The Chemical Bond in C2.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Understanding the nature of chemical bonds is fundamental in chemistry.
• The electronic structure and bonding in small molecules like C2 and acetylene (HC≡CH) are complex and have been subjects of theoretical investigation.
• Previous studies have proposed various bonding models for C2, but a consensus remains elusive.

Purpose of the Study:

• To investigate the bonding characteristics of the carbon-carbon bond in C2 and compare it with acetylene (HC≡CH) and nitrogen (N2).
• To determine the relative strengths of the C-C bonds in C2 and acetylene using advanced computational methods.
• To elucidate the nature of multiple bonding in C2 and its implications for its electronic structure.

Main Methods:

• Complete active space self-consistent field (CASSCF) calculations were performed for Hn CCHn (n=0-3) and N2.
• Quadratic force constants and stretching potentials were computed using the CASSCF/cc-pVTZ level of theory.
• Bond dissociation energies were calculated using explicitly correlated coupled-cluster configuration interaction (CASPT2-F12) with a cc-pVTZ-F12 basis set.

Main Results:

• The C-C bond in C2 was found to be weaker than the triple bond in acetylene.
• Analysis of CASSCF wavefunctions indicated four bonding components in C2, comprising two weakly bonding sigma (σ) bonds and two electron-sharing pi (π) bonds.
• Acetylene and N2 exhibited only three bonding components.

Conclusions:

• The bonding in C2 is better described as two σ and two π bonds, analogous to Be2 with enforced π bonding, rather than a simple quadruple bond.
• The assignment of quadruple bonding to C2 is misleading due to its lower bond strength compared to acetylene's triple bond.
• Quantum chemical calculations provide a detailed understanding of the complex bonding in C2, resolving ambiguities in its electronic structure.