Effect of Hydrostatic Pressure on Molecular Rotation in Solids

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Abstract
Measurements of the NMR spin—lattice relaxation time T1 have been carried out at various temperatures and pressures for eight solids that demonstrate rotational degrees of freedom. Apparatus is described which permits NMR pulse experiments to be performed over the temperature interval between 169° and 400°K under applied hydrostatic pressures between 1 and 680 atm. The intramolecular rotation of groups within molecules is insensitive to pressures within this range, and the rotation of entire molecules in the solid state is pressure dependent in varying degrees. The observed pressure coefficients of T1 in these solids are related to the thermal expansion coefficient of the lattice by a simple model that postulates a localized lattice expansion as an important step in the reorientation process.

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