High‐field multinuclear magnetic resonance relaxation studies of pivalic acid in liquid and solid phases

Abstract

AbstractProton, 13C, deuteron and 17O NMR line widths and spin‐lattice relaxation times (T1) were measured at a field strength of 9.4T over a wide temperature region in liquid and solid pivalic acid (PA). A striking feature of the NMR spectra is that the methyl proton signal is much wider than the carboxyl proton signal in solid I, whereas the opposite is true for the corresponding deuteron signals. Molecular self‐diffusion and overall molecular tumbling with activation energies of 59 and 31 kJ mol−1, respectively, are responsible for the considerable line narrowing observed for the methyl proton and deuteron resonances in solid I. The narrow carboxyl proton resonance and the low activation energy for the motional narrowing process (18 kJ mol−1) indicate that the acid hydrogen is moving through the lattice at a much faster rate than the rest of the molecule.An analysis of the T1 relaxation times of PA is reported. Rotational correlation times and activation parameters for the overall tumbling, and also the internal methyl and tert‐butyl reorientations (C3 and C3′ motions) taking place in the liquid and solid phases of PA, have been obtained for the first time. The results show that the slower motion is overall tumbling with an activation energy of 20–32 kJ mol−1, while the faster motions are internal C3′ and C3 reorientations with activation energies of 10–15 kJ mol−1. On freezing, the rate of overall tumbling is reduced by a factor of 47 from 1.6 × 1010 to 3.3 × 108 s−1, whereas the rates of C3′ and C3 rotations (1.1 × 1011 and 3.2 × 1010 s−1, respectively) are hardly affected.