The near equilibrium potential energy surface and the harmonic vibrations of linear C 11 have been studied by CCSD(T) calculations with large basis sets. In agreement with matrix IR spectroscopy, large intensities are calculated for the antisymmetric stretching vibrations ν 7 and ν 8 . The wavenumber of the former band is predicted to have an unusually large anharmonicity contribution. Wavenumber shifts and changes in IR intensities arising from 13 C substitution are discussed in detail. Linear C 11 exhibits no sign of floppiness and appears to behave like a fairly normal semi-rigid molecule.