Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n–, n ≈ 60–130, is investigated at temperatures T = 140–300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10–10 cm3 s–1 via exclusive formation of the hydrated F– anion and the SF5• radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol–1. Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S–F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol–1, in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5•, EA(SF5•) = 4.27 ± 0.25 eV.