressures. A comparison b etween methanol and water at similar thermodynamic conditions was also made to obtain new insight into these two important supercritical sol vents.