Comprehending the quadruple bonding conundrum in C(2) from excited state potential energy curves

The question of quadruple bonding in C(2) 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 C(2), N(2), Be(2) and HC[triple bond, length as m-dash]CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the (2S+1)Σ(g/u) (with 2S + 1 = 1, 3, 5, 7, 9) states of C(2) and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N(2) and HC[triple bond, length as m-dash]CH, the presence of a deep minimum in the (7)Σ(+) state of C(2) and CN(+) suggests a latent quadruple bonding nature in these two dimers. Our investigations reveal that the number of bonds in the ground state can be determined for 2(nd) row dimers by figuring out at what value of spin symmetry a purely dissociative PEC is obtained. For N(2) and HC[triple bond, length as m-dash]CH the purely dissociative PEC appears for the septet spin symmetry as compared to that for the nonet in C(2). This is indicative of a higher number of bonds between the two 2(nd) row atoms in C(2) as compared to those of N(2) and HC[triple bond, length as m-dash]CH. 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.