A coupled cluster study of the IR active bending vibrations of C2n chains (n = 2–5) in their electronic ground states

The IR active cis-bending vibrations of the linear carbon chains C2n(n = 2–5) in their 3 Σ g - electronic ground states have been studied by means of coupled cluster calculations with large basis sets. All four molecules investigated have rather steep and almost quadratic cis-bending potentials and are thus likely to behave like fairly normal semi-rigid molecules. Harmonic cis-bending vibrational wavenumbers and the corresponding absolute IR intensities were calculated for all four species. Very good agreement with argon matrix IR spectroscopy is obtained for linear C4 and the present calculations support the assignment of an astronomical feature at 57.5 μm (174 cm−1) to this molecule.