New Channels in the Reaction Mechanism of the Atmospheric Oxidation of Toluene

Abstract

Two different theoretical approaches are used to study the OH radical attack on toluene: the Møller-Plesset perturbation theory and the B3LYP density functional method. The critical points of the potential energy surface for the OH addition to toluene are determined, and rate-equilibrium relationships are discussed. A stable structure corresponding to a prereactive complex which is formed when the OH radical is at about 2.5 Å from toluene is obtained. The existence of this loosely bound system is necessary to explain the experimentally observed negative activation energy. The geometry of transition states and products are determined for addition at different positions in the ring, including the ipso position, which has not been considered in previous works. Energy results at the MP4 and coupled cluster levels calculated at the optimized MP2 and B3LYP geometries confirm that the ipso adduct is more stable than the ortho adduct by about 0.5 kcal/mol. Several routes are proposed for the subsequent reactions of the ipso adduct, which could explain the very high yield of o-cresol with respect to the other cresol isomers.

Theoretical study of mechanism and kinetics for the addition of hydroxyl radical to phenol

“…The CBS-APNO geometries are similar to those obtained from the B3LYP method. The C-C bonding characteristics are also observed for the OH-benzene adducts [38], OH-toluene adducts [26], OH-p-xylene [40], and OH-m-xylene adducts [41]. As in Table 1, the relative energies ∆E 0 of four adducts obtained by CBS-APNO range from 77.99 to 55.27 kJ mol 1 .…”

“…As the reactants approach each other, the reactant complexes, RC1 and RC2, are observed using B3LYP/6-311++G(2df, 2pd) (Figure 1 [38] and OH-toluene [26]. The equilibrium geometry of RC2 was not reported by Lundqvist et al [17].…”

Radical–Molecule Complexes: Changing Our Perspective on the Molecular Mechanisms of Radical–Molecule Reactions and their Impact on Atmospheric Chemistry

“…A prereactive complex has been found to be important for a number of other reactions involving the hydroxyl radical. These include OH radical plus C 2 H 6 , [78] C 2 H 5 X (X Cl,F), [78] halogenated and substituted ethenes, [79,80] C 3 H 5 , [81] toluene, [82] xylenes, [83] acetone, [84] and, most recently, aldehydes. [85] Included in the article of AlvarezIdaboy, Mora-Diez, and Vivier-Bunge [80] is a table summarizing the measured pre-exponential A factor and activation barriers for a number of OH radical and O( 3 P) reactions.…”

Theoretical Investigation of the OH.‐Initiated Oxidation of Benzaldehyde in the Troposphere

“…Low barriers and rather large reaction energies have also been observed in the past for the ipso OH addition path in toluene and the xylenes. [33,34,35] However, the entropy factor compensates these low energies so that, in fact, the ipso path is not favoured.…”