Rotational spectra of several isotopic species of the SiF,-NH, dimer were obtained with the Mark II Flygare-Balle FT-MW spectrometer. This is the first determination of the gas phase structure of a penta-coordinated silicon. The spectra indicate a symmetric top, trigonal bipyramid SiF, N structure with the lone electron pair of the NH, pointed at one face of the SiF, and the three equatorial F's splayed significantly away from the N. The LF,,-Si-F,, is about 12" less than tetrahedral. The SiN distance is determined to be 2.090 A. The experimental results are in excellent agreement with the SiF, deformation predicted by the ab initio calculations of Rossi and Jasinski [ Chem. Phys. Lett. 169, 399 (1990) 1. The measured dipole moment is 5.61 D, an enormous enhancement compared to the sum of the monomer moments, 1.47 D. The increase of 4.14 D is due largely to the geometric distortion of the tetrahedral SiF, molecule upon dimer formation, demonstrating that the Si-F bond is much more ionic than covalent. If the charge is simply partitioned between Si and F so as to obtain a moment of 4.14 D, the Si has a charge of + 3.52 and each F,-0.88. This conclusion is consistent with recent applications of the atoms-in-molecules charge partition methodology developed by Bader and co-workers.

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