Consistent Force Field Calculations. II. Crystal Structures, Sublimation Energies, Molecular and Lattice Vibrations, Molecular Conformations, and Enthalpies of Alkanes

Abstract

A set of energy functions of internal coordinates and interatomic distances is used for calculating simultaneously and consistently many properties of alkanes. Comparison between calculated and experimental data is used as a means for systematically selecting energy functions and determining their parameters. Properties calculated are: unit-cell parameters; heat of sublimation; molecular and lattice vibrations; thermal expansion of the unit cell—all these for n-hexane and n-octane crystals. Enthalpy differences—for gauche–trans butane and for axial–equatorial methyl-cyclohexane. Excess enthalpies for the series cyclopentane through cyclodedcane and molecular conformations and vibrations for a selected number of cyclo- and n-alkanes molecules. Some of the results and conclusions from these calculations are: molecular vibrations in the crystal are in part higher than in the gas, and the corresponding enthalpy difference contributes to the heat of sublimation equally as much as the lattice vibrations. Anharmonicity of lattice and molecular vibrations contributes significantly to the calculated unit-cell parameters of crystals of n-hexane and n-octane at 0°K; thermal expansion is largest in the a1 direction. The Lennard-Jones and Coulomb (12–6–1) potential is inadequate to account simultaneously for intra- and intermolecular interactions; the 9–6–1 potential and the exp-6–1 potential are equally preferable. The intermolecular interactions exihibit an anisotropy not fully represented by functions of interatomic distances; anisotropy is the source of systematic discrepancies between the calculated and measured unit-cell dimensions.