Derivation of Physically Significant Nonbonded Interaction Constants in Hydrides by a Modified Urey—Bradley Analysis

Abstract

Previous Urey—Bradley (U.B.) analyses of vibrational spectra have suggested that intramolecular repulsions between atoms are generally large and similar, at a given distance, to intermolecular nonbonded interactions. Hydrogen, however, has been found to exhibit anomalously low intramolecular repulsions according to conventional U.B. analyses. In the present modified analysis completely general quadratic force fields, corrected for anharmonicity, are resolved into the sum of U.B. fields and additional interactions neglected in conventional U.B. analyses. The assumptions required for evaluation of the central force components are discussed. It is found for a series of hydrides that explicit inclusion of stretch—stretch interactions over and above those implied by nonbonded repulsions markedly alters the analysis and yields H···H interactions interpretable in terms of large repulsions entirely comparable with those calculated using intermolecular force laws. Such large repulsions are compatible with structural, thermochemical, and kinetic data. While they cannot be separated from effects of hybridization solely on the basis of quadratic force constants, they provide an alternative explanation for effects commonly ascribed to hybridization and more specialized interactions.