A Selected Ion Flow Tube Study of the Reactions of Several Cations with the Group 6B Hexafluorides SF6, SeF6, and TeF6

Abstract

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 14 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 among 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, or Te). For reagent ions whose recombination energies lie between the first dissociative ionization limit (XF6 → XF5+ + F + e-) and the onset of ionization 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 ionization, long-range electron transfer may occur depending on whether certain physical factors apply, for example, nonzero 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.