Hydrogen bonding of methanol in supercritical CO2 studied by C-13 nuclear spin-lattice relaxation

A sapphire high-pressure NMR cell, capable of independently controlling sample pressure, temperature, and concentration, is used to measure C-13 spin-lattice relaxation times for dilute methanol in dense carbon dioxide at pressures from 80 to 220 atm and temperatures from 287 to 334 K, These ranges of temperature and pressure provide relaxation data for methanol in both the liquid and supercritical fluid phases of CO2. The nuclear Overhauser effect (NOE), used for separating the contributions of the dipolar and spin-rotation mechanisms, has also been measured. Spin-rotation relaxation theory, together with proposed hydrogen-bonding models, is used to interpret the experimental data. The partial molar enthalpy changes for hydrogen bond formation agree well with those reported by Smith et al. using FTIR (J. Am. Chem. Soc. 1991, 113, 8327-8334) and commonly accepted hydrogen bond energies. A large negative change in the partial molar volume arising from hydrogen bond formation in methanol is observed near the critical temperature of the CO2-methanol mixture.