OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4‑Chlorobenzotrifluoride (<i>p</i>‑ClC₆H₄CF₃)

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride (p-chlorobenzotrifluoride, p-ClC6H4CF3, PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k1, for the OH + PCBTF reaction were measured over the temperature ranges 275–340 and 385–940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k1(298–353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k1(T) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k1(275–385 K) = (1.50 ± 0.15) × 10–14 exp((705 ± 30)/T) cm3 molecule–1 s–1, where k1(298 K) = (1.63 ± 0.03) × 10–13 cm3 molecule–1 s–1 and a positive temperature dependence at elevated temperatures given by k1(470–950 K) = (5.42 ± 0.40) × 10–12 exp(−(2507 ± 45) /T) cm3 molecule–1 s–1. The present k1(298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k2, was also measured in THALAMOS using a relative rate technique that yielded k2(298 K) = (7.8 ± 2) × 10–16 cm3 molecule–1 s–1. As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3–4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH–PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available.