Bonding Conundrums in the C2 Molecule: A Valence Bond Study

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Molecular Spectroscopy

Background:

• The electronic structure and bonding in the C2 molecule have been a subject of theoretical interest.
• Understanding the bonding in C2 is crucial for comprehending the behavior of small carbon clusters.

Purpose of the Study:

• To investigate the electronic structure and bonding of the C2 molecule in its ground state using ab initio Valence Bond (VB) methods.
• To determine the dominant bonding configurations and their contributions to the overall molecular stability.

Main Methods:

• Ab initio Valence Bond (VB) calculations were performed.
• A large set of 78 chemically relevant VB structures were included.
• Valence Bond Configuration Interaction (VBCIS) calculations were used to obtain spectroscopic parameters.

Main Results:

• VB calculations accurately predict the bonding energy of C2.
• A triply bonded structure is identified as the major contributor to the ground state, being the lowest in energy.
• An ethylene-like structure with a σ + π double bond is the second most significant contributor.
• The calculated spectroscopic parameters closely match full Configuration Interaction (CI) results.

Conclusions:

• The ground state of C2 is well-described by a combination of VB structures, with a dominant triple bond.
• The bonding scheme is consistent with molecular orbital shapes and the multireference character of the ground state.
• The study provides insights into the nature and strength of π and σ bonds in C2, including covalent-ionic resonance contributions.