Hydrocarbon enthalpies of formation and ab initio calculations

Simple protocols to convert calculated HF ab initio energies for hydrocarbons to accurate heats of formation are described. The optimum procedures use the numbers of carbon and hydrogen atoms, and 6-31G∗ energies as independent variables. The 6-31G∗ energies are calculated at either 6-31G∗ or STO-3G optimized geometries. The data set of 65 compounds includes planar and nonplanar polycyclic aromatics, alkyl-substituted benzenes, highly strained alkenes and alkanes, and alkynes. The experimental ΔHf0(g) cover a range of −50 to +150 kcal/mol. The mean deviation between experimental and calculated ΔHf0(g) is 1.1 kcal, and the correlation coefficient is 0.9998 for the 6-31G∗ // STO-3G calculation.