Structural Phase Transition and Molecular Motion of p-Chlorobenzamide as Studied by the Temperature Variations of 35Cl NQR Frequencies, Chlorine Nuclear Quadrupolar Relaxation Times, and 1H Magnetic Dipolar Relaxation Times

Abstract

The temperature dependences of 35Cl NQR frequencies, 35Cl (37Cl) nuclear quadrupolar relaxation times (T1ClQ), 1H NMR absorption, and of 1H NMR relaxation times were observed for p-chlorobenzamide, p-ClC6H4CONH2 and its N-deuterated analog. The existence of three 35Cl NQR lines already reported was confirmed for the known low temperature phase below 316 K. A single sharp 35Cl NQR line was detected in the high temperature phase up to 447 K just below the melting point 451 K. At lower temperatures, three different 35Cl T1ClQ values attributable to the fluctuation of the electric field gradient due to rotational oscillation were observed. With increasing temperature, these T1ClQ decreased rapidly and yielded almost the same value immediately below Ttr suggesting the onset of a slow motion. The 1H T1, of the high temperature phase gave a T1, minim um and could be explained through the 180° flip motion of the phenyl ring with an activation energy o f 44 kJ mol-1.