Proton Resonance Study of Molecular Motion and Phase Behavior of Solid H2S and H2Se

Measurements of longitudinal and transverse proton relaxation in the three solid phases of H2S and H2Se are reported. In the lowest temperature phase it appears that the molecules execute order-preserving 180° flips about their C2 axes. In the intermediate phase there are, in addition, molecular reorientations through other angles and about axes other than C2. Over much of the high-temperature phase the relaxation rates are dominated by translational self-diffusion. The lower transition in H2Se, which is gradual in nature, is unusual in that it is accompanied by a cooperative lowering of the barriers to three-dimensional reorientation through the region which is associated with a linewidth transition.