Theoretical study of the oxidation mechanisms of naphthalene initiated by hydroxyl radicals: the O2 addition reaction pathways

Abstract

Atmospheric oxidation of the naphthalene–OH adduct [C₁₀H₈OH]˙ (R1) by molecular oxygen in its triplet electronic ground state has been studied using density functional theory along with the B3LYP, ωB97XD, UM05-2x and UM06-2x exchange–correlation functionals. From a thermodynamic viewpoint, the most favourable process is O₂ addition at the C₂ position in syn mode, followed by O₂ addition at the C₂ position in anti mode, O₂ addition at the C₄ position in syn mode, and O₂ addition at the C₄ position in anti mode, as the second, third and fourth most favourable processes. The syn modes of addition at these positions are thermodynamically favoured over the anti ones by the formation of an intramolecular hydrogen bond between the hydroxyl and peroxy substituents. Analysis of the computed structures, bond orders and free energy profiles demonstrate that the reaction steps involved in the oxidation of the naphthalene–OH adduct by O₂ satisfy Hammond's principle. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been supplied, using transition state and RRKM theories. By comparison with experiment, these data confirm the relevance of a two-step reaction mechanism. Whatever the addition mode, O₂ addition in C₄ position is kinetically favoured over O₂ addition in C₂ position, in contrast with the expectations drawn from thermodynamics and reaction energies. Under a kinetic control of the reaction, and in line with the computed reaction energy barriers, the most efficient process is O₂ addition at the C₄ position in syn mode, followed by O₂ addition at the C₂ position in syn mode, O₂ addition at the C₄ position in anti mode, and O₂ addition at the C₂ position in anti mode as the second, third and fourth most rapid processes. The computed branching ratios also indicate that the regioselectivity of the reaction decreases with increasing temperatures and decreasing pressures.