A cryosolution infrared study of the complexes of fluoroform with ammonia and pyridine: Evidence for a C-H...N pseudo blue-shifting hydrogen bond.

Mid-infrared spectra of mixed solutions in liquid xenon containing fluoroform and either ammonia or pyridine have been investigated at temperatures between 173 and 213 K. For both Lewis bases, a new band is found in the CH stretching region at a frequency approximately 5 cm(-1) higher than that of monomer fluoroform, which is assigned to a complex between fluoroform and the Lewis base. A detailed analysis of the nu1/2nu(4) Fermi resonance in the proton donor shows that the blue shifts observed for the complexes are not caused by a strengthening of the CH bond during the complexation, but are due to the changes in the Fermi resonance interactions. Information on the nu1/2nu(4) Fermi resonance was also obtained for the complexes of fluoroform with dimethyl ether and trimethyl amine.