The Orientational Freedom of Molecules in Crystals: I. Joint Interpretation of Thermal and N.M.R. Data in an “All or Nothing” Correlation of Orientation Theory

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Abstract
It is shown quantitatively that the Frenkel model (step reorientations among potential wells) is, in most cases, a very good approximation to the motion in crystals presenting a molecular orientational freedom. In contrast to n.m.r., thermodynamics is not sensitive to the dynamies of the motion, but rather to the appearance of the orientational disorder, whatever its rate of rearrangement. A classification accounting for the different observable temperature relations between n.m.r. line width narrowing and thermal data is proposed. This classification is based on the comparison between the symmetry of the molecule and the symmetry of its allowed orientations (potential wells). It applies to crystals of rigid molecules, in thermodynamic equilibrium, in which the correlation of orientation of the molecules is either very large (low temperature) or very small (high temperature).

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