Knowledge Bank

The rovibrational spectra of both the free ($\nu 1\prime$) and hydrogen bonded ($\nu 2\prime$) C-H stretches of $(HCN)2$ have been measured using the molecular beam bolometric detection technique. In the case of the free stretch, the transitions have instrument limited linewidths, indicating a very long vibrational predissociation lifetime. For the hydrogen bonded stretch, however, lifetime broadening of the transitions is observed. In the $\nu 2\prime$ band, many of the transitions associated with low J' states are significantly broader than those associated with higher J'. This effect is believed to result from perturbations due to Coriolis or anharmonic couplings with nearby vibrational states. In addition, several bands associated with higher clusters have been observed. One of these has been assigned as the free C-H stretch of a linear confomer of $(HCN)3$. Due to the small coupling between the free C-H stretch and the van der Waals bond, transitions associated with this band are found to be narrow. An additional band is observed to the red of the hydrogen bonded dimer band and is believed to correspond to the hydrogen bonded linear trimer stretches. In this case, however, no rotational fine structure is resolved. One final band is observed midway between the free and hydrogen bonded dimer bands. This spectrum is clearly associated with an oblate symmetric top and is most likely the cyclic conformer of the trimer. Further work is underway in an attempt to verify this latter assignment and to ensure that this band is not due to a higher order cluster.