Bonding conundrums in the C₂ molecule: A valence bond study

The ab initio VB study for the electronic structure of the C₂ molecule in the ground state is presented in this work. VB calculations involving 78 chemically relevant VB structures can predict the bonding energy of C₂ quite well. Sequentially, a VBCIS calculation provides spectroscopic parameters that are very close to full CI calculated values in the same basis set. Furthermore, the analysis of the bonding scheme shows that a triply bonded structure is the major one in terms of weights, and the lowest in energy at the equilibrium distance. The second structure in terms of weights is an ethylene-like structure, displaying a δ + π double bond. The structure with two suspended π bonds but no δ bond contributes only marginally to the ground state. This ordering of weights for the VB structures describing the C₂ molecule is shown to be consistent with the shape of the molecular orbitals and with the multireference character of the ground state. With the triply bonded bonding scheme, the natures of the π and δ bonds are investigated, and then the corresponding "in situ" bond strengths are estimated. The contribution of the covalent-ionic resonance energy to π and δ bonding is revealed and discussed.