Knowledge Bank

The vibrational spectra of $HCCl3,HCF3,HCCl2F$. and $HCClF2$ have been $measured1$ in the vapor from the CH stretching fundamental through to the fifth overtone (i.e. $v1=6$), using FTIR and photoacoustic spectrometers. Instead of a single strong band for each CH overtone, several strong bands appear, which are attributable to Fermi resonant combination tones involving the stretch and the CH bend. Some of the observed bands are also markedly broadened by near resonances. A simple effective spectroscopic Hamiltonian produces good fits to the observed data; a similiar treatment has recently been applied by Quack et. al. to $HCCl2F,2HCF3,3HCD3,4HC(CF3)3,5$ and $HCF(CF3)22$ using primarily FTIR spectra measured up to the third overtone. Strong mode-mode coupling between the CH stretch and bend has also been invoked by Sibert, Reinhardt, and $Hynes6$ to explain the overtone spectrum of benzene. Our work confirms the usefulness of considering separately the large stretch-bend off-diagonal term, and clarifies the physical origin of this large, ubiquitous, and practically important coupling.