Reaction mechanisms and kinetics of the isomerization processes of naphthalene peroxy radicals

The isomerization processes of naphthalene peroxy radicals [C10H8–OH] –O2 into bicyclic peroxy or oxy hydroperoxide radicals via ring closure and intramolecular hydrogen transfers have been studied computationally using density functional theory, along with various exchange–correlation functionals and an extremely large basis set. The calculated energy profiles have been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The cyclization of the R1-2OO-syn peroxy radical into the R1-2,9OO-syn bicyclic peroxy radical through formation of an O–O bridge is endothermic and reversible. Both from a thermodynamic and kinetic view points, the two most favorable processes for the R1-2OO-syn peroxy radical are ring closure into the R1-2,9OO-syn bicyclic peroxy radical species, and conversion through hydrogen transfer into the R1-P2O1-syn oxy hydroperoxide radical. Among all studied reaction channels, the latter process is the kinetically most competitive one. Also, in view of the computed rate constants, the R1-2OO-syn peroxy radical appears to be chemically much more reactive than the R1-4OO-syn species. All in all, the atmospheric oxidation mechanisms of naphthalene appear at this reaction stage to be quite different from that of benzene and its derivatives.