Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms and OH Radicals with Fluorobenzene at 296 K

Abstract

Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms and OH radicals with fluorobenzene, C6H5F, in 700 Torr of N2 or air diluent at 296 K. Reaction of Cl atoms with C6H5F proceeds via two pathways: H-atom abstraction to give HCl and the C6H4F radical and adduct formation to give the C6H5F−Cl adduct. At 296 K the rate constant for the abstraction channel is k 5a(Cl + C6H5F) = (1.1 ± 0.1) × 10-17 cm3 molecule-1 s-1. The C6H5F−Cl adduct undergoes rapid (k ∼ 108 s-1) decomposition to reform C6H5F and Cl atoms and reaction with Cl atoms via a mechanism which, at least in part, leads neither to production of C6H5Cl nor to reformation of C6H5F. As the steady-state Cl atom concentration is increased, the fraction of the C6H5F−Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for the reaction of C6H5F with Cl atoms. The equilibrium between Cl atoms, C6H5F, and the C6H5F−Cl adduct is established rapidly and has an equilibrium constant estimated to be K 5b=[C6H5F−Cl]/[C6H5F][Cl] = (3.2 ± 2.4) × 10-18 cm3 molecule-1. An upper limit of k 9 < 6 × 10-17 cm3 molecule-1 s-1 was established for the reaction of the C6H5F−Cl adduct with O2. The reaction of OH radicals with C6H5F was studied and a rate constant of k(OH + C6H5F) = (7.9 ± 2.2) × 10-13 cm3 molecule-1 s-1 was determined. The results are discussed with respect to the available literature concerning reaction of Cl atoms and OH radicals with aromatic compounds. As part of this work, rate constants for reaction of OH radicals with 2-, 3-, and 4-fluorophenol of (6.3 ± 1.3) × 10-12, (2.3 ± 0.5) × 10-11, and (2.5 ± 0.5) × 10-11 cm3 molecule-1 s-1 were determined.

Theoretical study on the mechanism and kinetics of addition of hydroxyl radicals to fluorobenzene

“…Errors in modeling kinetic parameters are attributed primarily to the possible errors in calculated frequencies due to the harmonic oscillator approximation and to the overestimated G3 reaction barriers or/and underestimated prereaction complex well depth. Furthermore, the yield of 4-fluorophenol was 10 6 3% following the reaction of OH radicals and fluorobenzene, [13] whereas the yield for 2-fluoro or 3-fluorophenol was below the detection limits (>3%). G3 calculations predict that atmospheric oxidation of fluorobenzene is channeled into the ortho and para reaction channels, with significant contribution of meta channel, which is in accordance with experiment and can be rationalized within the framework of electrophilic aromatic substitutions guided by halogen substituent.…”

“…Furthermore, to computationally confirm that the reverse (adduct to prereaction complex) reaction is a negligible process under environmental conditions (230-310 K) [13] , we have calculated the forward and reverse reaction rate constants at room temperature (300 K), and those results are given in Table S13 (Supporting Information). Gibbs free energy values are given in parentheses.…”

“…[1] The main atmospheric degradation pathway for fluorobenzene is initiated by hydroxyl (OH) radicals, because these atmospheric species have a highenough oxidation potential for reaction with aromatic system. [7][8][9]12] The more recent measurements [13] suggest somewhat higher reaction rate for fluorobenzene reaction with OH radical at ambient temperatures, that is, 7.9 6 2.2 Â 10 À13 cm 3 molecule À1 s À1 . In the temperature region that is of environmental concern, 230-310 K, the reaction rate constant for fluorobenzene seems to be temperature independent (6.9 6 1.38 Â 10 À13 cm 3 molecule À1 s À1 ), [8] and similar behavior was found for many other aromatic hydrocarbons.…”

“…[2][3][4][5][6][7][8] Available kinetic data [7][8][9][10][11] show two distinctive temperature regimes for this reaction. All those rate constants have been determined under the high-pressure limit regime and are independent of total pressure or the nature of diluent gas in this pressure range, [1,[7][8][9][10][11][12][13] that is, the pressure range that is relevant for tropospheric reactions. [7][8][9]12] The more recent measurements [13] suggest somewhat higher reaction rate for fluorobenzene reaction with OH radical at ambient temperatures, that is, 7.9 6 2.2 Â 10 À13 cm 3 molecule À1 s À1 .…”

The ν19a band of fluorobenzene

“…The fluorobenzene molecule has received considerable attention from quite different fields of research, ranging from its interest in cation-p interactions in biological molecules [1], as environmentally relevant species among other aromatic molecules [2], or in the study of van der Waals systems, formed by aromatic molecules linked to a rare gas atom [3][4][5] or to other species like methanol, N 2 or water [6][7][8]. On the other hand, the vibrotational spectroscopy of this molecule has been scarcely studied, mainly because the large moment of inertia of the molecule induces a crowded rovibrational spectrum, and makes the analysis complicated.…”