Proton magnetic relaxation study of molecular motion in anilinium chloride, bromide, iodide and sulphate

Abstract

Spin–lattice relaxation times of anilinium chloride, bromide, iodide, and sulphate have been measured at 31 MHz by pulsed n.m.r. spectrometry over appropriate temperature ranges between 80 and 550 K.

Each halide salt has a minimum in T₁ of 38 ms, which has been ascribed to reorientation of the –NH⁺₃ group about its C₃ axis. Activation energies for this motion are 37.1, 11.2 and 8.5 kJ mol^–1 for the chloride, bromide and iodide, respectively. The iodide has a second minimum of 178 ms at much higher temperature (513 K) and this has been ascribed to reorientation of the phenyl ring among more than two (probably four) potential wells about the C–N axis. Although this minimum is not reached by the bromide before decomposition and melting occur, a similar mechanism is likely and one may estimate an activation energy for this motion as 96 kJ mol^–1, compared with 75 kJ mol^–1 for the iodide. Irregular features of the variation of T₁ between about 200 and 300 K in the bromide and iodide can be interpreted in terms of a higher order phase change.

The sulphate shows three minima in T₁. The two at lower temperature are nearly equal at about 81 ms and have been attributed to relaxation by C₃ reorientation of ^–NH⁺₃ groups in two different crystallographic sites with activation energies of 11.2 and 27.4 kJ mol^–1. The third minimum, at higher temperature, has been assigned to a pseudo-C₄ reorientation of the phenyl group, with an activation energy of 59 kJ mol^–1.

The activation energy of the ^–NH⁺₃ reorientation has been found to be proportional to the temperature of the corresponding minimum in T₁, the constant of proportionality being related to τ^°_c for the motion. The activation energies of the ^–NH⁺₃ motion are compared with results for other compounds and their variation in magnitude discussed in terms of hydrogen bonding strength and the symmetry of the environment of the group.