Prediction of hydrocarbon enthalpies of formation by various thermochemical schemes

The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔH(f)'s) for 60 closed-shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000, 113, 9377). This set of thermodynamic values includes ΔH(f)'s for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCA's applicability to a variety of hydrocarbons. The ΔH(f)'s were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring-conserved isodesmic reactions were used to calculate the ΔH(f)'s. Using an atomization energy approach to determine the ΔH(f)'s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol(-1). The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA-P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol(-1). This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol(-1) obtained via the Gaussian-4 (G4), Gaussian-3 (G3), and Gaussian-3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions).