15min:

PETER BOTSCHWINA, Institut für Physikalische Chemie, Universität Göttingen, Tammanstraß e 6, D-37077 Göttingen, Germany.

\noindent 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 ( 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 renewed analysis of the experimental data and possible remeasurement is strongly recommended. Rovibrational transitions within the ₁₁ band of linear C₇ are predicted around 70 cm^-1 and thus rather close to transitions within the ₂ band of non-rigid C₃, but with very different spacing.