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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 (ΔHf,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 ΔHf,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 ΔHf,298values should be revised by significant amounts ranging from 6.5 and 24.4 kJ mol-1
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