Large gas-solid structural differences in complexes of haloacetonitriles with boron trifluoride

The structural properties of the singly halogenated derivatives of CH ₃CN-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, ClCH ₂CN-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, ClCH ₂CN-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 BrCH ₂CN-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.