Coupled cluster study of the linear carbon chains C2n+1 (n = 5-9)

Making use of the coupled cluster variant CCSD(T) in conjunction with Dunning's cc-pVTZ basis set, equilibrium structures and complete harmonic force fields have been calculated for the linear carbon chains of type C2n+1 with n = 5-9. With the exception of C3, which is a well-known "floppy" molecule with an extremely shallow bending potential, all members of the C2n+1 series up to C19 appear to behave like fairly normal semirigid molecules. The IR active bending vibrations of lowest wavenumber for C17 and C19, which may be of interest to forthcoming far-infrared astronomy, are predicted to occur at 13.1 and 11.1 cm(-1), respectively, with corresponding absolute IR intensities of 6.6 and 5.9 km mol-1. Huge IR intensities are calculated for one antisymmetric stretching vibration per chain (19,948, 29,632, and 30,040 km mol(-1) for C15, C17, and C19, respectively). A quantitative description of these vibrations may require the explicit consideration of anharmonicity effects and electronic structure calculations going beyond CCSD(T).