THE ELECTRON AFFINITIES OF THE SILICON FLUORIDES SIFN (N=1-5)

摘要

Several independent density functional methods have been employed to determine the molecular structures and total energies of SiFn and SiFn- (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 SiFn- are also reported. Trends in the predictions of the different DFI: methods are discussed. Self-consistent Kohn-Sham orbitals were obtained using various exchange correlation functionals and a double-zeta 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 AF(n) 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 art: 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 Angstrom longer than their respective neutral counterparts. The SiF3- anion is more strongly pyramidal than SiF3. The SiF4- species is predicted to lie energetically above SiF4, despite its experimental identification. No significantly bound minimum was found for the SiF5 molecule, although SiF5- is predicted to exist and M(+)SiF(5)(-) has been experimentally observed. The predicted vertical detachment energy of SiF5- is 8.54 eV with the BHLYP method. (C) 1995 American Institute of Physics. [References: 54]