I noted in an earlier blog, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C 2 . Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.[cite]10.26434/chemrxiv.11446224.v1[/cite] This looks at the so-called 7 Σ open shell state of the molecule where three electrons from one σ and two π orbitals are excited
The question of quadruple bonding in C2 has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C2, N2 and Be2 and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the 2S+1Σg/u (with 2S+1=1,3,5,7,9) states of C2 and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N2 and HC≡CH, the presence of a deep minimum in the 7Σ state of C2 and CN+ suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.