Accurate Heats of Formation for Polycyclic Aromatic Hydrocarbons: A High-Level Ab Initio Perspective

Polycyclic aromatic hydrocarbons (PAHs) are key reference materials for the validation and parameterization of computationally cost-effective procedures such as density functional theory (DFT), semiempirical molecular orbital theory, and molecular mechanics. We obtain accurate heats of formation (ΔH_f,298) for 20 PAHs with up to 18 carbon atoms by means of the explicitly correlated W1-F12 thermochemical procedure. The heats of formation are obtained via atomization reactions and quasi-isodesmic reactions involving CH₄, C₂H₄, and C₆H₆for which accurate experimental ΔH_f,298values are available from the active thermochemical tables (ATcT) network. We show that for large PAHs, the differences between W1-F12 heats of formation obtained from atomization and quasi-isodesmic reactions increase with the size of the system and range between 1.7 (C₇H₈) and 8.9 (chrysene, C₁₈H₁₂) kJ mol^-1. This suggests that atomization reactions should be used with caution for obtaining heats of formation for medium-sized systems even when highly accurate thermochemical procedures (such as W1-F12 theory) are used. For eight PAH compounds (toluene, indene, acenaphthylene, biphenyl, diphenylmethane, anthracene, pyrene, and chrysene), our best theoretical values agree with the best experimental values to within ∼1 kJ mol^-1; for six additional systems (indane, naphthalene, biphenylene, acenaphthene, phenanthrene, andm-terphenyl), the agreement between theory and experiment is good with deviations ranging between 2 and 4 kJ mol^-1. However, for four systems (p-terphenyl, fluorene, pyracene, and pyracyclene), our best W1-F12 values suggest that the experimental ΔH_f,298values should be revised by significant amounts ranging from 6.5 and 24.4 kJ mol^-1