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The MINDO/3 semiempirical SCF-MO method has been tested by calculations for a wide variety of hydrocarbons, for radicals and ions derived from them, and for several simple hydrocarbon reactions.The preceding paper1 described the development of an improved version (MINDO/3) of the MINDO3"5 semiempirical SCF-MO method. Here we report the results of calculations for a wide variety of hydrocarbons, for ions and radicals derived from them, and for several simple reactions of hydrocarbons.

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The stereochemical consequences of electron delocalization in substituted ethylenes (e.g., 1,2-difluoroethylene), propenes, butadienes and allyl and pentadienyl anions, as well as 1,2-disubstituted ethanes, are discussed. It is shown, on the basis of simple perturbation theory, that the generally greater stability of the bent (cis or gauche) conformations of these molecules may be attributed to a previously unrecognized conjugative destabilization which is transmitted through bonds and is greater for elongated, trans conformations than for bent, cis structures. This destabilization is due to the exchange (exclusion) repulsion associated with overlap between filled orbitals. The effect is conformation dependent in extended systems as a result of the fact that electrons delocalize preferentially in straight lines.

In the Winstein-Grunwald relationship for solvolysis reactions, log k¡k0 = mY, Fis a parameter taken to represent "solvent ionizing power" and not the nucleophilicity of the solvent or its ability to promote rate-determining elimination. Using r-butyl chloride, the reference compound chosen by Winstein and Grunwald to evaluate F, it is impossible to rule out these other mechanistic contributions. In order to assess the role of solvent, rate constants for solvolysis of 1-adamantyl bromide in fourteen solvents were determined; data for eight additional solvents were obtained from the literature. In such a bridgehead substrate, backside nucleophilic solvent attack and elimination are both impossible. In general, an excellent correlation between data for r-butyl chloride and 1-adamantyl bromide is found indicating that r-butyl chloride, in most instances, solvolyzes by a limiting mechanism, free from nucleophilic solvent participation and from rate-determining elimination. Significant dispersion is found for aqueous trifluoroethanol solvent systems; this deviation is discussed in terms of specific substrate and leaving group effects. The Winstein-Grunwald relationship for solvolysis reactions, log k/ko = mY, affords a useful although not entirely precise tool for the calculation of solvolysis rates.3 The parameter m is taken to be a measure

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