Gas-phase thermochemistry of polycyclic aromatic hydrocarbons: An approach integrating the quantum chemistry composite scheme and reaction generator

We introduce a protocol aimed at predicting the accurate gas-phase enthalpies of formation of polycyclic aromatic hydrocarbons (PAHs). Automatic generation of a dataset of equilibrated chemical reactions preserving the number of carbon atoms in each hybridization state on each side of equations is at the core of our scheme. The performed tests suggest the recommended enthalpy of formation to be derived via a two-step scheme. First, we consider the reactions with a minimal sum of the total number of particles involved, N, and the absolute difference between the total number of products and reactants, |ΔN|. Second, among these reactions, we identify the one with the smallest absolute reaction enthalpy change, ΔrH°m(g, 298.15 K). This approach has been applied to predict the gas-phase enthalpies of formation of 113 PAHs via the Feller-Peterson-Dixon approach. Our calculated ΔfH°m(g, 298.15 K) values provide the mean absolute deviations of 1.7, 1.9, 4.2, 8.1, and 18.5 kJ mol-1 with respect to the literature group-based error corrected (GBEC) G3MP2B3, ATOMIC (HC), group equivalent M06-2X, GBEC B3LYP, and G4MP2 values. Our predicted ΔfH°m(g, 298.15 K) values give the mean signed and mean absolute errors of -7.5 and 12.9 kJ mol-1 with respect to the experimental enthalpies of formation. The combination of our predicted and the experimental ΔfH°m(g, 298.15 K) values provide the solid-state enthalpies of formation, ΔfH°m(g, 298.15 K), which are not available for a few species. Approaching these values ΔfH°m(g, 298.15 K) as well as ΔfH°m(g, 298.15 K), producing large discrepancies from the experimental side, would be indispensable for testing and further tuning of computational chemistry approaches.