Extensive Quantum Chemistry Study of Neutral and Charged C$_4$N Chains. An Attempt to Aid Astronomical Observation

TL;DR

This study uses advanced quantum chemical methods to characterize neutral and charged C4N chains, providing data that supports their potential presence in space and aiding future astronomical observations.

Contribution

It offers comprehensive quantum chemical data on C4N chains, including formation pathways, to assist in their detection and understanding in astrochemical environments.

Findings

Quantum methods accurately reproduce experimental electron attachment energies.

Calculated thermodynamic data suggest C4N can exist in space.

Potential formation pathways for C4N chains are identified.

Abstract

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 (C$_4$N$^0$) and anion (C$_4$N$^-$) cyanopropynylidene chains, which were not yet identified in space although both the neutral (C$_3$N$^0$ and C$_5$N$^0$) and anion (C$_3$N$^-$} and C$_5$N$^-$) 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 momenta, electron attachment energies ($EA$) and ionization potentials ($IP$). The fact that (not only for C$_4$N but also for C$_2$N and C$_6$N) 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 C$_4$N is absent in space. To further support this idea, we analyze potential chemical pathways of formation of both C$_4$N$^0$ and C$_4$N$^-$, which include association and exchange reactions.