Electron attachment of SF_5CF_3 (296-563 K) and calculations of the neutral and anion thermochemistry

摘要

Moller-Plesset (MP0 perturbation theory and density functional theory (DFT) were used to examine the structure and bonding of trifluoromethyl sulfurpentafluoride, SF_5CF_3, and the corresponding anion, SF_5CF_3~-. The structural parameters, charge analysis, and energetics are all consistent with the anion having ion-dipole character (i.e., SF_5~- -CF_3). Results from G2(MP2) theory yield a neutral D_298~0(SF_5-CF_3) = 301 kJ mol~(-1( (3.12 eV),anion D_298~0(SF_5~--CF_3) = 21.5 kJ MOL~(-1) (0.22 eV), EA(SF_5CF_3) = 119 kJ mol~(-1) (1.24 eV), #DELTA#_fH_298~O (SF_5CF_3) = - 1639 kJ mol~(-1), and #DELTA#_fH_298(SF_5CF_3~-) = - 1750 kJ mol~(-1). The calculated value for the standard enthalpy of formation for SF_5CF_3 differs from the previous estimate by 78 kJ mol~(-1). DFT was found to perform poorly for quantities related to the neutral SF_5-CF_3 bond. Calculations were also carried out for SF_5, SF_5~-, CF_3, and CF_3~- fragments, and both DFT and G2(MP2) methods performed well for these open-shell species. Rate constants for electron attachment to SF_5CF_3 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) X 10~(-8) cm~3 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 SF_5~-. 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 SF_5CF_3 are discussed.