Predicting the Heats of Formation of Model Hydrocarbons up to Benzene

The heats of formation of benzene and seven other small hydrocarbons (allyl, allene, cyclopropene, propene, propyne, cyclopropane, and propane) have been calculated at high levels of ab initio molecular orbital theory. Geometries and frequencies were determined, in general, with coupled cluster theory, including a perturbative treatment of the connected triple excitations and with basis sets up through augmented quadruple-ζ in quality or, in some cases, augmented quintuple-ζ. Subsequent extrapolation of the total energies to the complete 1-particle basis set limit was performed, in an effort to further reduce the basis set truncation error. Additional improvements in the atomization energy were achieved by applying corrections for core/valence correlation, scalar relativistic, atomic spin−orbit, and higher-order correlation effects. Zero-point energies were based on an average of the vibrational energies obtained from the experimental fundamentals and theoretical harmonic frequencies. Using restricted open shell treatments for the atoms, we find the following heats of formation (kcal/mol) at 0 K: ΔHf(allene) = 48.1 ± 0.5 (calcd) vs 47.4 ± 0.3 (expt); ΔHf(cyclopropene) = 70.5 ± 0.5 (calcd) vs 68.3 ± 0.6 (expt); ΔHf(propyne) = 46.5 ± 1.5 (calcd) vs 46.0 ± 0.2 (expt); ΔHf(cyclopropane) = 17.4 ± 1.5 (calcd) vs 16.8 ± 0.1 (expt); ΔHf(propane) = −20.0 ± 1.6 (calcd) vs. −19.6 ± 0.1 (expt); ΔHf(propene) = 8.2 ± 1.5 (calcd) vs 8.4 ± 0.2 (expt); ΔHf(allyl) = 42.7 ± 1.5 (calcd) vs 42.7 ± 0.5 (expt); and ΔHf(benzene) = 24.7 ± 3 (calcd) vs 24.0 ± 0.2 (expt).