Atmospheric Oxidation Mechanism of p-Xylene: A Density Functional Theory Study Jiwen Fan and Renyi Zhang* Department of Atmospheric Sciences, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed: March 20, 2006; In Final Form: April 19, 2006 We report an investigation of the mechanistic features of OH-initiated oxidation reactions of p-xylene using density function theory (DFT). Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the p-xylene oxidation. OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.8 and 0.2 for ortho and ipso additions, respectively, and the calculated overall rate constant is in agreement with available experimental studies. Under atmospheric conditions, the p-xylene peroxy radicals arising from initial OH and subsequent O 2 additions to the ring are shown to cyclize to form bicyclic radicals, rather than to react with NO to lead to ozone formation. With relatively low barriers, isomerization of the p-xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O 2 addition to form bicyclic peroxy radicals. The study provides thermochemical and kinetic data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from p-xylene oxidation. 1. Introduction Aromatic compounds constitute 30-40% of hydrocarbon mass emitted into urban atmospheres and represent a significant source of urban ozone, photochemical smog and secondary organic aerosol (SOA) formation. 1,2 The SOA impacts human health and the climate. 3 Also, the likely formation of toxic epoxide products from aromatic oxidation is of considerable concern. 4 The aromatics are emitted into the atmosphere via automobile emissions and solvent use. 2 Although toluene is usually the most prominent aromatic compounds, the combined xylene isomers have contributed similar mass in most studies. 5-7 Xylene is one of the top 30 chemicals produced in the United States in terms of volume. Levels of xylenes measured in the air of industrial areas and cities of the United States range from 1 to 88 parts of xylenes per billion parts of air. 8 The reaction of aromatic hydrocarbons with hydroxyl radicals (OH) represents the major atmospheric loss process during daylight hours. Hydroxyl radicals react with aromatic com- pounds by abstracting hydrogen atoms from the alkyl group or by adding to the aromatic ring. 1,9 H atom abstraction is relatively minor for xylene isomers (less than 10%). 1,10,11 The addition of OH to the xylene ring forms OH-xylene adducts. Under atmos- pheric conditions, O 2 is expected to rapidly add to the OH- xylene adduct, forming primary peroxy radicals. 12,13 The fate of the xylene peroxy radicals is governed by competition be- tween reaction with NO to form alkoxy radicals and cyclization to form bicyclic radicals. For p-xylene-OH-O 2 peroxy radicals, the cyclization reaction is expected to be more important than reaction with NO at typical atmospheric levels, on the basis of a theoretical study. 13 The large yields of hexenedione, glyoxal, and methylglyoxal detected in the work by Smith et al. also support the preference of cyclization reaction to form bicyclic radicals. 1 Volkamer et al. also identified cyclization as the major pathway for the oxidation of p-xylene. 14 The bicyclic radicals then undergo unimolecular rearrangement to form epoxide radicals or bimolecular reaction with O 2 to form (secondary) bicyclic peroxy radicals. The mechanistic complexity of the p-xylene oxidation further arises from multiple isomeric path- ways at each reaction stage. Initial OH addition to p-xylene results in two distinct structural p-xylene-OH adduct isomers (i.e., ortho and ispo). Subsequent reactions of the p-xylene- OH adducts with O 2 and cyclization of the peroxy radicals also incur additional isomeric branching. Scheme 1 illustrates the likely pathways of p-xylene oxidation initiated by OH. Experimental studies have investigated the temperature- and pressure-dependent rate constant of the initial p-xylene-OH reaction. 11,12,15,16 The room-temperature rate constant for p- xylene reported by Bandow and Washida is 1.4 × 10 -11 cm 3 molecule -1 s -1 with an uncertainty factor about (2 at 300 K and 1 atm. 10,15 There has been considerable experimental work on the products from the reactions of OH with p-xylene. Environmental chamber studies have identified several major products consisting of both ring-retaining and fragmentation compounds. 1,2,4,14-18 The experimental work carried out by Volkamer et al. reported a significant yield (about 40 ( 10%) of glyoxal and indicated that the ring-cleavage pathway involv- ing the bicyclic peroxy radical represents the major pathway for the oxidation of p-xylene initiated by OH. 14 Experimental studies detected the large quantities of E- and Z-isomers of hex- 3-ene-2,5-dione, qualitatively consistent with the work of Smith et al., in which both isomers were identified as primary products, with the yields of 18% for Z-isomer and 5% for E-isomer. 1,2 Smith et al. also identified another unsaturated dicarbonyl product from the ring fragmentation of p-xylene, i.e., 2-methyl- 2-butenedial, with a yield of 7%. 1 At present, however, the detailed mechanism of p-xylene oxidation following the initial OH attack remain highly uncer- tain. Most of the aromatic intermediate radicals have not yet been detected directly in the gas phase. In previous experimental product studies of xylenes, typically less than 65% of the reacted carbon has been identified as products. 1,14-20 Interpretation of the identified reaction products is hindered because of the existence of multiple reaction pathways and steps. On the other * Corresponding author. E-mail: zhang@ariel.met.tamu.edu. Tel: 979- 845-7656. Fax: 979-862-4466. 7728 J. Phys. Chem. A 2006, 110, 7728-7737 10.1021/jp061735e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006