Molecular Motion in Solid 1,2-Dichloroethane

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Abstract
35Cl pure quadrupole spin—lattice relaxation has been studied in solid 1,2‐dichloroethane over the presently accessible temperature range below 90°K. T1 for 35Cl changes by about three orders of magnitude between 53° and 82°K, and shows the onset of a thermally activated rate process above 63°K. The activation energy for this motion is found to be 3.93 kcal/mole. The temperature dependence of the proton T1 was also examined. At values of H0 for which relaxation is not complicated by the temperature dependent cross relaxation with the chlorine spin system, the activation energy obtained from proton T1 data agrees with that from the pure quadrupole measurements. An anisotropic least‐squares refinement was made using the 133°K. X‐ray data of Reed and Lipscomb and amplitudes were assigned to chlorine atom motions. The physical picture that emerges from the combination of these data is that the amplitude of over‐all motion is small below the heat capacity anomaly, relaxation being effected by a reorientation between two (or several) close minima 30° to 40° apart and separated by the above barrier. Near the heat capacity anomaly this precursor motion goes over into a large scale motion, still about the Cl–Cl axis, which has been previously described.