THE STRUCTURE OF 1-CHLOROSILATRANE - AN AB-INITIO MOLECULAR-ORBITAL AND A DENSITY-FUNCTIONAL THEORY STUDY

The molecular geometries of the 1-chloro-, 1-fluoro-, 1-methyl-, and 1 -hydrogenosilatranes were fully optimized by the restricted Hartree-Fo ck (HF) method supplemented with 3-21G, 3-21G(d), 6-31G(d), and CEP-31 G(d) basis sets; by MP2 calculations using 6-31G(d) and CEP-31G(d) bas is sets; and by GGA-DFT calculations using 6-31G(d5) basis set with th e aim of locating the positions of the local minima on the energy hype rsurface. The HF/6-31G(cl) calculations predict long (>254 pm) and the MP2/CEP calculations predicted short (similar to 225 pm) equilibrium Si-N distances. The present GGA-DFT calculations reproduce the availab le gas phase experimental Si-N distances correctly. The solid phase ex perimental results predict that the Si-N distance is shorter in 1-chlo rosilatrane than in 1-fluorosilatrane. In this respect the HF results show a strong basis set dependence, the MP2/CEP results contradict the experiment, and the GGA-DFT results in electrolytic medium agree with the experiment. The latter calculations predict that 1-chlorosilatran e is more polarizable than 1-fluorosilatrane and also support a genera l Si-N distance shortening trend for silatranes during the transition from gas phase to polar liquid or solid phase. The calculations predic t that the ethoxy links of the silatrane skeleton are flexible. Conseq uently, it is difficult to measure experimentally the related bond len gths and bond and torsion angles. This is the probable origin of the s urprisingly large differences for the experimental structural paramete rs. On the basis of experimental analogies, nb initio calculations, an d density functional theory (DFT) calculations, a gas phase equilibriu m (r(e)) geometry is predicted for 1-chlorosilatrane. The semiempirica l methods predict a so-called exo minimum (at above 310 pm Si-N distan ce); however, the ab initio and GGA-DFT calculations suggest that this form is nonexistent. The GGA-DFT geometry optima were characterized b y frequency analysis. (C) 1996 by John Wiley & Sons, Inc.