Molecular Motion in Solid Pentachlorobenzenethiol by N. M. R.

Abstract

The nuclear magnetic resonance line width, second moment and the relaxation times T₁ and T_1p have been measured for protons in a pentachlorobenzenethiol (PCIBT) crystal, from 120 K to the melting point. The results have been interpreted in terms of the various molecular motions occurring in this compound. We observed successively as the temperature increases: the fast jump of the proton around the C-S bond. even at low temperature. The associated activation enthalpy is 7.95 ± 0.2KJ mole⁻¹ and correlation time at 125K is about ∼ 2.8 10⁻¹s. Then appears the in plane molecular reorientation around the pseudo hexad axis, with activation enthalpy equal to 48.5 ± 2 KJ mole⁻¹. and correlation time of 1.610⁻⁷s. Then appears the in plane molecular reorientation around the pseudo hexad axis, with activation enthalpy equal to 48.5 ± 2KJ mole⁻¹ and correlation time of 1.610⁻⁷s at ambient temperature. These last results are in good agreement with those obtained by Brot by dielectric methods. Before the transition (435 K) a slow motion, molecular self diffusion or tumbling occurs. The hindering energy for this process is found to be 160 ± 30 KJ mole⁻¹. The T_1p results allow the calculation of the correlation frequency for this motion: at the melting point. it approaches 150s⁻¹, which is still too slow to average the dipolar interactions and thus reduce the second moment. We do not observe the rigid and diffusive phases that were seen in a similar compound, pentachlorophenol. The pentachlorobenzenethiol behavior is closer to that of pentachlorophenol-hexachlorobenzene mixed crystals. At low temperature, the T_1D relaxation time is mainly due to dipolar “second kind” relaxation with chlorines, rapidly relaxed by quadrupolar interaction.