Atmospheric Oxidation Mechanism of Toluene

Abstract

The atmospheric oxidation mechanism of toluene initiated by OH radical addition is investigated by quantum chemistry calculations at M06-2X, G3MP2-RAD, and ROCBS-QB3 levels and by kinetics calculation by using transition state theory and unimolecular reaction theory coupled with master equation (RRKM-ME). The predicted branching ratios are 0.15, 0.59, 0.05, and 0.14 for OH additions to ipso, ortho, meta, and para positions (forming R1-R4 adducts), respectively. The fate of R2, R4, and R1 is investigated in detail. In the atmosphere, R2 reacts with O2 either by irreversible H-abstraction to form o-cresol (36%), or by reversible recombination to R2-1OO-syn and R2-3OO-syn, which subsequently cyclize to bicyclic radical R2-13OO-syn (64%). Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol and 39% for R4-35OO-syn, while reaction of R1 and O2 leads to R1-26OO-syn. RRKM-ME calculations show that the reactions of R2/R4 with O2 have reached their high-pressure limits at 760 Torr and the formation of R2-16O-3O-s is only important at low pressure, i.e., 5.4% at 100 Torr. The bicyclic radicals (R2-13OO-syn, R4-35OO-syn, and R1-26OO-syn) will recombine with O2 to produce bicyclic alkoxy radicals after reacting with NO. The bicyclic alkoxy radicals would break the ring to form products methylglyoxal/glyoxal (MGLY/GLY) and their corresponding coproducts butenedial/methyl-substituted butenedial as proposed in earlier studies. However, a new reaction pathway is found for the bicyclic alkoxy radicals, leading to products MGLY/GLY and 2,3-epoxybutandial/2-methyl-2,3-epoxybutandial. A new mechanism is proposed for the atmospheric oxidation mechanism of toluene based on current theoretical and previous theoretical and experimental results. The new mechanism predicts much lower yield of GLY and much higher yield of butenedial than other atmospheric models and recent experimental measurements. The new mechanism calls for detection of proposed products 2,3-epoxybutandial and 2-methyl-2,3-epoxybutandial.

Reassessing the atmospheric oxidation mechanism of toluene

“…The bicyclic radicals then undergo unimolecular rearrangement, followed by H abstraction, to form epoxide intermediates or react with O 2 to form secondary RO 2 , followed by ring cleavage to produce small α-carbonyl compounds such as glyoxal and methylglyoxal (16,21). The toluene−oxide/ methyloxepin channel remains speculative, and several previous quantum chemical calculations have shown a high barrier for this pathway (22)(23)(24)(25)(26). Numerous experimental studies have been performed to investigate the products from the OH−toluene reactions (2-5, 18, 27).…”

The Atmospheric Oxidation Mechanism of Benzyl Alcohol Initiated by OH Radicals: The Addition Channels

“…[8][9][10][11] For GLY formation, Harrison and Wells [3] proposed a mechanism involving an initial OH addition at the para-position, followed by reaction steps similar to those of benzene oxidation. However, our recent studies [12][13][14] found that this traditionally proposed mechanism might not represent the main reaction pathway, and suggested additional pathways for GLY formation in benzene oxidation. Our studies also revealed significant differences between the oxidation mechanisms of benzene, alkyl benzenes, [15] phenol, [16] and naphthalenes.…”

“…Nonetheless, the branching ratios for the ipso and ortho additions amongst the addition channels are 80 % and 20 %, respectively, using ROCBS-QB3 energies, and 81 % and 19 % for both the UCBS-QB3 and M06-2X energies. The dominance of OH addition to the ipso and ortho sites of BA differs from that of toluene or other alkyl benzenes, for which ortho addition dominates, followed by para and meta additions, according to theoretical [13,34,36,37] and experimental [38][39][40] studies. Note that the experimental studies propose negligible ipso addition of toluene.…”

“…This is similar to other substituted benzenes. [13,14,41] If one assumes a steady-state, the effective bimolecular rate constant for R1-26OO-s formation from R1 and O 2 via R1-2OO-s can be estimated as follows [Eq. (4)]:…”

Theoretical Determination of Enthalpies of Formation of Benzyloxy, Benzylperoxy, Hydroxyphenyl Radicals and Related Species in the Reaction of Toluene with the Hydroxyl Radical

“…As seen in Table 2, MP2 again fails badly for reaction (8). However, it gives a surprisingly good enthalpy of formation from reaction (7), undoubtedly a fortuitous result (the same was observed for the phenyl radical).…”