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 v(1)/2v(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 v(1)/2v(4) Fermi resonance was also obtained for the complexes of fluoroform with dimethyl ether and trimethyl amine.