Abstract
Nearly a half-century ago I published a paper which successfully calculated the relative stabilities of a large number of hydrocarbons. Calculations were based on empirical atom-atom potential energies I had introduced in 1960, which had been deliberately biased to take into account the differences between branched and u n b r a n c h e d hydrocarbon molecules. Little controversy arose until quite recently when Gronert revived this old work and entered into what evolved into a bitter controversy with the Grimme, Wiberg, and Schleyer accounts of stabilities of branched vs. unbranched hydrocarbons. Gronert invoked 1-3 nonbonded interactions to reproduce successfully the relative stabilities of the various hydrocarbons. His opponents carried out careful quantum computations including electron correlation energies (not feasible to calculate in 1960 when the empirical interaction potentials were formulated). The purpose of this paper is to resolve the controversy by showing that while the two approaches differ in verbal explanations; they are equivalent in concrete results.
Highlights
To get to the heart of what is involved, consider the fact that the relative stabilities of isomers are built into the force-fields of the molecules concerned
The idea that that geminal distances around a central atom are based on a 1960 model [2] invoking the close packing of repelling ligands, rather than by presumed effects of hybridization or even by the interactions proposed by the popular VSEPR model
This account has since been confirmed in detail by the author of the VSEPR papers, and his colleagues [3,4,5,6,7,8,9,10,11,12,13]
Summary
To get to the heart of what is involved, consider the fact that the relative stabilities of isomers are built into the force-fields of the molecules concerned. The idea that that geminal distances (bond angles) around a central atom are based on a 1960 model [2] invoking the close packing of repelling ligands, rather than by presumed effects of hybridization or even by the interactions proposed by the popular VSEPR model This account has since been confirmed in detail by the author of the VSEPR papers, and his colleagues [3,4,5,6,7,8,9,10,11,12,13]. To get quickly to the point of the present argument, the Gronert force field [14], based closely on Bartell’s empirical field of 1960 [2], incorporated nonbonded interactions, which inserted into Eq 1, deliberately biased them to make the C-H interactions more repulsive than the sum of the C-C and H-H interactions This stratagem successfully accounted for observations of a large number of unbranched, singly, and multiplies branched hydrocarbons [1]. No explanation was advanced to account for this bias except that it was needed to account for the extra stability of branched molecules
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