A temperature-dependent selected ion flow tube study of anions reacting with SF5CF3

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Abstract

Rate constants and product distributions were measured for the reactions of trifluoromethyl sulfurpentafluoride (SF5CF3) with 11 anions using two selected ion flow tube instruments. The reaction of O2 was studied from 240 to 500 K and was found to proceed at approximately 30% of the collisional rate, yielding SF5 as the only product ion, presumably via a dissociative electron transfer mechanism. The reaction of OH proceeds predominantly via nucleophilic attack at the carbon atom, displacing SF5 at approximately 25% of the collisional rate at 298 K. The rate constant exhibits a T−1.8 dependence from 240 to 500 K, suggesting the reaction occurs through a long-lived collision complex. The F reaction, which proceeds at 70% the collisional rate at 298 K, also occurs via nucleophilic attack at the carbon atom, leading to the SF5 product ion. The O ion reacts predominately by nucleophilic attack at the carbon atom and also via CF3 radical transfer to O. The product branching fraction for the O reaction did not vary with temperature, although the rate constant exhibits a T−1.6 dependence, similar to that of the OH reaction. The ions SF6, CF3, NO2, CO3, Cl, NO3, and SF5 were found to be unreactive toward SF5CF3, despite the fact that sufficiently exothermic reactive channels exist for many of these ions.

Introduction

A GC-MS analysis of stratospheric air samples by Sturges et al. [1] has indicated that trifluoromethyl sulfurpentafluoride (SF5CF3) is present in the stratosphere. The compound is thought to be exclusively anthropogenic in origin; Sturges et al. speculate that the source of atmospheric SF5CF3 may be the reaction of SF6 with fluoropolymers in electrical devices. Although present at a level of only 0.12 ppt in 1999, the atmospheric abundance of SF5CF3 is reportedly increasing by 6% per year, tracking the increase of atmospheric SF6. This is significant because both SF6 and SF5CF3 are powerful greenhouse gases. The global warming potential (GWP) of SF5CF3 is currently estimated at 18,000 times that of CO2, making it one of the largest of any molecules.

The stratospheric profile that was measured by Sturges et al. suggests this compound is long-lived in the atmosphere. We have recently speculated that, because the compound is not broken down by UV photodissociation [2] and there are no known atmospheric sinks, ion–molecule reactions and electron attachment reactions will play a significant role in the atmospheric chemistry of SF5CF3. Previous reports from our laboratories on the electron attachment rate to SF5CF3 place an upper limit of 1000 years on the compound’s atmospheric lifetime [3], [4].

As a further exploration of this molecule, Miller et al. [4] have performed ab initio calculations on SF5CF3 and its corresponding anion, obtaining equilibrium structures, an electron affinity (EA), and neutral and anion bond dissociation energies and heats of formation. Kennedy and coworkers [5], [6] have reported on the reactions of SF5CF3 with a variety of cations of atmospheric and industrial importance. In this paper, we continue our investigations of SF5CF3, reporting the first negative ion chemistry of this molecule. We present temperature-dependent rate coefficients and product ion distributions for the reactions of SF5CF3 with several anions of atmospheric importance. Because of the anthropogenic nature of this compound and the speculation that it may be formed in industrial processes, we have also examined the reactions of SF5CF3 with several anions of industrial importance.

Section snippets

Experimental

Measurements were made using two separate selected ion flow tube (SIFT) instruments, one at the Air Force Research Laboratory (AFRL) and one at the University of Birmingham (UB). Both of these instruments, as well as the technique, have been described previously in detail [7], [8], [9]. Therefore, only a brief description of the method and details pertinent to the present study are given here. In each apparatus, the reactant ions were formed in a remote ion source region and were mass selected

Results

Reaction rate constants and product branching fractions are shown in Table 1 for 11 anions of atmospheric or industrial plasma importance reacting with SF5CF3. The reactions are ordered according to the corresponding neutral atom or molecule’s EA. Note for comparison that the EA of SF5CF3 has been calculated to be 1.24 eV [4]. The rate constants listed in Table 1 represent an average of two to six independent measurements. The notation “n.r.” indicates no reaction products were detected; an

O2

The reaction of O2 with SF5CF3 proceeds at approximately 30% of the collision rate, yielding SF5 as the only product ion. Excluding the 298 K measurement, the temperature dependence of the rate constant from 240 to 500 K mirrors that of the collision rate, meaning the reaction remains ∼30% efficient throughout this temperature range. The likely reaction mechanism is dissociative electron transfer, although we cannot rule out the possibility of a chemically reactive channel, producing CF3O2

Conclusion

Rate constants and product ion distributions are reported for the reactions of O2, O, OH, and F with SF5CF3. The F reaction proceeds with an efficiency of approximately 70%. The O2, O, and OH reactions all proceed at 25–30% of the collisional rate at 298 K. The rate constant for the O2 reaction exhibits essentially no temperature dependence, while the rate constants for the O and OH reactions exhibit temperature dependences similar to one another, T−1.6 and T−1.8, respectively. The

Acknowledgements

CAM is grateful for the assistance of Ms. C. Atterbury during the SIFT measurements and acknowledges financial support provided by the UK Technological Plasma Initiative Program, EPSRC, under Grant Reference GR/L82083. The AFRL authors acknowledge technical support provided by John Williamson and Paul Mundis and financial support provided by the US Air Force Office of Scientific Research under Project no. 2303EP4. T.M.M. was supported through Visidyne Inc. (Burlington, MA) under contract number

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