Gas-phase molecular structure of 1,1,1,2-tetrabromo-2,2-dimethyldisilane: theoretical and experimental investigation of a super-halogenated disilane and computational investigation of the F, Cl and I analogues

The molecular structure of 1,1,1,2-tetrabromo-2,2-dimethyldisilane (Br₃SiSiBrMe₂) has been determined in the gas phase by electron diffraction and ab initio molecular-orbital calculations. The computational investigation was used to augment the experimental investigation using the Structure Analysis Restrained by Ab initio Calculations for Electron diffractioN (SARACEN) method. The structure was found to adopt a staggered structure with C _s symmetry by both theory and experiment. Important structural parameters (r _h1) include: rSi–Si 235.6(5) pm, rSi–C 185.4(3) pm, rSi–Br_av 220.3(1) pm, ∠Si–Si–Br(14) 106.1(4)°, ∠Si–Si–C 109.2(8)° and ?Br–Si–Si–Br 180.0°(fixed). These experimental observations are supported by theoretical predictions obtained at the MP2/6-311+G* level. An analogous theoretical investigation was also performed for the series X₃SiSiXMe₂ (X = F, Cl and I) and structural trends identified. The Si–X bond was observed to lengthen as a function of the halogen substituent, with corresponding changes to the Si–Si–X bond angles in the SiX₃ groups. The Si–Si–X bond angle in the SiXMe₂ groups displayed rather different behaviour, and was relatively stable to substitution until X = I. The flexible nature of bond angles about silicon atoms was observed, even in this relatively sterically unhindered system.