Negative ion thermochemistry: The sulfur fluorides SFn/SFn- (n=1-7)

The molecular structures and total energies of SFn and SFn- (n = 1-7) have been predicted using density functional methods. Three significant measures of electron affinity are reported: the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vent)), and the vertical detachment energy of the anion (VDE). The first S-F ligand dissociation energies D(Fn-1S-F), D(Fn-1S--F), and D(Fn-1S-F-) are also reported. Self-consistent Kohn-Sham orbitals were obtained using various functional forms and a double-zeta plus polarization (DZP) basis set. The addition of diffuse s- and p-type functions to the basis set lowered the energies of the anions relative to their neutral species and significantly improved the values of the electron affinities. The method (BHLYP) based upon Becke's half-and-half exchange functional and the Lee-Yang-Parr correlation functional predicted molecular geometries and electron affinities in best agreement with experiment, while the other methods tended to produce bond lengths that were slightly longer and electron affinities which were larger. The BHLYP electron affinities are typically a few tenths of an electronvolt above experiment. Neutral SF7 was found to have no structures that were significantly bound with respect to dissociation. SF7- structures with C-4 upsilon-symmetry and C-3 upsilon-symmetry were found to lie very close in energy. The adiabatic electron affinities calculated here are in good agreement with experimental results for SF and SF3, but the predicted electron affinities for SF6 vary widely between functionals and are much larger than the accepted experimental value. The density functional results favor a value EA(ad)(SF4) near 1.8 eV. This is a slightly larger value than that predicted by recent experiments which concluded that EA(ad)(SF4) = 1.5 +/- 0.2 eV. The neutral bond dissociation energies D(Fn-1S-F) tend to confirm experiment except for SF5 where the DFT methods predict D(F4S-F) = 1.09-1.72 eV, which is less than an experimental value of 2.30 +/- 0.26 eV. The bond dissociation values of the anions D(Fn-1S--F) and D(Fn-1S-F-) are in agreement with experimentally determined values with the exception of D(F5S-F-) which is predicted to be at least 1 eV larger than an experimental value of 1.10 eV. All density functional schemes predict a vertical detachment energy for SF6- that is at least 0.5 eV more than the recently reported experimental value of 3.16 eV. It is concluded that the density functional methods, while very useful in establishing trends, are not yet quantitatively reliable. However, additional (SFn-SFn-) experiments are required to precisely establish the reliability of the different density functional methods.