INTERMOLECULAR FORCE EQUATIONS FOR SINGLE CRYSTAL STRUCTURES AND CALCULATION OF THE HEAT OF FUSION
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Date
1974
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Ohio State University
Abstract
The use of vector analysis of single crystals was demonstrated by Patterson in 1935 when he improved upon the previously used radial distribution function to generate the analysis which now bears his name. The use of molecular packing analysis for studying non-bonded interactions in single crystals has lead us to formulation of vector equations which make use of the static equilibrium nature of these crystals so that we may solve ``the force equal to zero” problem. One consequence of this technique is the reduced dependence on X-ray data. This system of analysis was instrumental in the correction of the crystal structure of ethylene. Our general results Include (1) the ability to distinguish between correct and incorrect packing models; (2) the calculation of the true lattice energy equal to the heat of melting; (3) a definition of the pair summation limit based upon summation to the balance of attractive and repulsive pair force components; and (4) the future use of this procedure to improve pair potential functions in a rigorous fashion. Our calculation of the heat of melting (lattice energy) is based upon recognition that the heat of sublimation (previously used as a standard) may be broken down by Hess’ Law into the heats of melting plus vaporization, or qualitatively, the lattice energy plus the liquid energy equal the total intermolecular binding energy. The lattice stabilization energy is only the heat of fusion.
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Author Institution: Department of Chemistry, Macalester College; Department of Chemistry, Michigan State University