Trends in Strong Chemical Bonding in C2, CN, CN-, CO, N2, NO, NO+, and O2.

The strong chemical bonds between C, N, and O play a central role in chemistry, and their formation and cleavage are critical steps in very many catalytic processes. The close-lying molecular orbital energies and large correlation effects pose a challenge to electronic structure calculations and have led to different bonding interpretations, most notably for C2. One way to approach this problem is by strict benchmark comparison of related systems. This work reports reference electronic structures and computed bond dissociation enthalpies D0 for C2, CN, CN-, CO, N2, NO, NO+, O2 and related systems C2+ and C2- at chemical accuracy (∼1 kcal/mol or 4 kJ/mol) using CCSD(T)/aug-cc-pV5Z, with additional benchmarks of HF, MP2, CCSD, explicitly correlated F12 methods, and four density functionals. Very large correlation and basis set effects are responsible for up to 93% of total D0. The order of the molecular orbitals 1πu and 3σg changes, as seen in textbooks, depending on total and effective nuclear charge. Linear trends are observed in 2σu-2σg orbital splitting (R2 = 0.91) and in D0 of C2, C2-, and C2+ (R2 = 0.99). The correlation component of D0 of C2 is by far the largest (∼93%) due to a poor HF description. Importantly, density functional theory fails massively in describing this series consistently in both limits of effective nuclear charge, and Hartree-Fock exchange or meta functionals do not remedy this 100 kJ/mol error, which should thus be addressed in future density functional developments as it affects very many studies involving cleavage or formation of these bonds.