Intramolecular effects on the kinetics of unimolecular reactions of β-HOROO˙ and HOQ˙OOH radicals

Abstract

Intramolecular effects involved in the unimolecular decomposition of oxygenated radicals formed during the low temperature combustion of alcohols and alkenes were investigated and kinetic correlations were proposed.

Abstract

A theoretical study describing the influence of intramolecular effects on the energy barriers and rate constants of unimolecular reactions involving β-HOROO˙ and HOQ˙OOH radicals is proposed. The reactions considered are HO ₂˙ elimination, the Waddington mechanism, H-shift, cyclic ether formation and β-scission. All the calculations are performed at the CBS-QB3 level of theory along with canonical transition state theory and statistical thermodynamics, including a specific treatment of hindered rotors. Several structural parameters are investigated, such as the location of the hydroxyl function in the cyclic transition states or the substitution of H atoms by alkyl groups on carbon atoms involved in the reaction coordinate. It is shown that these molecular systems involve numerous transition states, especially for reactions such as 1,5 or 1,6 H-shift, and that, a priori simplification is not possible. It is also shown that the position of the –OH group in the transition state can largely modify both the barrier heights and the rate constants. However, opposite trends can be observed depending on the competition between energetic and entropic effects. Similar observations are made when H atoms are replaced by methyl or alkyl groups. These results can largely be explained by intramolecular effects such as hydrogen bonds, stabilization effects (from –OH or –CH ₃ groups), steric influences and by the coupling between them. The last point renders the classic establishment of the structure–reactivity relationship challenging.