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

Abstract

Proton, I3C, deuteron and "0 NMR line widths and spin-lattice relaxation times (T,) 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 selfdifision and overall molecular tumbling with activation energies of 59 and 31 kJ mol-', 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-') 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 T, relaxation times of PA is reported. Rotational correlation times and activation parameters for the overall tumbling, and also the internal methyl and tert-hutyl reorientations (C, and C,' 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-

Effects of Pressure and Temperature on the Dynamics of Liquid tert-Butyl Alcohol

“…All the values in Table are in reasonable agreement, but the activation energies determined from the slopes of the high-pressure 1 H NMR relaxation time measurements (ln T 1 vs 1/ T (K -1 )) do appear to be lower than the values reported by Aksnes and Kimtys and Margalit at 1 bar, which also appear in Table . , The activation energy determined by high-pressure 1 H NMR is comparable to that reported for pivalic acid (20.3 kJ/mol) which exists as cyclic dimers . One could postulate from these data that rotational relaxation in tert -butyl alcohol is controlled by small cyclic hydrogen-bonded aggregates.…”

“…3,16 The activation energy determined by high-pressure 1 H NMR is comparable to that reported for pivalic acid (20.3 kJ/ mol) which exists as cyclic dimers. 26 One could postulate from these data that rotational relaxation in tert-butyl alcohol is controlled by small cyclic hydrogen-bonded aggregates. There is evidence of cyclic trimers and cyclic dimers in solution studies of alcohol aggregates based on 1 H chemical shift measurements as a function of concentration.…”

Dynamic1H and2H NMR investigations of acetonitrile confined in porous silica

“…However, the confined samples were only measured for increasing temperatures to avoid this complication. If the 1 H and 2 H relaxation mechanisms are governed by dipolar and quadrupolar interactions, respectively, modulated by a single reorientational process with correlation time t i , the appropriate intramolecular relaxation rates are given by 27,28 1…”

Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

“…5. The 'H and 13 C spin-lattice relaxation of these tert-butyl compounds is dominated by the dipole-dipole interactions, and if a single reorientational correlation time rr is involved, the appropriate relaxation rates are given by the formulae below [24,25] (2) where Mz H is the second moment modulated by the motion considered and r CH is the C-H bond length (0.110 nm). In Eq.…”

“…(2) b = 3/10, 4/15 and 8/45 for isotropic tumbling, C3 rotations and C3 rotations, respectively [26]. The spectral density functions, g(ooH , z-) and f(w1, arc, r), are given by the usual expressions [24]. Previous NMR and neutron scattering measurements of tert-butyl compounds [24][25][26][27][28] indicate that the C 3' motion is usually significantly faster than the rotation of the methyl group (C 3 motion) in the ordered phase.…”