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

Abstract

We measured 1H NMR chemical shifts (delta H) and 1H and 2H NMR spin-lattice relaxation times (1H- and 2H-T1) 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 ( Pr = P/ Pc) 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, 1H- 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 (Tc), 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 (tauc(D)) obtained from 2H- 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 (DeltaEa) for molecular reorientational motions in sc alcohols was smaller compared with liquid alcohols, being about 1 order of magnitude.

Operando MAS NMR Reaction Studies at High Temperatures and Pressures

“…When the calculated rotor temperature reaches 240 °C, the pairs of lines merge (methyl: 24.3 ppm; methine: 63.7 ppm), showing that there are no longer distinct chemical environments for the liquid and vapor phases. At 235.3 °C and 47 bar, 2-PrOH enters its supercritical phase . At the highest temperature shown in Figure , the spectrum is consistent with the appearance of this supercritical phase.…”

On the artifact behavior of simple liquids in the inversion‐recovery experiments under mas conditions at high spinning rates

“…The room-temperature 2 H T 1 measurements performed for CD 3 OD, placed into a static 4-mm NMR rotor, shows exponential relaxation for groups CD 3 (Fig. 3) with the 2 H T 1 time of 3.2(5) s in accord with (9,15). Protons in the same static CD 3 OD sample also relax exponentially (Fig.…”

“…3) with the 2 H T 1 time of 3.2(5) s in accord with (9,15). The room-temperature 2 H T 1 measurements performed for CD 3 OD, placed into a static 4-mm NMR rotor, shows exponential relaxation for groups CD 3 (Fig.…”

“…Since the methanol protons relax by dipolar interactions and the spin-rotation mechanism develops only at very high pressure and temperature (14), the linear dependences in Fig. 2 could be attributed to this pressure effect (9,15). For example, the experiments performed in Ref.…”

Excited-State Proton Transfer of 5,8-Dicyano-2-naphthol in High-Temperature and High-Pressure Methanol: Effect of Solvent Polarity and Hydrogen Bonding Ability

“…Supercritical fluids (SCFs) have been well-studied because of their unique physicochemical properties such as diffusion coefficient, refractive index, and dielectric constants around the critical point. − Among the many studies on SCFs, the solvent–solvent interaction in a hydrogen bonding (HB) solvent has attracted the interest of many chemists, and experimental − and theoretical − studies on the behavior of HB in water, methanol, ethanol, etc. at high temperature and pressure have accumulated.…”