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)_<i>n</i>^–, <i>n</i> ≈ 60–130, is investigated at temperatures <i>T</i> = 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 Δ<i>H</i>_R,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 Δ<i>H</i>_298K = 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^•, <i>EA</i>(SF₅^•) = 4.27 ± 0.25 eV