VIBRATIONAL SPECTROSCOPY OF PARTIALLY BONDED COMPLEXES: A MATRIX ISOLATION INFRARED STUDY OF CH3CN - BF3
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Date
2001
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Ohio State University
Abstract
The complex formed from acetonitrile and boron trifluoride has been called a ``partially bonded molecule'' since its B-N bond distance and N-B-F bond angle are intermediate between those characteristic of a weak, non-bonded interaction and a bona fide donor-acceptor $bond.^{a}$ Furthermore, complexes among this class have been shown to be quite sensitive to chemical medium, and in particular, undergo dramatic structural rearrangement upon crystallization, which includes a substantial contraction of the donor - acceptor bond. In fact, the C-N stretching frequency of $CH_{3}CN - BF_{3}$ was recently observed in solid $argon^{b}$ at a value very near that of the crystalline $complex.^{c}$ This seemed to indicate that the inert matrix environment caused a nearly full contraction of the B-N dative bond. We have undertaken a re-examination of the complex, with an eye toward vibrational modes involving motion of the $BF_{3}$ moiety, which should be rather sensitive to the effects of complexation. We have observed and identified several new absorption features in argon matrices seeded with 0.1 to 1.0 percent $CH_{3}CN$ and $BF_{3}$ that we assign to the 1:1 complex. The basis for these assignments is: dependence on both component species, consistent relative intensities across a wide range of conditions, and isotope shifts consistent with recent $calculations^{d}$ on the gas-phase complex. Our observations of the $BF_{3}$ antisymmetric stretching mode $(\nu_{13})$ indicate that, contrary to the previous study, matrix-isolated $CH_{3}CN - BF_{3}$ more closely resembles the gas-phase complex than the crystalline species. Recently obtained spectra of two additional isotopic forms of the complex provide further support for our assignments, and will also be discussed.
Description
$^{a}$Leopold, K. R.; Canagaratna, M.; Phillips, J. A. Accts. Chem. Res. 1997, 30, 57. $^{b}$Beattie, I. R.; Jones, P. J. Agnew. Chem. Int. Ed. Engl. 1996, 35, 1527. $^{c}$Swanson, D. F.; Schriver, D. F. Inorg. Chem. 1970, 9, 1406. $^{d}$Cheong, B.; Cho, H. J. Mol. Struct. (Theochem) 2000, 486, 185.
Author Institution: University of Wisconsin-Eau Claire; Department of Chemistry, University of Wisconsin-Eau Claire
Author Institution: University of Wisconsin-Eau Claire; Department of Chemistry, University of Wisconsin-Eau Claire