MICROWAVE SPECTRUM AND STRUCTURE OF $CH_{3}-CN-BF_{3}$

The gas phase adduct $CH_{3}CN-BF_{3}$ has been observed by rotational spectroscopy. The structure is that of a symmetric top with the nitrogen end of the $CH_{3}CN$ toward the boron. The B-N bond length is $2.010\\pm 0.020$ {\\AA} and the NBF angle is $95.3^{0}\\pm 1.4^{0}$. The structure is reminiscent of the classical dative bond chemistry known for adducts of $BF_{3}$ with nitrogen containing donors, and of related weakly bound complexes of $BF_{3}$, but is extremely unusual in that the bond length and the NBF angle are intermediate between the limits normally observed for van der waals or covalently bonded systems. Moreover, the B-N bond length is $0.4$ {\\AA} longer than that determined in the solid state by X-ray crystallography, indicating a significant influence of neighboring molecules in the crystal on the structure of a single dimeric unit. We draw analogy with well known crystallographic work of Dunitz and coworkers, and interpret the structure as a snapshot along the reaction path for the formation of the boron -- nitrogen dative bond.