NMR RELAXATION AND SELF-DIFFUSION STUDIES OF H-BOND EXCHANGE AND MOLECULAR-MOTION IN PIVALIC ACID AND BINARY-SOLUTIONS OF PIVALIC ACID AND CARBON-TETRACHLORIDE

H-1 NMR relaxation times (T1) and, in particular, self-diffusion coefficients in the liquid and plastic phases of neat pivalic acid (PA) and binary mixtures of PA and carbon tetrachloride are reported. The slight increase in T1 of the methyl protons on dilution is ascribed to reduced viscosity. The T1 measurements of PA monitor reorientations of complete dimers rather than monomers in the liquid. The large increase in T1 of the carboxy proton on dilution, followed by a corresponding reduction in the activation energy, is the result of a decreasing contribution from intermolecular interactions and enhanced mobility. The diffusion coefficients (approximately 10(-9) m2 s-1), obtained from the methyl and carboxy resonances, are mutually identical in both PA and the most dilute sample (6.4 mol%). This indicates that the carboxy hydrogen is diffusing at the same rate as the rest of the molecule. The average life time of the cyclic dimer thus appears to be longer than the mean time between diffusional jumps in the liquid state. In the disordered solid the diffusion coefficient of the carboxy hydrogen (8 x 10(-12) m2 s-1 at 286 K) is about three orders of magnitude larger than the whole-molecule diffusion coefficient (approximately 10(-14) m2 s-1), Whereas the corresponding activation energies are 18 and 59 kJ mol-1. These observations show that the carboxy hydrogen is moving through the lattice approximately a thousand times faster than the rest of the molecule.