Thermochemistry of neutral and anionic sulfur fluorides SFn (n = 1–6): Revisited with theoretical G4/W1/W2 composite methods and the roles of metastable conformer of SF4− anion

Abstract

To augment previous theoretical studies of thermochemical properties such as the electron affinity (EA) and bond dissociation enthalpy (D298o) of neutral and anionic SFn (with n = 1–6), further extensive theoretical computations using Gaussian-4 and Weizmann-1 and Weizmann-2 (G4/W1/W2) methods were carried out with extensive consideration of the role of the metastable conformational isomer of the SF4− anion. The energy of the metastable conformer is 39 kJ/mol higher than that of the global minimum structure, and the barrier height between the metastable conformer and its global minimum was calculated to be 27 kJ/mol by the CCSD(T)/Aug-cc-pvQZ+d//MP2/cc-pvQZ+d method. Many of the discrepancies that have persisted between previous theoretical and experimental data can be explained more adequately by considering the metastable conformer. The difference in the EA calculated using the Gaussian-3 (G3) vs the W2 method for SFn with n = 2–6 ranges from 0.12 eV to 0.21 eV, which is much larger than ±0.041 eV of the expected error for the G3 method. The difference in D298o calculated using G3 vs W2 was also significant in several cases, especially for hypervalent fluorides with n = 3–6. The final results obtained with the W2 procedure are sufficiently converged to a chemical accuracy of ±4 kJ/mol ≈ ±0.04 eV for not only the EA but also D298o, for all neutral and anionic SFn species with n = 1–6.