Theoretical Studies of Silicon - Containing Molecules

We report herein, a series of ab initio calculations on various SiH, and Si,H, species using generalized valence bond and configuration interactions methods. These results allow systematic estimation of the heats of formation and the enthalpies of reactions for these species. For SiH, compounds, the calculated and experimental energetics are in excellent agreement; we obtain D,,, (H,SiH) = 91.6 kcal mol-* (experiment, 90.3 f l), D,,, (SiH) = 68.2 kcal (experiment <70 kcal), and an energy of atomization for SiH, of 303.4 kcal mol-’ (experiments yield 307.9). However, for silylene, we find AHf = 67 kcal moll, 9 kcal mol-* higher than the (indirect) experimental value and suggest re-examination of these experiments. We calculate D,,, (H,Si-SiH,) = 71.7 kcal mol-L, in good agreement with the experimental value of 73.7 f 2 kcal mol-‘. Particularly interesting is disilene, where it is found that (i) the ground state is nonplanar (trans) (with a barrier to planarization of 1 kcal mall’), (ii) disilene is stable relative to the isomer silylsilylene (H,Si-SiH) by about 6 kcal mol-I, and (iii) the Si=Si double bond energy is 56 kcal mol-*, which is 16 kcal mole1 weaker than the Si-Si single bond in disilane! The origin of this effect is explained in terms of GVB bonding.