Rotational Diffusion Rates in Molecular Crystals Relations to Molecular Structure and to Crystal Packing Density

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Abstract
The rotational diffusion rates of molecules and of mobile molecule segments in crystalline solids are expressed as relaxation rates relative to the corresponding torsional oscillation rate of the molecule or molecule segment. The resulting dimensionless relaxation time τi(ΔH s/I i )1/2 turns out to be a function of packing density (φ*) of the crystal and of the characteristic temperature fRT/ΔHs , where f is the number of external degrees of freedom per molecule. The dimensionless activation energy ΔE/ΔHs is a uniform function of the packing density for crystals composed of rigid molecules and another function of φ* for crystals composed of flexible molecules. The rate determining factor for molecular rotation in crystals containing hydrogen bonds appears to be the breaking energy of the hydrogen bond.

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