Extensive Quantum Chemistry Study of Neutral and Charged C₄N Chains: An Attempt To Aid Astronomical Observations

Many molecular species can presumably still be observed in space if they are adequately characterized chemically. In this paper, we suggest that this could be the case of the neutral (C₄N⁰) and anion (C₄N^–) cyanopropynylidene chains, which were not yet identified in space although both the neutral (C₃N⁰ and C₅N⁰) and anion (C₃N^– and C₅N^–) neighboring members of the homologous series were observed. Extensive data obtained from quantum chemical calculations using density functional theory (DFT), coupled-cluster (CC), and quadratic configuration interaction (QCI) methods for all charge and spin states of interest for space science (doublet and quartet neutrals, triplet and singlet anions, and singlet and triplet cations) are reported, e.g., bond metric and natural bond order data, enthalpies of formation, dissociation and reaction energies, spin gaps, rotational constants, vibrational properties, dipole and quadrupole moments, electron attachment energies (EAs), and ionization potentials (IPs). The fact that (not only for C₄N but also for C₂N and C₆N) the quantum chemical methods utilized here are able to excellently reproduce the experimental EA value, which is often a challenge for theory, is particularly encouraging, since this indicates that theoretical estimates of chemical reactivity indices (which are key input parameters for modeling astrochemical evolution) can be trusted. The presently calculated enthalpies of formation and dissociation energies do not substantiate any reason to assume that C₄N is absent in space. To further support this idea, we analyze potential chemical pathways of formation of both C₄N⁰ and C₄N^–, which include association and exchange reactions. In view of the substantially larger dipole moment (D_anion ≫ D_neutral), we suggest that astronomical detection should first focus on C₄N^– chains rather than on neutral C₄N⁰ chains.