Electron affinities of silicon hydrides:: SiHn (n=0-4) and Si2Hn (n=0-6)

The molecular structures and electron affinities of the SiHn/SiHn- (n = 1-4) and Si2H(n)/Si2Hn- (n = 0-6) molecules, as well as the silicon atom, have been investigated using density functional theory (DFT) and hybrid Hartree-Fock/density functional theory. Specifically, four different types of electron affinities are reported in this work: the adiabatic electron affinity (EA(ad)), zero-point corrected EA(ad) (EA(zero)), the vertical electron affinity (EA(vert)), and vertical detachment energy (VDE). The basis set used in this work is of double-zeta plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP(++). Of the six different density functionals used in this work, the BHLYP functional predicted the best molecular structures, and the B3LYP functional predicted the best electron affinities. When compared to the available six experimental electron affinities, the B3LYP functional has an average absolute error of just 0.06 eV. We predict the unknown electron affinities for Si2H2 (dibridged), Si2H3, Si2H4 (disilene), and Si2H5 (disilyl radical) to be 0.45, 2.21, 1.34, and 1.85 eV, respectively. These predictions assume that the (unknown) molecular structure of each anion is analogous to the known structure of the corresponding neutral molecule. The most interesting aspect of the present research is that for Si2H2- and Si2H4-, the lowest energy structures are qualitatively different from those of the neutrals. For Si2H2- the disilavinylidene structure H2SiSi-, is predicted to lie 24 kcal/mol below the dibridged or "butterfly" anion. For Si2H4- the silylsilylene anion H3SiSiH-, is predicted to lie 3 kcal/mol below the disilene anion structure H2SiSiH2-. The zero-point corrected adiabatic electron affinities of disilavinylidene, silylsilylidyne, and silylsilylene are 1.87, 1.01, and 1.71 eV, respectively. The saturated closed shell systems SiH4 and Si2H6 do not have conventional electron affinities.