Nuclear magnetic spin relaxation in solid solutions of hydrogen chloride in deuterium chloride

The following measurements have been made: the proton linewidth for HCl, in both solid phases by both steady state and Bloch decay methods, the proton spin-lattice relaxation time for HCl and for 50% HCl/DCl in the solid state, at resonance frequencies of 21·5, 20·8, 14·3 and 9·0 MHz, and the proton spin-lattice relaxation time in the rotating frame for radio-frequency fields of 14·5, 12·0 and 5·3 G. The measurements have been interpreted in terms of the various nuclear spin interactions and important information has been obtained about the molecular motion. It is found that for HCl in the upper temperature phase below the melting point the molecules are reorienting rapidly and diffusing slowly, and the corresponding correlation times and diffusion constants are derived. On melting the reorientation rate hardly changes but the molecules become free to translate much more rapidly. It is possible that there is a small spin-rotation contribution to spin-lattice relaxation in the upper solid phase in HCl. In the lower temperature part of the upper phase the proton relaxation, both conventional and rotating frame, contains a contribution due to dipolar relaxation of the second kind. In the lower temperature phase we have difficulty in interpreting the spin-lattice relaxation times but they and the linewidths are consistent with very little, if any, large-scale molecular motion in this phase. It seems likely that the proton relaxation is due to dipolar relaxation of the second kind with chlorine nuclei controlled by quadrupole interaction.