Dipole-bound states contribution to the formation of anionic carbonitriles in the ISM: Calculations using multireference methods for C<inf>3</inf>N⁻

Abstract

We report the results of fixed-nuclei calculations for the excited, dipole-supported anionic states of the C3N− anion using the multireference configuration interaction (MRCI) method with Davidson perturbative correction, plus other similar multireference methods. Within the adiabatic approximation, we find that there are several low-lying bound states of 3Σ+ and 1Σ+ symmetry that lie in energy below the neutral molecule when we employ specifically selected multiconfigurational self-consistent field (MCSCF) reference functions for the subsequent MRCI. We further find that only one set of 3Σ+ and 1Σ+ states are present when using complete active space SCF (CASSCF) reference functions. No 1,3Π states were found to be bound by the present calculations. Furthermore, when employing very diffuse basis sets, we found that there are many states lying in the vicinity of the neutral species due to the large number of the neutral virtual molecular Hartree-Fock orbitals with energies close to the zero. On the other hand, when additionally considering that the rotational constant of the parent neutral species is about 0.02 meV, only a smaller number of such states have a computed binding energy above this threshold and could therefore be considered physically viable. We therefore surmise that the present calculations are helping us to shed more light on the physical features of these, very special anionic states of the title molecule, states also present in a larger class of linear carbon chain with supercritical dipoles.