Knowledge Bank

A new pulsed-nozzle Fourier transform microwave spectrometer has been used to study the complex $HF-BF3$. A symmetric top spectrum is observed with $B=3327.3361(24)MHz,DJ=0.01097(36)MHz,DJK=0.0504(10)$ MHz, and $eqQaa=2.7833(53)$ MHz for the $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B$ species, and $B=3334.9078(12)$ MHz, $DJ=0.01117(12)$ MHz, $DJK=0.05183(39)$, and $eqQaa=5.765(11)$ for the $\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}B$ species. The data are consistent with a fluorine-bonded structure with and unexpectedly short intermolecular B-F bond length of 2.51 {\AA}. {Ab initio} calculations, which greatly facilitate location of the spectrum, further predict an approximately $2\circ$ out-of-plane distortion of the $BF3$ in the complex, and a B-F-H bond angle of $108\circ$. This latter result suggests that the complex is a symmetric top due to only to rapid vibrational averaging of the HF orientation. The short bond length and the small but non-zero distortion of the $BF3$ are indicative of an incipient donor-acceptor interaction, which is somewhat surprising for fluorine.