# ROTATIONAL SPECTRUM, STRUCTURE, AND DIPOLE MOMENT OF THE $SiF_{4}$ $NH_{3}$ SYMMETRIC TOP

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 Title: ROTATIONAL SPECTRUM, STRUCTURE, AND DIPOLE MOMENT OF THE $SiF_{4}$ $NH_{3}$ SYMMETRIC TOP Creators: Ruoff, R. S.; Emilsson, T.; Jaman, A. I.; Germann, T. C.; Gutowsky, H. S. Issue Date: 1991 Abstract: It has been suggested recently that Si-F bonds are highly ionic in $character,^{1}$ and theoretical electron density analysis supports a strongly ionic Si-H bond in, for example, $SiH_{4}$.$^{2}$ Microwave spectroscopy can address to some extent the nature of the Si-F and Si-H bonds. High quality ah initio calculations indicate a strongly bound axial dimer $SiF_{4}-NH_{3}$ with a calculated well depth of 10.5 kcal and a short Si-N bond $distance.^{3}$ We have obtained the rotational spectra of various isotopically substituted $SiF_{4}-NH_{3}$ dimers (as well as of $SiH_{4}-NH_{3}$) and assigned them as symmetric tops with internal rotation present. For the main isotopic species of $SiF_{4}-NH_{3}$ we find $B_{0} = 2502.4777 MHz$. Analysis of the structure indicates that the $F_{ax}-Si-F_{eq}$ angle is about $97^{0}$ differing greatly from the tetrahedral $109.5^{0}$- This change in geometry should give a large dipole moment for the dimer if the Si-F bond dipoles are large, as one would expect for an ionic bond. Ab initio calculations indicate a dimer dipole moment of 4.2 $D,^{4}$ which exceeds the sum of monomer moments by 2.7 D. We plan to report the dipole moment, compare the experimentally derived geometry with the ab initio calculated geometry, and discuss the internal rotation of the $SiF_{4}$ and $NH_{3}$ tops. Spectra were observed with a Balle/Flygare pulsed nozzle, Fourier transform spectrometer. Assignment of the internal rotation states was aided by observing a time profile of the population of the state as the gas pulse evolves, with the internal rotor state populations remarkably dependent on whether the front, middle or end of each gas pulse is probed. This effect will also be discussed. URI: http://hdl.handle.net/1811/12324 Other Identifiers: 1991-FA-1