A selected ion flow tube study of the reactions of several cations with the group 6B hexafluorides SF₆, SeF₆, and TeF₆

The first investigation of the ion chemistry of SeF₆ and TeF₆ is presented. Using a selected ion flow tube, the thermal rate coefficients and ion product distributions have been determined at 298 K for the reactions of fourteen atomic and molecular cations, namely H₃O⁺, CF₃⁺, CF⁺, CF₂⁺, H₂O⁺, N₂O⁺, O⁺, CO₂⁺, CO⁺, N⁺, N₂⁺, Ar⁺, F⁺ and Ne⁺ (in order of increasing recombination energy), with SeF₆ and TeF₆. The results are compared with those from the reactions of these ions with SF₆, for which the reactions with CF⁺, CF₂⁺, N₂O⁺ and F⁺ are reported for the first time. Several distinct processes are observed amongst the large number of reactions studied, including dissociative charge transfer, and F^-, F, F₂^- and F₂ abstraction from the neutral reactant molecule to the reagent ion. The dissociative charge transfer channels are discussed in relation to vacuum ultraviolet photoelectron and threshold photoelectron-photoion coincidence spectra of XF₆ (X = S, Se, and Te). For reagent ions whose recombination energies lie between the first dissociative ionisation limit (XF₆ → XF₅⁺ + F + e^-)and the onset of ionisation of the XF₆ molecule, the results suggest that if dissociative charge transfer occurs, it proceeds via an intimate encounter. For those reagent ions whose recombination energies are greater than the onset of ionisation, long-range electron transfer may occur depending on whether certain physical factors apply, for example non-zero Franck-Condon overlap. From the reaction kinetics, limits for the heats of formation (in kJ mol-1) of SeF₄, SeF₅, TeF₄ and TeF₅ at 298 K have been obtained;  ΔfH°(SeF₄) < −369, ΔfH°(SeF₅) < −621, ΔfH°(TeF₄) > −570, and ΔfH°(TeF₅) < −822.