Knowledge Bank

The rotational spectrum of $CH3F-BF3$ has been observed via pulsed-nozzle Fourier transform microwave spectroscopy. A dense spectrum is observed, and transition arising from two internal rotor states of the complex have been identified. The spectra are consistent with the expected geometry in which the flurine of the $CH3F$ approaches the $BF3$ on or near its $C3$ axis, with a B-F-C angle of $120\circ$. The intermolecular B--F bond length is about 2.4 \AA, a value considerably shorter than both the 2.7 \AA distance expected for a van der Waals bond length and the 2.54 \AA distance previously observed in HF-$BF3$. Complexes of this type have been invoked as intermediatea in solution phase Fridel-Crafts alkylation reactions, and the relevance of the current observations to this chemistry will be discussed.