N. M. R. Investigations of Phase Transitions in Pure and Mixed Crystals of Pentachlorophenol

Abstract

Abstract The nuclear magnetic resonance line width, second moment and the relaxation times T 1 and T 1D have been measured for protons in a pentachlorophenol (PClPh)-hexachlorobenzene (HClB) mixed crystal (70% in concentration of PClPh), from 120 K to the melting point. The results have been interpreted in terms of the various molecular motions occurring in this crystal. We first observed, as the temperature increased, the fast jump of the proton around the C–O bond, even at low temperature. The activation enthalpy for this motion is 7.5 ± 0.2 KJ mole−1 and its correlation time of ≃ 2.3.10−8 s at ambient temperature. Before the transition (410 K), a slow motion occurs, molecular self diffusion or tumbling, which exhibits a discontinuity in its correlation time at the transition. The hindering enthalpy for this process is found to be 100 ± 20 KJ mole−1 above the transition. The associated correlation frequency is estimated from T 1d to be 50 s−1 near the melting point. These resultscan be extended to other mixed PCIPh-HCIB crystals, for which we have made T I and line-width measurements. Pure pentachlorophenol presents a different picture with two transitions. Below the first (355 K), the structure is rigid, the molecules being linked by hydrogen bonds, in agreement with the X-ray results of Sakurai. Above it, its behavior is similar to that observed for the mixed crystals, up to the high transition (435 K). Above this tem erature up to the me1tingpoint.a diffusive phase evident; the diffusion coefficient being 2 10−3 cm2/s at 460 K. At low temperatures, the dipolar relaxation time is mainly due to dipolar “second kind” re1axation with chlorine nuclei which are strongly relaxed by quadrupolar interactions.