A COUPLED CLUSTER STUDY OF LINEAR $C_{7}$: NO EVIDENCE OF FLOPPINESS

Abstract

Linear $C_{7}$ 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 $C_{7}$ 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 $Saykally^{a}$. A renewed analysis of the experimental data and possible remeasurement is strongly recommended. Rovibrational transitions within the $\nu_{11}$ band of linear $C_{7}$ are predicted around $70 cm^{-1}$ and thus rather close to transitions within the $\nu_{2}$ band of non-rigid $C_{3}$, but with very different spacing.