The dicarbon bonding puzzle viewed with photoelectron imaging

Area of Science:

• Quantum Chemistry
• Molecular Spectroscopy
• Chemical Bonding Theory

Background:

• The electronic structure and bonding in the dicarbon (C2) molecule remain a subject of scientific debate.
• Previous theoretical models proposed various dicarbon bond orders, including single, double, or triple bonds, leading to conflicting interpretations of its ground state.

Purpose of the Study:

• To experimentally determine the electronic configuration and bonding characteristics of the neutral dicarbon molecule (C2).
• To resolve the controversy surrounding the bond order in the ground state of C2.

Main Methods:

• High-resolution photoelectron spectroscopy of the C2 anion was performed.
• Photoelectron imaging techniques were utilized across a range of wavelengths.
• Analysis of electron angular anisotropies provided insights into orbital character.

Main Results:

• Photoelectron spectra revealed distinct ground (X 3Πu) and excited (A 3Πu) electronic states of neutral C2.
• Electron detachment patterns indicated the involvement of predominantly sigma (σ) and pi (π) orbitals.
• The observed orbital characteristics contradicted theoretical predictions for high bond-order models involving significant sigma-pi mixing.

Conclusions:

• The experimental findings strongly suggest a double-bonded configuration for the ground state of C2.
• The bonding in C2 is best described by two sigma (σ) bonds and one pi (π) bond, refuting models with higher bond orders.
• This study clarifies the fundamental bonding nature of the simplest neutral dicarbon molecule.