Knowledge Bank

We report the microwave spectrum and structure of the partially bonded donor - acceptor adduct $H2O-BF3$. $H2O$ and $BF3$ react to form two compounds, a monohydrate and a dihydrate, both with well defined stoichiometries and melting points. For the monohydrate, previous X-ray diffraction $worka$ indicates a B-O bond distance of 1.532 {\AA} (equal to the sum of covalent radii) and a tetrahedral geometry at the boron. In the gas, however, we find a bond length of 1.90 {\AA} and an out-of-plane distortion of the $BF3$ of only $8\circ$. Thus, while the B-O bond in the solid is an ``ordinary'' dative linkage, the bond in the gas phase system appears to be only partially developed. The large changes in bond length and bond angle which occur upon crystallization (0.4 {\AA} and $9\circ$, respectively) will be discussed in comparison with other partially bound systems which have been investigated in our laboratory. Ab initio calculations place the gas phase binding energy at 9.6 kcal/mol, which is a very reasonable value for a partially formed dative bond.