High-field multinuclear magnetic resonance studies of molecular motions in the liquid and solid phases of 3-chloropivalic acid

Abstract

The phase behaviour and the translational and rotational dynamics of the different phases of 3-chloropivalic acid (CPA) were studied using high-field multinuclear magnetic resonance. Two components of CPA with very different line widths were observed within a large part of the investigated temperature region between the melting point (316.7K) and 160K. The coexisting narrow line and broad line components are ascribed to the disordered and ordered solids I and II, respectively. The relatively narrow NMR signals of solid I allowed observation of the individual resonances from the methyl, chloromethyl and carboxyl groups.The diffusion in the liquid and solid I phases of CPA appears to involve the migration of the strongly hydrogen-bonded dimers with activation energies of 18 and 84kJ mol−1, respectively. The temperature variation of the spin-lattice relaxation time T1 of the carboxyl deuteron reflects the overall tumbling motion with activation energies of 33 and 53kJ mol−1 for the liquid and solid I phases, respectively. 1H T1 of solid II was also measured over a large temperature region from 163 to 299K. The broad and shallow T1 minimum indicates that the rotations of the alkyl groups are correlated in this phase.