Abstract
We investigated the isomeric variations of the enthalpies of formation of octanes by use of orthogonalized connectivity indices as molecular descriptors. With two omega descriptors the multivariate regression coefficient of 0.932 and the standard error of 0.478 were obtained. The residual of the multivariate regression based on the two orthogonalized connectivity indices could not be reduced by an explicit use of terms attributed to simple steric effects, while at the same time the contributions to the enthalpies empirically assigned to steric terms show a very high correlation with the omega indices (i.e. orthogonalized connectives) 1Ω and 2Ω. Hence the dom- inant contributions of steric terms in octane isomers are well accounted for by the connectivity indices, the graph invariants that apparently parallel the particular three-dimensional characteristics of alkane molecules.
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