Structures, thermochemistry, and electron affinities of the disilicon fluorides, Si2Fn/Si2Fn- (n=1-6)

A systematic investigation of Si2Fn/Si2Fn- systems is carried out with five density functional (DFT) methods in conjunction with DZP++ basis sets. For each system, various structures, including minima, transition states, and energetically low lying saddle points, are optimized. The geometries and th e relative energies are discussed and compared. Three kinds of electron aff inity and dissociation energy pertaining to the global minimum for each com pound are reported. The theoretical predictions are in good agreement with the limited experimental results. The zero-point vibrational energy (ZPVE) corrected adiabatic electron affinities (EA(ad)) are predicted as 1.97 (Si2 F), 1.92 (Si2F2), 2.39 (Si2F3), 2.02 (Si2F4), 2.68 (Si2F5), and 0.73 (Si2F6 )eV by the BHLYP method, which is considered to be the most reliable method in the present study for predicting the EAs. These theoretical predictions are quite different from those for the analogous silicon hydrides and fluo rocarbons. For example, both Si2F2 and its anions have vinylidene-like (Si- SiF2) global minima. The anion SiSi bond distance is about 0.1 Angstrom sho rter than that for the Si-SiF2 neutral. Both Si2F3 and its anion have carby ne-like (FSi-SiF3) global minima, with the anion SiSi distance about 0.05 A ngstrom shorter. Both Si2F4 and its anion have carbene-like (FSi-SiF3) glob al minima, again with the negative ion SiSi distance similar to0.05 Angstro m shorter. Surprisingly, doubly bridged structures of Si2F4 are energetical ly competitive. For the ethyl-radical-like Si2F5 the expected longer SiSi d istance (by 0.13 Angstrom) for the anion is predicted. Whereas Si2H4, C2F4, Si2H6, and C2F6 do not have significant electron affinities, Si2F4 and Si2 F6 do bind an electron. However, the unexpected Si2F6 species has a signifi cantly longer SiSi distance (by 0.15 Angstrom) than that of neutral Si2F6.