Knowledge Bank

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 $CH3CN-BF3$ was recently observed in solid $argonb$ 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 $BF3$ 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 $CH3CN$ and $BF3$ 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 $calculationsd$ on the gas-phase complex. Our observations of the $BF3$ antisymmetric stretching mode $(\nu 13)$ indicate that, contrary to the previous study, matrix-isolated $CH3CN-BF3$ 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.