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

Abstract

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-C₅F₈ 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 H₂O⁺ for which the reaction efficiency is only 55%. Apart from H₂O⁺ 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-C₅F₈ 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-C₅F₈⁺ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C₅F₈ recorded from 12–23 eV shows detection of the parent ion and the daughter ions C₄F₆⁺ and C₅F₇⁺, 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-C₅F₈ at 298 K has been determined to be −1495 kJ mol⁻¹.