Hydrogen bonding in the gas phase: the infrared spectra of complexes of hydrogen fluoride with hydrogen cyanide and methyl cyanide

Abstract

The infrared spectra of the gas phase complexes HCN-HF, DCN-DF and four isotopic species of CH3CN-HF have been measured over the range 200 to 4000 cm -1 . Two bands have been observed, one associated with the stretching vibration of HF in the complex and the other with a bending vibration of the hydrogen bond itself. At higher resolution both bands show fine structure which has been interpreted as being a series of hot bands associated with transitions from excited levels of another low-frequency bending vibration of the hydrogen bond. In the first band the peaks are P branch bandheads in the individual hot bands and in the second band they are sharp Q branches. From temperature studies of these bands and from the effects of isotopic substitution on the spacing of the fine structure the frequency of the lower bending vibration has been determined. Further structure in the first band gives the frequency of the stretching vibration of the hydrogen bond itself. A complete assignment of all the vibrations associated with the hydrogen bond has therefore been made. From the frequencies of the two bending motions (555 and 70 cm -1 for the HCN-HF complex) values of the bending force constants have been calculated. Several anharmonic constants have also been measured and the effect of anharmonicity on the breadth of bands associated with the hydrogen bond is discussed.