New additivity schemes for hydrocarbon energies

[structure: see text] A new isodesmic additivity scheme based on the energetic relationships among the simplest hydrocarbon molecules reproduces the experimental heats of formation for a broad range of unstrained hydrocarbons with remarkable accuracy. The stabilizations of radicals, double, and triple CC bonds by alkyl substituents (hyperconjugation), as well as the stabilization by 1,3-alkyl group interactions at the same carbon (branching), support conventional interpretations. Statistical data fitting can also be achieved by using only four adjustable parameters.