Fragmentation of state selected SF5CF3+ probed by threshold photoelectron-photoion coincidence spectroscopy: the bond dissociation energy of SF5-CF3, and its atmospheric implications
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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 SF5CF3, a new anthropogenic greenhouse gas. The ground state of SF5CF3+ is repulsive in the Franck-Condon region, the parent ion is not observed, and the onset of ionization can only give an upper limit to the energy of the first dissociative ionization pathway of SF5CF3, to CF3+ + SF5 + 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 SF5CF3+. Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionization energy for SF5CF3 of 12.9 +/- 0.4 eV. A similar experiment for CF4 (to CF3+ + F + e(-)) and SF6 (to SF5+ + F + e(-)) yielded values for their dissociative ionization energies of 14.45 +/- 0.20 and 13.6 +/- 0.1 eV, respectively, in agreement with previous data on the CF3 and SF5 free radicals. The enthalpy of formation at 0 K of SF5CF3 is determined to be -1770 +/- 47 kJ mol(-1), and the dissociation energy of the SF5-CF3 bond at 0 K to be 392 +/- 43 kJ mol(-1) or 4.06 +/- 0.45 eV. The implication of this bond strength is that SF5CF3 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 SF5CF3 have been determined. CF3+ and SF3+ are the most intense fragment ions, with SF5+, SF4+, and CF2+ observed very weakly. Energetic constraints require that SF3+, SF4+, and CF2+ can only form with CF4 + F, CF4, and SF6, respectively, so that fragmentation of SF5CF3+ to these ions involves migration of a fluorine atom across the S-C bond.
|Number of pages||10|
|Journal||The Journal of Physical Chemistry A|
|Early online date||24 Aug 2001|
|Publication status||Published - 20 Sep 2001|