AB-INITIO MOLECULAR-ORBITAL STUDY OF 1-FLUOROSILATRANE

The molecular geometry of 1-fluorosilatrane was optimized fully by res tricted Hartree-Fock (HF) calculations using the 3-21G, 3-21G(d) and 6 -31G(d) basis sets, with the aim of locating the positions of the loca l minima on the energy hypersurface. The optimized geometries were com pared with available experimental (X-ray and ED) and semiempirical dat a. The ab initio calculations using polarized basis sets are in good a greement with those of previously reported semiempirical calculations, giving a slightly longer equilibrium Si-N distance (approximately 256 pm) in the case of the endo minimum. However, the exo minimum predict ed by the semiempirical methods is not supported. There was no experim ental evidence for the existence of this exo minimum, and the present ab initio calculations suggest that it is highly unstable. There is co nsiderable disagreement among the experimental results in the C-N and C-C bond lengths in various silatranes, their differences being as lar ge as 13 pm. The present calculations predict that these differences m ay appear because the silatrane skeleton is flexible with low-energy, large-amplitude internal motions which introduce considerable uncertai nties into the position of ring carbon atoms. (C) 1994 by John Wiley & Sons, Inc.