DENSITY-FUNCTIONAL STUDIES ON AMINO-SUBSTITUTED METHANE AND SILANE

In all-electron density functional calculations on mono-and tetraamino substituted methane and silane the coordination around the nitrogen ce nter is found, in complete agreement with experiment, to be less pyram idal in the silicon compounds. The calculated barrier to inversion at nitrogen is only 0.6 kcal/mol in silylamine, while in methylamine it i s 5.5 kcal/mol. The larger flexibility is attributed to tile greater i onic character of the Si-N bond compared to that of the C-N bond. In t etra(amino)methane, inversion of one amino group leads to a local mini mum (calculated inversion barrier: 6.4 kcal/mol), while a correspondin g structure (or barrier) does not exist for the silicon compound. Due to the steric influence of the amino groups, the barrier to rotation a round the C-N bond is larger in the tetrasubstituted compound (6.4 kca l/mol compared to 2.3 kcal/mol in methylamine). While the average bind ing energy of the amino groups is similar in both tetra(amino)compound s, the binding energy of the fourth ligand is about 25% larger in the silicon compound for both homolytic and heterolytic bond cleavage. Thi s difference arises from the relative stabilization of the tri(amino)m ethyl species, due to some pi contribution to the C-N bond.