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 SeF\(_6\) and TeF\(_6\) is presented. Using a selected ion flow tube, the thermal rate coefficients and ion product distributions have been determined at 300 K for the reactions of fourteen atomic and molecular cations, namely H\(_3\)O\(^+\), CF\(_3^+\), CF\(^+\), CF\(_2^+\), H\(_2\)O\(^+\), N\(_2\)O\(^+\), O\(^+\), CO\(_2^+\), CO\(^+\), N\(^+\), N\(_2^+\), Ar\(^+\), F\(^+\) and Ne\(^+\) (in order of increasing recombination energy), with SeF\(_6\) and TeF\(_6\). The results are compared with those from the reactions of these ions with SF\(_6\), for which the reactions with CF\(^+\), CF\(_2^+\), N\(_2\)O\(^+\) 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, F\(_2^-\) and F\(_2\) 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 XF\(_6\) (X = S, Se, and Te). For reagent ions whose recombination energies lie between the first dissociative ionisation limit, XF\(_6\) \(\rightarrow\) XF\(_5^+\) + F + e\(^-\), and the onset of ionisation of the XF\(_6\) 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 of SeF\(_4\), SeF\(_5\), TeF\(_4\) and TeF\(_5\) at 298 K have been obtained; \(\Delta_f\)H\(^o\)(SeF\(_4\)) -570 kJ mol\(^{-1}\), and \(\Delta_f\)H\(^o\)(TeF\(_5\)) < -822 kJ mol\(^{-1}\).