Laboratory measurements of the 12C/13C kinetic isotope effects in the gas-phase reactions of unsaturated hydrocarbons with Cl atoms at 298 ± 3 K

Abstract

Abstract. The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k 12 /k 13 , are greater than unity or normal KIEs.The KIEs, reported in per mil according to Cl ε = (k 12 /k 13 -1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1);propene, 5.56 ± 0.18 (2); 1-butene, 5. transition state structure. IntroductionThe impact of chlorine atom chemistry on the removal of unsaturated hydrocarbons from the atmosphere has been studied in a significant number of publications (Boudries and Bottenheim, 2000;Jobson et al., 1994;Ramacher et al., 1999;Rudolph et al., 1999;Wingenter et al., 1999). Although reactions with hydroxyl radicals and ozone are far more important on the global scale, particularly in continental mid-latitudes, it is agreed that at high latitudes during Polar Sunrise and in coastal environments in the marine boundary layer, where concentrations of Cl atoms can reach levels of up to 10 5 cm -3 (Chang et al., 2004;Singh et al., 1996), Cl-atom chemistry can significantly increase the removal rate of NMHC from the atmosphere.The usefulness of stable carbon isotope ratio measurements for investigating the OH chemistry of both saturated and unsaturated nonmethane hydrocarbons (NMHC) has been demonstrated recently (Rudolph et al., 2000;Rudolph et al., 2002;Rudolph et al., 2003;Saito et al., 2002;Thompson et al., 2003;Tsunogai et al., 1999). To accurately interpret the isotope ratio measurements of unsaturated hydrocarbons, knowledge of the kinetic isotope fractionations associated with their chemical removal is necessary.Measurements have been reported of the 12 C/ 13 C-KIEs in the reactions of alkenes and aromatic hydrocarbons with OH radicals (Anderson et al., 2004a;Anderson et al., 2004b;Rudolph et al., 2000) and in the reactions of alkenes with ozone (Iannone et al., 2003).To best interpret measurements of NMHC from environments with elevated Cl-atom ExperimentThe method for measuring carbon KIEs for the reactions of NMHC + Cl is similar to the method used for measuring OH-reaction KIEs which has been described in detail in a previous publication . The method required only a few minor changes to generate Cl atoms rather than OH radicals, described recently ( For each experiment, before the Cl-atom reaction was initiated, two or three measurements of the reaction chamber contents were made to verify sufficient stability in both the concentration and stable isotope ratios of the hydrocarbons. From all of the hydrocarbons, the mean relative standard deviation of the individual hydrocarbon concentration measurements was 1.5%, with relative standard deviations ranging from 0.5-3.0%. The mean standard deviation of the stable carbon isotope compositions...

Measurements of the 12C/13C Kinetic Isotope Effects in the Gas-Phase Reactions of Light Alkanes with Chlorine Atoms

“…Thus measurement of the stable carbon isotope ratios of saturated NMHC is less promising for the elucidation of tropospheric Cl atom chemistry than isotope ratio studies for unsaturated hydrocarbons. 49 For the interpretation of tropospheric measurements of the stable carbon isotope ratios of NMHC, the most important consequence of our findings is that, in general, reactions with Cl atoms will have a very limited impact on the measured isotope ratio. Possible exceptions are conditions with a dominating influence of Cl atom chemistry such as ozone depletion episodes in the boundary layer during polar sunrise.…”

Rate coefficients and products for gas‐phase reactions of chlorine atoms with cyclic unsaturated hydrocarbons at 298 K

“…Except for one rate coefficient measurement for the reaction with cyclopentene [18], there are no previous rate coefficient data, at least to our knowledge, for the reactions of chlorine atoms with cycloalkenes. Even in the case of cyclopentene, the reported value (Table II) is at least two times higher than the presently measured rate coefficient.…”

“…[17]. However, there are not many studies on the rate coefficients, or mechanism of reactions of Cl atoms with these cyclic alkenes, except for a measurement of Cl atom reactions with cyclopentene by Anderson et al [18]. In the present study, the rate coefficients of reactions of Cl atoms with three cyclic alkenes, cyclopentene, cyclohexene, and cycloheptene, are determined using the relative rate method, to assess the significance of these reactions in the troposphere.…”

Experimental and Computational Investigation on the Gas Phase Reaction of p-Cymene with Cl Atoms

“…At room temperature, the total rate coefficient for the Cl atom reaction with p -cymene is found to be 2 times larger than the ones measured for o -, m -, and p -xylene and 3 times larger than the one measured for toluene. Wallington et al, Shi and Bernhard, and Anderson et al reported the rate coefficients for the Cl atom reaction with o -, m -, and p -xylene which fall in the range (1.2–1.5) × 10 –10 cm 3 molecule –1 s –1 . The reported values for the Cl atom reaction with toluene at room temperature fall in between 5.6 and 6.1 × 10 –11 cm 3 molecule –1 s –1 . ,,− The experimental results on the reaction products and isotope effects indicate that the reactions of Cl atom with alkyl aromatics like toluene, xylene, alkyl naphthalenes, etc., proceed via an abstraction reaction, whereas we have calculated the rate coefficients of the p -cymene + Cl reaction only for addition channels.…”

“…At room temperature, the total rate coefficient for the Cl atom reaction with p-cymene is found to be 2 times larger than the ones measured for o-, m-, and p-xylene and 3 times larger than the one measured for toluene. Wallington et al, 57 Shi and Bernhard, 58 and Anderson et al 59 reported the rate coefficients for the Cl atom reaction with o-, m-, and p-xylene which fall in the range (1.2−1.5) × 10 −10 cm 3 molecule −1 s −1 . The reported values for the Cl atom reaction with toluene at room temperature fall in between 5.6 and 6.1 × 10 −11 cm 3 molecule −1 s −1 .…”