Coupled cluster calculations for (potential) interstellar anions: The C2nH− series (n = 2–6)

The coupled cluster variant CCSD(T) in conjunction with a large basis set has been used to calculate spectroscopic properties for anions of type C2nH− (n = 2–6), three of which were recently observed in the interstellar medium by means of radio astronomy. Accurate equilibrium structures and electric dipole moments are predicted for all five species. A variety of spectroscopic constants (vibrational wavenumbers, vibration–rotation coupling constants, l-type doubling constants and centrifugal distortion constants) are reported for various isotopomers of C4H− and C6H−. The harmonic bending vibrations of chains up to C10H− and HC9N are analyzed by means of “mass plots” which draw the wavenumbers of interest as a function of the “hydrogen” mass, which is regarded as a continuous variable.