High-Pressure Limit Rate Rules for α‑H Isomerization of Hydroperoxyalkylperoxy Radicals

Hydroperoxyalkylperoxy
(OOQOOH) radical isomerization is an important
low-temperature chain branching reaction within the mechanism of hydrocarbon
oxidation. This isomerization may proceed via the migration of the
α-hydrogen to the hydroperoxide group. In this work, a combination
of high level composite methodsCBS-QB3, G3, and G4is
used to determine the high-pressure-limit rate parameters for the
title reaction. Rate rules for H-migration reactions proceeding through
5-, 6-, 7-, and 8-membered ring transitions states are determined.
Migrations from primary, secondary and tertiary carbon sites to the
peroxy group are considered. Chirality is also investigated by considering
two diastereomers for reactants and transition states with two chiral
centers. This is important since chirality may influence the energy
barrier of the reaction as well as the rotational energy barriers
of hindered rotors in chemical species and transition states. The
effect of chirality and hydrogen bonding interactions in the investigated
energies and rate constants is studied. The results show that while
the energy difference between two diastereomers ranges from 0.1–3.2
kcal/mol, chirality hardly affects the kinetics, except at low temperatures
(atmospheric conditions) or when two chiral centers are present in
the reactant. Regarding the effect of the H-migration ring size,
it is found that in most cases, the 1,5 and 1,6 H-migration reactions
have similar rates at low temperatures (below ∼830 K) since
the 1,6 H-migration proceeds via a cyclohexane-like transition state
similar to that of the 1,5 H-migration.