Rotational spectra, dipole moment, and structure of the SiF ₄-NH₃ dimer

Abstract

Rotational spectra of several isotopic species of the SiF 4-NH3 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 SiF3 N structure with the lone electron pair of the NH 3 pointed at one face of the SiF4 and the three equatorial F's splayed significantly away from the N. The 〈Fax-Si-F eq is about 12° less than tetrahedral. The Si-N distance is determined to be 2.090 Å. The experimental results are in excellent agreement with the SiF4 deformation predicted by the ab initio calculations of Rossi and Jasinski [Chem. Phys. Lett. 169, 399 (1990)]. 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 SiF4 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. © 1992 American Institute of Physics.