Pressure and Temperature Effects on the Hydrogen-Bond Structures of Liquid and Supercritical Fluid Methanol

Abstract

The proton spin-lattice relaxation times and proton chemical shifts for the hydroxyl and methyl protons in methanol were measured at liquid and supercritical densities using capillary high-pressure NMR spectroscopy. The pressure range for the proton nuclear relaxation measurements was between 50 and 3500 bar over a temperature range of 298-573 K. The proton chemical shifts of methanol were investigated for a pressure range of 50-3500 bar and a temperature range of 298-773 K. Attempts were made to separate the contributions of the dipolar and spin-rotation 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 and the apparent activation energy of the methyl group internal rotation have been extracted from the experimental relaxation data. The extracted quantities show a moderate pressure dependence in addition to temperature effects, which suggest that molecular packing effects on hydrogen-bonded methanol are important at higher pressures. A comparison between methanol and water at similar thermodynamic conditions was also made to obtain new insight into these two important supercritical solvents.

NMR Studies on Effects of Temperature, Pressure, and Fluorination on Structures and Dynamics of Alcohols in Liquid and Supercritical States

“…Herein, we utilized C eff value of CH 4 in place of the values of the pure alcohols. 25,38,39 The calculated relaxation rates (1/T 1cal ) were estimated according to eqs 4-6 and compared with the experimental data (1/T 1exp ). As shown in Figure 4, the temperature-dependence of 1/T 1cal values of OH and CH 3 groups in MeOH was similar to that of 1/T 1exp values from liquid to sc conditions.…”

“…The relaxation processes of 2 H nuclei in solutions are mainly dominated by quadrupole interactions; that is, the contribution of the chemical shift anisotropy, scalar coupling, and SR interaction to 2 H-T 1 is a few percent. 25 For examining the molecular reorientational relaxation of each group in sc alcohols, it should be meaningful to measure 2 H-T 1 of deuterated alcohol with a quadrupole moment. We measured the 2 H-T 1 values of deuterated hydrocarbon (CD n , n ) 1-3) groups in CD 3 OD, CD 3 CD 2 OD, (CD 3 ) 2 CDOD, CF 3 CD 2 OD, and (CF 3 ) 2 CDOD in the temperature range of 298 to 673 K at P r ) 1.23 and 3.14 by using the inversion recovery method.…”

Inelastic X-ray Scattering of Sub- and Supercritical Methanol

“…Raman spectral 1) , NMR chemical shift 2) , and NMR relaxation 3) measurements, and MD simulations 4) for SCM have revealed that the hydrogen bonds of methanol decrease with increasing temperature but remain even in the supercritical state. Detailed information on the cluster formation in SCM has been reported by small angle neutron scattering measurements.…”

Hydrogen bonding in liquid and supercritical 1-octanol and 2-octanol assessed by near and midinfrared spectroscopy

“…Therefore, fundamental studies mainly devoted to lower alcohols such as methanol and ethanol have been performed to elucidate the structure and dynamics of pure alcohols in the supercritical state. Molecular dynamics simulations, 2 NMR experiments, 3,4 neutron diffraction, 5 and vibrational spectroscopies ͑Raman 6 and infrared [7][8][9] ͒ came to the converging conclusion that hydrogen bonding still exists in the supercritical domain even at high temperatures. As a matter of fact, at about 330°C and at intermediate reduced densities r = / c =2 ͑where c is the critical density͒, the degree of hydrogen bonding was found to be about 30% that existing in liquid methanol and ethanol at room temperature.…”