Coupling of CH stretching and bending vibrations in trihalomethanes

The vibrational spectra of HCCl3, HCF3, HCCl2F, and HCClF2 have been measured in the vapor from the CH stretching fundamental through to the fifth overtone (i.e., v1=6, where v1 is the number of quanta in the CH stretching mode), using FTIR and photoacoustic spectrometers. Instead of a single strong CH overtone progression, additional strong bands attributable to Fermi-resonant combination tones are prominent in the spectra. The CH stretch is found to be most strongly coupled to the CH bending mode and much less strongly coupled to the heavy atom motions. The resolved band structures are analyzed in the following paper to give quantitative stretch–bend coupling constants for these CH oscillators.