Structure and electronic spectra of the C anion

Ab initio calculations are employed for determining structures, spectroscopic parameters and transitions to the excited electronic states of the astrophysical relevant C6 anion, which is a potential intermediate of chemical processes involving carbon chains and polycyclic aromatic hydrocarbons. Calculations confirm the prominent stability of linear carbon chain anions which guarantees their formation. The lowest doublet potential energy surface of C6 displays six isomers, whereas in the lowest quartet potential energy surface, a unique minimum energy geometry is found. The most stable form is the linear anion l-C6 (X 2 u) followed by the c-C − 6 (X A1’) at 9291 cm−1 above in energy, which presents a distorted D3h structure. The other isomers are also located at more than 9000 cm−1 above l-C6 (X 2 u). For each isomer, a set of spectroscopic parameters including their equilibrium structures, rotational constants at equilibrium, harmonic wavenumbers, dipole moments and electron affinities are predicted. For linear C6 , eight electronic states lying below the electron detachment threshold, are computed. Four doublet electronic states, which play important roles in the C6 reactivity, lie below 2 eV. Spin–orbit constants for the upper electronic states of l-C6 are predicted.