Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C\(_5\)F\(_8\) with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H\(_2\)O\(^+\) for which the reaction efficiency is only 55 %. Apart from H\(_2\)O\(^+\) and N\(^+\), the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of \(c\)-C\(_5\)F\(_8\) measured by TPEPICO spectroscopy occurs at 12.25 ± 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 ± 0.10 eV), He (I) photoionisation (11.30 ± 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 ± 0.002 eV). The ground electronic state of \(c\)-C\(_5\)F\(_8^+\) has very weak intensity under threshold electron conditions. The TPEPICO spectrum of \(c\)-C\(_5\)F\(_8\) recorded from 1223 eV shows detection of the parent ion and the daughter ions C\(_4\)F\(_6^+\) and C\(_5\)F\(_7^+\), with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of \(c\)-C\(_5\)F\(_8\) at 298 K has been determined to be \(\sim\)1495 kJ mol\(^{-1}\)