15min:

NATHAN P. WELLS AND JAMES A. PHILLIPS, Department of Chemistry, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702-4004.

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.\footnoteLeopold, K. R.; Canagaratna, M.; Phillips, J. A. Accts. Chem. Res. \textbf1997, 30 , 57. 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 \chemCH₃CN - BF₃ was recently observed in solid argon\footnoteBeattie, I. R.; Jones, P. J. Agnew. Chem. Int. Ed. Engl. \textbf1996, 35 , 1527. at a value very near that of the crystalline complex.\footnoteSwanson, D. F.; Schriver, D. F. Inorg. Chem. \textbf1970, 9 , 1406. 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 \chemBF₃ 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 \chemCH₃CN and \chemBF₃ 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\footnoteCheong, B.; Cho, H. J. Mol. Struct. (Theochem) \textbf2000, 486 , 185. on the gas-phase complex. Our observations of the \chemBF₃ antisymmetric stretching mode ( ₁₃) indicate that, contrary to the previous study, matrix-isolated \chemCH₃CN - BF₃ 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.