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

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 HO2˙ 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 -CH3 groups), steric influences and by the coupling between them. The last point renders the classic establishment of the structure-reactivity relationship challenging.

Revisiting 1-hexene low-temperature oxidation

“…In this study, the rate constants for the Waddington mechanism are adopted from the recent theoretical work of Lizardo-Huerta et al [14]. Figure 3 displays a comparison between the rate constants of Bounaceur et al [8] and that of Lizardo-Huerta et al [14] for reaction (1). This figure also shows that the change of these rate constants has a slight effect on reactivity, but a more significant one on aldehyde formation.…”

“…The kinetics of several reactions important for alkene lowtemperature oxidation (e.g. reactions of alkenes with OH radicals [13] or reactions of peroxyradicals including a hydroxyl function [14]) was recently investigated using theoretical calculations. However, the last detailed kinetic models of hexene oxidation were published before 2011 by Mehl et al [7,9] and by Bounaceur et al [8], except in the case of 3-hexene [11].…”

Combustion and Pyrolysis Kinetics of Chloropicrin

“…This methodology has proven its precision with molecules containing hetero-atoms 15,16,24 . In addition, the low frequency vibration modes, corresponding to internal rotations, were treated as hindered rotors instead of harmonic oscillators, using THERMROT 25 . This in-house code allows a one-dimensional hindered rotor (1D-HR) treatment of internal rotations and avoids the coupling of torsional modes encountered in the frequency analysis made by Gassian09, using the method proposed by Vansteenkiste et al 26 another one dedicated to chlorinated species, and a mechanism specific to the decomposition and oxidation of chloropicrin and derived products.…”

Thermal cracking of n-butylbenzene at high pressure: Experimental study and kinetic modelling

“…All of the thermokinetic parameters have been calculated using the THERMROT software [32]. The first is a detailed primary mechanism of n-butylbenzene.…”