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

The phase behaviour and the translational and rotational dynamics of the di fferent 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 temperatu re region between the melting point (316.7 K) and 160 K. The coexisting nar row line and broad line components are ascribed to the disordered and order ed solids I and II, respectively. The relatively narrow NMR signals of soli d I allowed observation of the individual resonances from the methyl, chlor omethyl and carboxyl groups.The diffusion in the liquid and solid I phases of CPA appears to involve the migration of the strongly hydrogen-bonded dim ers with activation energies of 18 and 84 kJ mol(-1), respectively. The tem perature variation of the spin-lattice relaxation time T-1 of the carboxyl deuteron reflects the overall tumbling motion with activation energies of 3 3 and 53 kJ mol(-1) for the liquid and solid I phases, respectively. H-1 T- 1 of solid II was also measured over a large temperature region from 163 to 299 K. The broad and shallow T-1 minimum indicates that the rotations of t he alkyl groups are correlated in this phase. (C) 1999 Elsevier Science B.V . All rights reserved.