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The molecular geometry of 1-fluorosilatrane was optimized fully by restricted 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 local minima on the energy hypersurface. The optimized geometries were compared with available experimental (X-ray and ED) and semiempirical data. The ab initio calculations using polarized basis sets are in good agreement with those of previously reported semiempirical calculations, giving a slightly longer equilibrium Si-N distance (-256 pm) in the case of the endo minimum. However, the exo minimum predicted by the semiempirical methods is not supported. There was no experimental evidence for the existence of this ex0 minimum, and the present ab initio calculations suggest that it is highly unstable. There is considerable disagreement among the experimental results in the C-N and C-C bond lengths in various silatranes, their differences being as large as 13 pm. The present calculations predict that these differences may appear because the silatrane skeleton is flexible with low-energy, large-amplitude internal motions which introduce considerable uncertainties into the position of ring carbon atoms. 0 1994 by John Wiley & Sons, Inc.bond. Two basic forms of the structure are possible: the endo form, in which the lone pair of the nitrogen is directing toward the silicon and the Si-N distance is relatively short (Figs. 1 and 2), and the exo form, in which the Si-N distance is much longer T which have the MNDof and the nitrogen lone pair is pointing outward, away from the cage structure of the molecule. The aforementioned semiempirical methods, which did not consider the d orbitals, may be disadvantageous in describing the Si-N hansannular interaction. A b initio theory is known to give reasonably reliable geometry in most of the cases if polarized