Molecular motions in the liquid and solid phases of 3,3 '-dichloropivalic acid studied by high-field multinuclear magnetic resonance

The translational and rotational dynamics of the liquid and solid phases of 3,3'-dichloropivalic acid (DPA) are studied using high-field multinuclear magnetic resonance. The nuclear magnetic resonance (NMR) data are consisten t with a phase transition from the undercooled liquid to the disordered sol id I at 320 K followed by a second transition to the ordered solid II at 28 1 K. On warming the sample, solid II persisted up to the melting point. Two components of DPA with significantly different T-2 values were observed in a limited temperature region below the transition point at 320 K. The diffusion in the liquid and disordered phases appears to involve the mi gration of dimers with activation energies of 39 and 113 kJ mol(-1), respec tively. However, the narrow carboxyl proton signal in solid I implies that these protons are moving through the lattice at a much faster rate than the rest of the molecule. The temperature variation of the deuteron Ti relaxat ion time (and line width) reflects the overall tumbling motion with activat ion energies of 39 and 60 kJ mol(-1) in the undercooled liquid and solid I phases, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.