The C9 cluster: structure and infrared frequencies

In this Letter we announce the first structural characterization of C9, the largest cluster predicted to have a linear ground state. We have used infrared diode laser absorption spectroscopy of a supersonic carbon cluster jet to measure 51 lines of the nu 6 (sigma u) antisymmetric stretch fundamental transition within the linear 1 sigma g(+) ground electronic state of C9. The measured ground state rotational constant 429.30(50) MHz] indicates an effective C-C bond length of 1.278 68 (75) angstroms, in good agreement with ab initio theory and consistent with cumulenic bonding in this cluster. In addition, the molecule is found to have unusually large ground-state and upper-state distortion constants, and several discrete perturbations are observed in the upper state.