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 approximate to 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 x 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F- anion and the SF5 center dot radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of Delta H-R,H-298K = 234 +/- 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result hi an F5S-F bond dissociation enthalpy of Delta H-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 center dot, EA(SF5 center dot) = 4.27 +/- 0.25 eV.