Electronic Structure and Bonding of the Fastidious Species CN₂ and CP₂: A First-Principles Study

The transient molecular species CN₂ (CNN, NCN, c(yclic)-CN₂) and CP₂ (CPP, PCP, c(yclic)-CP₂), along with the isoelectronic to CNN and isovalent to CPP, CCO, have been studied theoretically through the ab initio methodologies multireference configuration interaction (MRCI) and RCCSD(T) coupled with augmented correlation-consistent quintuple and sextuple basis sets. For the CNN, NCN, and c-CN₂ molecules, the examined states are [X̃³Σ^-, ã¹Δ, b̃¹Σ⁺, óΠ, and c̃¹Π], X̃³Σ_g^-, and X̃¹A₁, respectively. The analogous phosphorous system CPP has been studied theoretically for the first time. Our results show that the symmetries ³Σ^-, ¹Δ, and ¹Σ⁺ are not stationary states; thence, the ground state of CPP is of ³Π symmetry and of similar electronic structure to that of the Ã³Π state of CNN. For most of the symmetries studied, we have constructed fully optimized potential energy profiles or "cuts" through the corresponding surfaces at the MRCI level of theory in an effort to follow the (valence) electronic distributions from the "selected" adiabatic species to equilibrium. Our numerical results are in excellent agreement with existing experimental data and previous, although limited, high-level ab initio calculations. Finally, it should be said that some of our findings like dissociation energies, permanent electric dipole moments, and bonding considerations are addressed for the first time.