Carbon chains of type C2n+1N(-) (n=2-6): A theoretical study of potential interstellar anions.

Linear anions of type C(2n+1)N(-) (n=2-6), which are expected to be good candidates for experimental investigation by microwave spectroscopy and radio astronomy, were studied by means of the coupled cluster variant CCSD(T). Making use of corrections taken over from HC(3)NC(3)N(-) and HC(5)N, accurate equilibrium structures ( approximately 0.0005 A accuracy in bond lengths) have been established for all five anions. The electric dipole moments increase strongly with increasing chain length. For C(13)N(-), a very large equilibrium dipole moment of 16.53 D (with respect to center-of-mass coordinate system, negative end of dipole at terminal carbon site) is predicted. The lowest vertical detachment energies, leading to (2)Sigma states of the radicals for C(3)N(-) and C(5)N(-) and to (2)Pi states in the case of the larger anions, are calculated to lie in the range of 4.40-4.63 eV. The ground-state rotational and quartic centrifugal distortion constants of C(5)N(-) are predicted to be 1389.4 MHz and 33.8 Hz, respectively. All anions studied appear to be fairly normal semirigid linear molecules. Throughout, good agreement with available matrix isolation IR spectroscopic data is obtained and many predictions of spectroscopic properties are made.