Formation of Peroxy Radicals from OH−Toluene Adducts and O₂

Abstract

The gas-phase reaction of OH radicals with toluene and toluene-d 8 and reactions of the resulting OH adducts with O 2 were studied in N 2 /O 2 mixtures at atmospheric pressure and room temperature. OH production was performed by pulsed 248 nm photolysis of H 2 O 2 . Cw UV-laser long-path absorption at 308 nm was used for time-resolved detection of OH and OH-toluene adducts. The reaction OH + toluene was studied in N 2 and O 2 and similar rate constants of (5.70 ( 0.19) × 10 -12 cm 3 s -1 (N 2 ) and (5.60 ( 0.14) × 10 -12 cm 3 s -1 (O 2 ) were obtained at 100 kPa. For toluene-d 8 the corresponding rate constants are (5.34 ( 0.34) × 10 -12 cm 3 s -1 (N 2 ) and (5.47 ( 0.14) × 10 -12 cm 3 s -1 (O 2 ). Absorption cross-sections of (1.1 ( 0.2) × 10 -17 cm 2 were determined for both the OH-toluene and OH-toluene-d 8 adduct at 308 nm. Rate constants of (4.7 ( 1.4) × 10 -11 cm 3 s -1 and (1.8 ( 0.5) × 10 -10 cm 3 s -1 were determined for the OH-toluene adduct self-reaction and the OH-toluene adduct + HO 2 reaction, respectively. Upper limits e8 × 10 -15 cm 3 s -1 were estimated for any reactions of the OH-toluene adduct with H 2 O 2 or toluene. Adduct kinetics in the presence of O 2 is consistent with reversible formation of peroxy radicals with an estimated rate constant of (3 ( 2) × 10 -15 cm 3 s -1 and an equilibrium constant of (3.25 ( 0.33) × 10 -19 cm 3 (toluene-d 8 : (3.1 ( 1.2) × 10 -19 cm 3 ). The effective adduct loss from the equilibrium can be explained by (i) an additional, irreversible reaction of the adduct with O 2 with a rate constant of (6.0 ( 0.5) × 10 -16 cm 3 s -1 (toluene-d 8 : (4.7 ( 1.2) × 10 -16 cm 3 s -1 ), or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (1.85 ( 0.15) × 10 3 s -1 (toluene-d 8 : (1.5 ( 0.4) × 10 3 s -1 ). Consequences for the atmospheric degradation of toluene will be discussed.

Consecutive reactions of aromatic-OH adducts with NO, NO<sub>2</sub> and O<sub>2</sub>: benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline

“…In that experiment, however, an underlying background absorption with unknown formation kinetics has been subtracted, influencing the simultaneous determination of k 6 and the equilibrium constant (7/-7). The result obtained by Bohn (2001) for toluene, (6.0±0.5)×10 −16 cm 3 s −1 , is in excellent agreement with this work, that is further confirmed by Johnson et al (2005).…”

“…Although k −7 is somewhat uncertain, it is much faster than k −1 so that the steady-state approximation can be used to derive what is already obvious from Fig. 4 of : The saturation effect of the equilibrium on the effective rate constant k 6 is negligible under our conditions (about 1% for benzene at 299 K, even less at higher temperatures, similar for toluene, where, according to Bohn (2001), K eq is only slightly larger). For the other aromatics of this study, the peroxy radical may be more stable, but also less O 2 was needed in the experiments.…”

“…These aspects have been studied by laser photolysis/laser absorption and laser photolysis/UV absorption by Zellner et al (1985), , Bohn (2001), Johnson et al (2002Johnson et al ( , 2005, Raoult et al (2004) and Grebenkin and Krasnoperov (2004); by discharge-flow/laser induced fluorescence (Goumri et al, 1990); in flash photolysis/resonance fluorescence studies by Perry et al (1977), and Knispel et al (1990) for benzene, toluene and phenol; and in product studies by , Atkinson et al (1991), Atkinson and Aschmann (1994), Klotz et al (1998), Smith et al (1998, Bethel et al (2000), Berndt and Böge (2001a, 2001b, 2003 and Volkamer et al (2002a,b), Klotz et al (2002), Olariu et al (2002), Zhao et al (2005) and Coeur-Tourneur et al (2006) for benzene, toluene, o-, m-and p-xylene, 1,2,3-, 1,2,4-and 1,3,5-trimethylbenzene, HMB, phenol and o-, m-and p-cresol. In the following, we present a kinetic study of the consecutive reactions of aromatic-OH adducts in the presence of variable amounts of either NO, NO 2 or O 2 . Most of the results on benzene, toluene, p-xylene, phenol and m-cresol were obtained using the VUV flash photolysis/resonance fluorescence (FP/RF) technique, introduced by Stuhl and Niki (1972).…”

Characteristics of carbonyl compounds in public vehicles of Beijing city: Concentrations, sources, and personal exposures

“…The evident leakage of the exhaust may be the main reason for the higher levels of benzaldehyde in TA cars. In addition, photochemical reactions of aromatic compounds might make a minor contribution to benzaldehyde, such as the reactions of toluene with OH and styrene with OH or O 3 (Atkinson et al, 1982;Bohn, 2001). Acetone is widely used as chemical solvent in various industries such as paint and perfume.…”

Predicting arene rate coefficients with respect to hydroxyl and other free radicals in the gas-phase: a simple and effective method using a single topological descriptor

“…Ohta and Ohyama (1985) q. Calvert et al (2002) b. Bohn (2001 j. Atkinson (1986) r. Atkinson et al (1984) c. Atkinson and Aschmann (1989) k. Corchnoy and Atkinson (1990) s. Bolzacchini et al (1999) d. Atkinson and Aschmann (1988) l. Baulch et al (1994) t. Triebert et al (1998) e. Bignozzi et al (1981 m. Frerichs et al (1989) u.…”