A Theoretical Study of the Kinetics of the Benzylperoxy Radical Isomerization

Abstract

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

A consistent chemical mechanism for oxidation of substituted aromatic species

“…Canneaux et al [35] computed the rate constant for the fourcentered isomerization of the benzylperoxy radical to form A 1 CHO 2 H (a hydroperoxide radical) using an elaborated CASPT2 method. They noted that this reaction is the rate determining step in the conversion of a benzylperoxy radical into benzaldehyde as this involves a strained four-centered cyclic transition state.…”

“…The rate constants for these reactions have been derived assuming that the benzylperoxy radical is in steady state. In this calculation, the forward and backward reaction rates of benzyl + O 2 forming a benzylperoxy radical, and the rate of phenoxy radical formation from the benzylperoxy radical are derived from Murakami et al [34]; the reaction rate for the formation of benzaldehyde from the benzylperoxy radical is obtained from Canneaux et al [35]. The newly derived rate constants are given in Table 2.…”

The Benzylperoxyl Radical as a Source of Hydroxyl and Phenyl Radicals

“…20 kcal mol À1 ), [14] and thus reversible at higher temperatures. Theoretical studies have focused on some of the reactive intermediates, such as 2, [15][16][17] the benzoxyl radical, [18] and the autoxidation of toluene. [19] The combustion and tropospheric oxidation of toluene are highly complex processes involving a very large number of intermediates.…”

“…20 kcal mol −1 ),14 and thus reversible at higher temperatures. Theoretical studies have focused on some of the reactive intermediates, such as 2 ,15–17 the benzoxyl radical,18 and the autoxidation of toluene 19…”

Density functional study of the phenylethyl + O2 reaction: Kinetic analysis for the low‐temperature autoignition of ethylbenzenes

“…1), the reaction schemes for these two isomers were investigated in this study. As there have been no previous investigations on the isomerization reaction pathways for the phenylethylperoxy radicals, we have inferred the isomerization reaction pathways from our previous investigations on the isomerization reactions of benzylperoxy radicals [16] as well as the previous works carried out by Canneaux et al [17,18]. In addition to these isomerization reaction pathways, we have additionally calculated the HO 2 elimination reaction channels (E, Q) and also the H-atom migration reactions within the aromatic side chain (D, O), which were introduced by the presence of the longer aromatic side chains in the phenylethylperoxy radicals.…”