The IR active cis-bending vibrations of the linear carbon chains C-2n(n = 2-5) in their (3)Sigma(-)(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 C-4 and the present calculations support the assignment of an astronomical feature at 57.5 mu m (174 cm(-1)) to this molecule. (c) 2005 Elsevier B.V. All rights reserved.