Is There a Quadruple Bond in C2?

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• The chemical structure of the C2 molecule, particularly the existence of a fourth covalent bond in its ground state, has been a long-standing debate in chemistry.
• Previous studies have proposed conflicting theories regarding the nature of bonding in C2.

Purpose of the Study:

• To definitively determine whether a quadruple bond exists in the ground state of the C2 molecule.
• To clarify the nature of the proposed "fourth" covalent bond in C2.
• To provide a robust theoretical framework for understanding chemical bonding in small molecules.

Main Methods:

• Application of the generalized product function energy partitioning (GPF-EP) method to calculate interference energy (IE).
• Analysis of IE for standard sigma and pi bonds, as well as the purported "fourth" bond in C2.
• Comparative analysis using the (3)Σ(-) excited state of the C3 molecule to validate findings.
• Examination of orbital overlaps along the bond axis to correlate with force constants.

Main Results:

• The interference energy (IE) analysis indicates that the "fourth" bond in C2 is a destabilizing factor, not a stabilizing covalent bond.
• Standard sigma and pi bonds in C2 exhibit expected interference energy behavior.
• Analysis of the C3 molecule's excited state supports the conclusion that the "fourth" interaction in C2 is not a conventional covalent bond.
• Differences in force constants between C2 and acetylene are explained by variations in internuclear charge density and orbital overlaps.

Conclusions:

• The C2 molecule does not possess a quadruple bond in its ground state.
• The proposed "fourth" bond is an artifact of the calculation method and represents a destabilizing interaction.
• The GPF-EP method provides a reliable tool for dissecting covalent bond contributions.
• Understanding internuclear charge density and orbital interactions is crucial for interpreting molecular bonding.