Electron attachment to SF5CF3 (296–563 K) and calculations of the neutral and anion thermochemistry

Abstract

Møller–Plesset (MP) perturbation theory and density functional theory (DFT) were used to examine the structure and bonding of trifluoromethyl sulfurpentafluoride, SF5CF3, and the corresponding anion, SF5CF3−. The structural parameters, charge analysis, and energetics are all consistent with the anion having ion-dipole character (i.e., SF5−–CF3). Results from G2(MP2) theory yield a neutral D2980(SF5–CF3)=301 kJ mol−1 (3.12 eV), anion D2980(SF5−–CF3)=21.5 kJ mol−1 (0.22 eV), EA(SF5CF3)=119 kJ mol−1 (1.24 eV), ΔfH2980(SF5CF3)=−1639 kJ mol−1, and ΔfH298(SF5CF3−)=−1750 kJ mol−1. The calculated value for the standard enthalpy of formation for SF5CF3 differs from the previous estimate by 78 kJ mol−1. DFT was found to perform poorly for quantities related to the neutral SF5–CF3 bond. Calculations were also carried out for SF5, SF5−, CF3, and CF3− fragments, and both DFT and G2(MP2) methods performed well for these open-shell species. Rate constants for electron attachment to SF5CF3 were measured over the temperature range 296–563 K in 133 Pa helium gas using a flowing afterglow Langmuir probe apparatus. The 296 K rate constant is (8.6±2.2)×10−8 cm3 s−1, which agrees within uncertainties with the estimate reported by Kennedy and Mayhew [Int. J. Mass Spectrom. 206, i–iv (2001)]. The only ionic product of attachment is SF5−. The temperature dependence of the electron attachment rate constant implies an activation energy of 25 meV. Implications of this work for estimating the atmospheric lifetime of SF5CF3 are discussed.