A coupled cluster study of linear C7: no evidence of floppiness

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 (ν11) 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 renewed analysis of the experimental data and possible remeasurement is strongly recommended. Rovibrational transitions within the ν11 band of linear C7 are predicted around 70 cm−1 and thus rather close to transitions within the ν2 band of non-rigid C3, but with very different spacing.