OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4‑Chlorobenzotrifluoride (p‑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.

取代芳烃与羟基自由基（OH radical）、氯原子（Cl atom）的气相反应动力学机制，仍是大气与燃烧化学领域的研究热点。4-氯三氟甲苯（对氯三氟甲苯，p-ClC6H4CF3，缩写为PCBTF）是溶剂型涂料中常用的取代芳烃类挥发性有机化合物（VOC），因此PCBTF被归类为挥发性化学产品（VCP），其排放进入大气后可能对空气质量造成影响。

本研究依托法国奥尔良ICARE/法国国家科学研究中心（CNRS）实验室，采用低压放电流管反应器耦合质谱检测器，在275~340 K与385~940 K两个温度区间内，测定了羟基自由基与PCBTF反应的速率常数k1；同时采用相对速率法，在法国杜埃的温控大气模拟舱（THALAMOS）中，测定了298~353 K区间内的k1值。

速率常数k1(T)表现出非阿累尼乌斯（non-Arrhenius）温度依赖性：在275~385 K区间内呈现负温度相关性，拟合表达式为k1(275~385 K) = (1.50 ± 0.15) × 10^-14 exp((705 ± 30)/T) cm³·分子^-1·s^-1，其中298 K下的k1值为(1.63 ± 0.03) × 10^-13 cm³·分子^-1·s^-1；在470~950 K的高温区间则呈现正温度相关性，拟合表达式为k1(470~950 K) = (5.42 ± 0.40) × 10^-12 exp(-(2507 ± 45)/T) cm³·分子^-1·s^-1。本研究测得的298 K下k1值，与两项此前在296 K、760 Torr（合成空气为载气）条件下开展的相对速率测定结果具有合理的一致性。

本研究同时采用相对速率法，在THALAMOS模拟舱中测定了氯原子与PCBTF反应的速率常数k2，得到298 K下k2 = (7.8 ± 2) × 10^-16 cm³·分子^-1·s^-1。作为本研究的一部分，团队还在美国科罗拉多州博尔德的美国国家海洋和大气管理局（NOAA）实验室中，测定了PCBTF的紫外与红外吸收光谱。基于测得的紫外吸收光谱，假设光解量子产率为1，本研究估算得到PCBTF在中纬度夏季近地面大气中的瞬时紫外光解寿命为3~4天。

针对275~950 K区间内羟基自由基与PCBTF反应的非阿累尼乌斯行为，本研究通过分析OH-PCBTF加合物的形成过程及其热化学稳定性，对其反应机制进行了解释。本研究结果可为仅具备有限温度依赖动力学数据的卤代芳烃类化合物的羟基自由基及氯原子反应动力学研究提供参考。