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Selected ion flow tube study of the gas-phase reactions of CF+, CF2+, CF3+ and C2F4+ with C2H4, C2H3F, CH2F2 and C2HF3

Simpson, M. J. and Tuckett, R. P. (2012) Selected ion flow tube study of the gas-phase reactions of CF+, CF2+, CF3+ and C2F4+ with C2H4, C2H3F, CH2F2 and C2HF3. The Journal of Physical Chemistry A, 116. pp. 8119-8129. ISSN 1089-5639 (In Press)

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URL of Published Version: http://dx.doi.org/10.1021/jp304768n

Identification Number/DOI: 10.1021/jp304768n

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF\(^+\), CF\(_2^+\), CF\(_3^+\) and C(\_2\)F\(_4^+\)) with ethene (C\(_2\)H\(_4\)), monofluoroethene (C\(_2\)H\(_3\)F), 1,1-difluoroethene (CH\(_2\)CF\(_2\)), and trifluoroethene (C\(_2\)HF\(_3\)) have been studied in a Selected Ion Flow Tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Since the recombination energy of CF\(_2^+\) exceeds the ionisation energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of non-dissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF\(^+\), CF\(_3^+\) and C\(_2\)F\(_4^+\) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF\(^+\) and CF\(_3^+\) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF\(_3^+\) and C\(_2\)F\(_4^+\) are significantly slower than those of CF\(^+\) and CF\(_2^+\), with adducts being formed with the former cations. The reactions of C2F\(_4^+\) with the four neutral titled molecules are complex, giving a range of products. All can be characterised by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ\(_f\)H°\(_2\) (C\(_3\)F\(_{298}\)H\(_3^+\), with structure CF\(_2\)=CH-CH\(_2^+\)) of 628 kJ mol\(^{-1}\) and a lower limit for Δ\(_f\)H°\(_{298}\) (C\(_2\)F\(_2\)H\(^+\), with structure CF\(_2\)=CH\(^+\)) of 845 kJ mol\(^{-1}\) are determined.

Type of Work:Article
Date:August 2012 (Publication)
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Chemistry
Subjects:QD Chemistry
Institution:University of Birmingham
Copyright Holders:The Authors, American Chemical Society
ID Code:1213
Refereed:YES
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