Correlation between MO eigenvectors and enthalpies of formation for alkanes

A new model for the calculation of enthalpies of formation of alkanes (up to C8) is presented. An additive bond energy scheme, using the experimental methane and diamond values for the CH and CC bond energies, respectively, is supplemented by correction for the CC π antibonding character of the highest occupied molecular orbitals (HOMOs), effectively adjusting the CC bond energies. The effect is calculated by the summation of products of appropriate eigenvectors from semiempirical PM3 or HF/STO-3G calculations, after orthogonal transformation. The enthalpy of formation can then be expressed in terms of only one adjustable parameter. With HF/STO-3G eigenvectors, the mean discrepancy between experimental and calculated enthalpies of formation, after a one-parameter correction for 1,4 steric interactions, is 2.2 kJ mol−1, comparable with more highly parameterized models. The results using PM3 eigenvectors are less satisfactory, probably on account of the neglect of overlap in the semiempirical scheme.