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Fragmentation of energy-selected SF5CF3+ probed by threshold photoelectron photoion coincidence spectroscopy: bond dissociation energy of SF5-CF3, and its atmospheric implications

Chim, R. Y. L. and Kennedy, R. A. and Tuckett, R. P. and Zhou, W. and Jarvis, G. K. and Collins, D. J. and Hatherly, P. A. (2001) Fragmentation of energy-selected SF5CF3+ probed by threshold photoelectron photoion coincidence spectroscopy: bond dissociation energy of SF5-CF3, and its atmospheric implications. The Journal of Physical Chemistry A, 105 (37). pp. 8403-8412. ISSN 1089-5639

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URL of Published Version: http://dx.doi.org/10.1021/jp0116050

Identification Number/DOI: doi:10.1021/jp0116050

Using tunable vacuum-UV radiation from a synchrotron in the range 12-26 eV, we have measured the threshold photoelectron and threshold photoelectron – photoion coincidence spectrum of SF\(_5\)CF\(_3\), a new anthropogenic greenhouse gas. The ground state of SF\(_5\)CF\(_3^+\) is repulsive in the Franck-Condon region, the parent ion is not observed, and the onset of ionisation can only give an upper limit to the energy of the first dissociative ionisation pathway of SF\(_5\)CF\(_3\), to CF\(_3^+\) + SF\(_5\) + e\(^-\). We have determined the kinetic energy released into the two fragments over a range of photon energies in the Franck-Condon region of the ground state of SF\(_5\)CF\(_3^+\). Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionisation energy for SF\(_5\)CF\(_3\) of 12.9 \(\pm\) 0.4 eV. A similar experiment for CF\(_4\) to CF\(_3^+\) + F + e\(^-\)) and SF\(_6\) (to SF\(_5^+\) + F + e\(^-\)) yielded values for their dissociative ionisation energies of 14.45 \(\pm\) 0.20 and 13.6 \(\pm\) 0.1 eV, respectively, in agreement with previous data on the CF\(_3\) and SF\(_5\) free radicals. The enthalpy of formation at 0 K of SF\(_5\)CF\(_3\) is determined to be -1770 \(\pm\) 47 kJ mol\(^{-1}\), and the dissociation energy of the SF\(_5\)-CF\(_3\) bond at 0 K to be 392 \(\pm\) 48 kJ mol\(^{-1}\) or 4.06 \(\pm\) 0.45 eV. The implication of this bond strength is that SF\(_5\)CF\(_3\) is very unlikely to be broken down by UV radiation in the stratosphere. In addition, over the complete energy range of 12-26 eV, coincidence ion yields of SF\(_5\)CF\(_3\) have been determined. CF\(_3^+\) and SF\(_3^+\) are the most intense fragment ions, with SF\(_5^+\), SF\(_4^+\) and CF\(_2^+\) observed very weakly. Energetic constraints require that SF\(_3^+\), SF\(_4^+\) and CF\(_2^+\) can only form with CF\(_4\) + F, CF\(_4\) and SF\(_6\), respectively, so that fragmentation of SF\(_5\)CF\(_3^+\) to these ions involves migration of a fluorine atom across the S-C bond.

Type of Work:Article
Date:2001 (Publication)
School/Faculty:Schools (1998 to 2008) > School of Chemical Sciences
Department:School of Chemistry, School of Physics
Subjects:QD Chemistry
QC Physics
Institution:University of Birmingham
Copyright Holders:American Chemical Society
ID Code:746
Refereed:YES
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