Large Gas−Solid Structural Differences in Complexes of Haloacetonitriles with Boron Trifluoride

The structural properties of the singly halogenated derivatives of CH3CN−BF3 (X−CH2CN−BF3: X = F, Cl, Br, I)
have been investigated via single-crystal X-ray crystallography, solid-state infrared spectroscopy, and correlated
electronic−structure theory. Taken together, these data illustrate large differences between the gas-phase and
solid-state structures of these systems. Calculated gas-phase structures (B3PW91/aug-cc-pVTZ) of FCH2CN−BF3,
ClCH2CN−BF3, and BrCH2CN−BF3 indicate that the B−N dative bonds in these systems are quite weak, with
distances of 2.422, 2.374, and 2.341 Å, respectively. However, these distances, as well as other calculated structural
parameters and normal-mode vibrational frequencies, indicate that the dative interactions do become slightly stronger
in proceeding from F− to Br−CH2CN−BF3. In contrast, solid-state structures for FCH2CN−BF3, ClCH2CN−BF3, and
ICH2CN−BF3 from X-ray crystallography all have B−N distances that are quite short, about 1.65 Å. Thus, the B−N
distances of the F- and Cl-containing derivatives contract by over 0.7 Å upon crystallization. Large shifts in the
vibrational modes involving motions of the BF3 subunit parallel these structural changes. An X-ray crystal structure
could not be determined for BrCH2CN−BF3(s), but the solid-state IR spectrum is consistent with those obtained
previously for related complexes and suggests that the solid-state structure resembles those of the others, and in
turn, implicates a large gas−solid structural difference for this species as well.