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

Abstract

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.

Hydrogen shift isomerizations in the kinetics of the second oxidation mechanism of alkane combustion. Reactions of the hydroperoxypentylperoxy OOQOOH radical

“…Figure 10 shows the final high-pressure-limit MP-CVT/SCT rate constants in the temperature range 298−1500 K and compares them to previous predictions. There are three noteworthy aspects: (i) R1 and R2 both have lower rate constants than other empirical results [20,25] or theoretical studies [89]. In the temperature range 500−1000 K, the calculated rate constants by Mohamed et al [89] using 1D hindered rotor treatment and classical transition state theory for R2 are a factor of 8.0−9.6 faster than our computed data.…”

“…And R1 begins to dominate with the increasing temperature, which will facilitate the extensive auto-oxidation mechanism. Comparisons between our calculated rate constants with others [20,25,89] for R1 and R2 at 298−1500 K. MP denotes MP-VTST/SCT. Figure 11 shows the results of using the SS-QRRK theory to predict the falloff effects in the temperature range 298−1500 K. For 298−700 K, there is negligible pressure dependence for either reaction.…”

Estimation of rate coefficients and branching ratios for reactions of organic peroxy radicals for use in automated mechanism construction

“…Vereecken and Peeters, 2004;Peeters et al, 2009;Crounse et al, 2013;Ehn et al, 2014Ehn et al, , 2017Jokinen et al, 2014;Rissanen et al, 2015;Jørgensen et al, 2016;Praske et al, 2017Praske et al, , 2019Otkjaer et al, 2018;Mohammed et al, 2018), and new information is constantly emerging on this important aspect of peroxy radical chemistry (e.g. Bianchi et al, 2019;Xu et al, 2019;Møller et al, 2019). The present section provides a summary of selected classes of isomerization reactions that are currently being considered and represented in ongoing mechanism development work.…”

Intramolecular Hydrogen Shift Chemistry of Hydroperoxy-Substituted Peroxy Radicals

“…Intramolecular H-shifts in these radicals have been the subject of previous studies, but only at elevated temperature. [36][37][38][39][40][41][42][43] In this study, we oxidize 2-hydroperoxy-2-methylpentane with OH under conditions relevant to the atmosphere. As shown in Scheme 1, the RO 2 formed following hydrogen abstraction and O 2 addition lack an abstractable α-OOH hydrogen due to the presence of the methyl substituent.…”