Knowledge Bank

Linear $C7$ has been studied by means of coupled cluster calculations including connected triple substitutions (variant CCSD(T)) and a large basis set of 385 contracted Gaussian type orbitals. The calculations indicate that linear $C7$ is a fairly normal semi-rigid molecule with no sign of floppiness. In particular, excitation of the bending vibration with lowest wavenumber $(\nu 11)$ produces only a 0.2% change in the rotational constant. The results of the present calculations are at variance with an experimental study by Heath and $Saykallya$. A renewed analysis of the experimental data and possible remeasurement is strongly recommended. Rovibrational transitions within the $\nu 11$ band of linear $C7$ are predicted around $70cm-1$ and thus rather close to transitions within the $\nu 2$ band of non-rigid $C3$, but with very different spacing.