Improper or classical hydrogen bonding?: A comparative cryosolutions infrared study of the complexes of HCClF2, HCCl2F, and HCCl3 with dimethyl ether

Complexes of haloforms of the type HCC\F-n(3-n) (n = 1-3) with dimethyl ether have been studied in liquid argon and liquid krypton, using infrared spectroscopy. For the haloform C-H stretching mode, the complexation causes blue shifts of 10.6 and 4.8 cm(-1) for HCCIF2 and HCCl2F, respectively, while for HCCl3 a red shift of 8.3 cm(-1) is observed. The ratio of the band areas of the haloform C-H stretching in complex and monomer was determined to be 0.86(4) for HCCIF2, 33(3) for HCC2F, and 56(3) for HCCl3. These observations, combined with those for the HCF3 complex with the same ether (J. Am. Chem. Soc. 2001, 123, 12290), have been analyzed using ab initio calculations at the MP2=FC/6-31G(d) level, and using some recent models for improper hydrogen bonding. Ab initio calculations on the haloforms embedded in a homogeneous electric field to model the influence of the ether suggest that the complexation shift of the haloform C-H stretching is largely explained by the electric field effect induced by the electron donor in the proton donor. The model calculations also show that the electric field effect accounts for the observed intensity changes of the haloform C-H stretches.