Comment on “Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces”: The 7Σ heptet excited states for related molecules.

I noted in an earlier blog, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C₂. Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.[1] This looks at the so-called ⁷Σ open shell state of the molecule where three electrons from one σ and two π orbitals are excited into the corresponding σ^* and π^* unoccupied orbitals. The argument is presented that these states are not dissociative, showing a deep minimum and hence a latent quadruple bonding nature. They also note that the isoelectronic BN molecule IS dissociative.^† Thus to quote: “Hence, the proof of existence of a minimum in the ⁷Σ_u+ for C₂ and the absence of such a minimum in the equivalent case for BN is likely to corroborate our findings on quadruple bonding in these two cases.“

Although a PES (potential energy surface) is shown for ⁷Σ C₂, no vibrational wavenumber is reported. So in the spirit of a commentary on this pre-print, I have calculated these values (CCSD(T)/Def2-TZVPP) for the molecules noted in the article and a few other isoelectronic species. The results are collected at DOI: 10.14469/hpc/6599

The two singly occupied σ-orbitals are shown below and are in part responsible for the non-dissociative behaviour.

The results using this method (the article reports many more methods), reveal that C₂ does indeed exist in a minimum for the heptet state, whilst the isoelectronic BN is only very weakly bound, in effect dissociative. Another dissociative isoelectronic is BO¹⁺, whilst N₂²⁺ despite the coulombic repulsions of a double positive charge, still manages to be bound with a relatively short bond length. A further neutral diatomic BeO is also clearly non-dissociative.

Lets hope a spectroscopist somewhere tries to take a look at these heptet excited states of such molecules so that further experimental insight can be cast on this fascinating problem.

^‡The orbital occupancy of this species is different from the others. ^†As are the neutral ten valence electron N≡N (confirmed here) and HC≡CH.

References

1. I. Bhattacharjee, D. Ghosh, and A. Paul, "Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective", 2019. http://dx.doi.org/10.26434/chemrxiv.11446224.v1