DENSITY-FUNCTIONAL DESCRIPTION OF TRANSITION STRUCTURES USING NONLOCAL CORRECTIONS SILYLENE INSERTION REACTIONS INTO THE HYDROGEN MOLECULE

The insertion reactions of SiH2, SiHF, and SiF2 into the hydrogen molecule have been investigated using density functional methods. Local spin density (LSD) calculations were performed with the LSD exchange functional and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (B-P); Becke and the correlation energy functional of Lee, Yang, and Parr (B-LYP); and Perdew and Wang (PW) generalized gradient exchange-correlation energy functional. Reactants, transition structures, and products were fully optimized at the LSD and NLSD levels. For each of these reactions, relative energies have been calculated with density functional methods and also at the quadratic configuration interaction with single, double, and triple excitations [QCISD(T)]/6-31G(2df,pd) level. Vibrational frequencies were also computed with local and nonlocal approximations as well as at the second-order Moller-Plesset (MP2)/6-31G (d,p) level. As previously reported in the Hartree-Fock case, density functional methods also predict a dramatic increase in the barrier height with fluorine substitutions. The best estimated barrier heights for SiH2 + H-2, SiHF + H-2, and SiF2 + H-2 are - 3.52 +/- 3, 16.07 +/- 3, and 50.49 +/- 2 kcal/mol, respectively.