NMR studies on effects of temperature, pressure, and fluorination on structures and dynamics of alcohols in liquid and supercritical states

We measured H-1 NMR chemical shifts OH) and 1H and 1H NMR spin-lattice relaxation times (H-1- and H-2-T-1) of methanol, ethanol, 2-propanol, 2,2,2-trifluoroethanol, and 1,1,1,3,3,3-hexafluoro-2-propanol in the temperature range from 298 to 673 K at reduced pressures (P-r = P/P-J of 1.22 and 3.14. The delta(H) values showed that the degree (X-HB) of hydrogen bonding decreased in the order of methanol > ethanol > 2-propanol > H2O, and that the hydrogen bonding was much affected by fluorination, because of the intramolecular H-F interactions in supercritical (sc) states. Moreover, H-1-T-1 measurements revealed that the relaxation processes of OH groups in nonfluoroalcohols are controlled by dipole-dipole (DD) and spin-rotation (SR) mechanisms below and above the critical temperature (T,), while the cross-correlation effects connected with intramolecular DD interactions between a carbon atom and an adjacent proton played an important role for hydrocarbon groups (CHn, n = 1-3) under sc conditions. This interpretation was also supported by two other results. The first is that the intramolecular H-F interactions strongly inhibit the internal rotation of CH and CH2 groups of sc fluoroalcohols, and the second is that the molecular reorientational correlation times (T-c(D)) obtained from H-2-T-1 values of deuterated hydrocarbon groups (CDn) at temperatures above T-c have significantly less temperature dependence than those of OD groups. Actually, the apparent activation energy (Delta E-a) for molecular reorientational motions in sc alcohols was smaller compared with liquid alcohols, being about 1 order of magnitude.