A kinetic study of gas-phase reaction of thiophene with NO 3 by using absolute and relative methods B. Caba~ nas * , P. Mart  ın, S. Salgado, M.T. Baeza, M.R. L opez, E. Mart  ınez Facultad de Qu  ımicas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain Received 21 December 2001; in final form 6 March 2002 Abstract The gas-phase rate coefficient for the reaction of thiophene with nitrate radical, NO 3 , has been determined, using relative (gas chromatography as analytical tool) and absolute methods at 298 K. The absolute experiments were carried out using a fast-flow-discharge technique, with detection of the NO 3 radical by Laser Induced Fluorescence (LIF). The proposed rate coefficient at 298 K is, k ¼ð3:16  0:27Þ 10 14 cm 3 molecule 1 s 1 . The temperature dependence was studied by the absolute technique in the range 260–433 K, observing that in the interval of 260–277 K the rate coefficient decreases as the temperature increases and from 298 K onwards the rate coefficient increases with temperature. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Nitrate, (NO 3 ) and hydroxyl (OH) radicals to- gether with ozone (O 3 ) are important oxidant agents in the Earth’s atmosphere [1–6]. Their re- actions are the principal degradation routes for many organic compounds. The OH radical reac- tion is normally the main sink for numerous compounds during the daytime. However the NO 3 radical that undergoes photolysis by sunlight [7] is mainly involved in night-time atmospheric chem- istry. Many kinetic and mechanistic studies of the oxidation of organics by OH, O 3 and NO 3 have been reported [4–6,8–14]. The reactions of some heterocycles with OH and O 3 have been the subject of different studies [14–24]. However, the investi- gation about the reactivity of these compounds, with the NO 3 radical, is specially scarce, and more kinetic data for the NO 3 reaction with heterocycle compounds are needed to get a better under- standing on its role in the atmospheric chemistry. Thiophene is a five-member heterocycle ring that is emitted in processes of energy production such as coal gasification, fuel oil, natural gas and coal combustion and also it is detected over sea- weed fields [25–28]. Thiophene is a sulphur-con- taining compound and the study of its chemistry is important to understand the whole role of the sulphur compounds in the atmospheric global sulphur cycle. The reaction of thiophene with OH and O 3 has been reported in different works [6,14,15,21], but the night-time reaction of the NO 3 radical with thiophene has been less investi- gated [4,5,29]. So far the available rate constant 7 June 2002 Chemical Physics Letters 358 (2002) 401–406 www.elsevier.com/locate/cplett * Corresponding author. E-mail address: beatriz.cabanas@uclm.es (B. Caba~ nas). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)00494-3