Structural and Energetic Properties of Haloacetonitrile - GeF₄ Complexes

The 1:1 and 2:1 complexes of FCH₂CN and ClCH₂CN with GeF₄ have been investigated by M06/aug-cc-pVTZ calculations, low-temperature, thin-film IR spectroscopy, and an x-ray structure has been obtained for (FCH₂CN)₂-GeF₄. Theoretical structures and binding energies for FCH₂CN-GeF₄ and ClCH₂CN-GeF₄ demonstrate that halogen substitution significantly weakens the Ge-N dative bonds. The Ge-N distances for the F-and Cl-complexes (2.447 and 2.407 Å, respectively) are about 0.2 Å longer than in CH₃CN-GeF₄, and the binding energies (6.5 and 6.9 kcal/mol) are 2 to 3 kcal/mol less. Furthermore, the Ge-N potential curves are flatter for the halogenated complexes, exhibit a greater response to dielectric media, and thus these systems are more prone to structural change in condensed-phases. For the 2:1 complexes, experimental and theoretical structure and frequency data are consistent with differences in the (calculated) gas-phase and solid-state structures. For (FCH₂CN)₂-GeF₄ the calculated gas-phase structure has Ge-N distances about 0.3 Å longer those in the x-ray structure (2.366 Å vs. 2.059 Å (ave)). Also, low-temperature IR spectra of CH₃CN/GeF₄, FCH₂CN/GeF_4, and ClCH₂CN/GeF₄ thin films are consistent with the presence of 2:1 nitrile:GeF₄ complexes, and the splitting patterns of the GeF-stretching bands (~700 cm^-1) match predictions for the corresponding complexes, but are red-shifted relative to the gas-phase predictions, and reflect Ge-N bonds that are compressed in the solid-state, relative to predicted gas-phase structures.