The electron affinities of the silicon fluorides SiFn (n=1–5)

Abstract

Several independent density functional methods have been employed to determine the molecular structures and total energies of SiFn and SiF−n (n=1–5). Three significant measures of neutral–anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first Si–F ligand dissociation energies D(Fn−1Si–F), D(Fn−1Si−–F), and D(Fn−1Si–F−) as well as the harmonic vibrational frequencies of SiFn and SiF−n are also reported. Trends in the predictions of the different DFT methods are discussed. Self-consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double-ζ plus polarization basis set augmented with diffuse s-type and p-type functions. The method (BHLYP) based upon the Becke half-and-half exchange functional and the Lee–Yang–Parr correlation functional predicts molecular geometries in best agreement with experiment, while the other methods tend to produce bond lengths that are slightly longer. The BHLYP vibrational frequencies are also superior to those obtained via the other three DFT methods utilized. In previous studies of AFn molecules, the density functional methods have predicted electron affinities a few tenths of an electron volt above experiment with the BHLYP value being the best. Although experimental information concerning the electron affinities of the SiFn series is scarce, the BHLYP method does continue to predict electron affinities that are lower than the other methods. The BHLYP adiabatic electron affinities are 0.83 eV (SiF), 0.42 eV (SiF2), 2.50 eV (SiF3), and −0.22 eV (SiF4). The (unknown) Si–F bond distances for the anions are 0.076–0.088 Å longer than their respective neutral counterparts. The SiF−3 anion is more strongly pyramidal than SiF3. The SiF−4 species is predicted to lie energetically above SiF4, despite its experimental identification. No significantly bound minimum was found for the SiF5 molecule, although SiF−5 is predicted to exist and M+SiF−5 has been experimentally observed. The predicted vertical detachment energy of SiF−5 is 8.54 eV with the BHLYP method.