Energy Ordering of Molecular Orbitals

Area of Science:

• Physical Chemistry
• Surface Science
• Quantum Chemistry

Background:

• Molecular orbitals are fundamental to understanding chemical bonding.
• Computational chemistry methods often rely on calculating molecular orbitals.
• The real-world existence and experimental observability of molecular orbitals remain key questions.

Purpose of the Study:

• To experimentally investigate the physical reality of molecular orbitals.
• To compare experimental orbital energies with theoretical calculations.
• To understand the role of electron correlation in molecular orbital ordering.

Main Methods:

• Photoemission tomography was used to study 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) monolayers on three silver surfaces.
• Photoelectron angular distributions were analyzed to assign specific molecular orbitals.
• Density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations were performed for comparison.

Main Results:

• Experimental observation of molecular orbitals was achieved through photoemission tomography.
• Deviations in energy ordering were found between DFT calculations and experimental data.
• CASSCF calculations successfully explained the experimentally observed orbital ordering, indicating the significance of static electron correlation.

Conclusions:

• The study provides experimental evidence for the physical reality and robustness of the molecular orbital concept.
• It highlights the limitations of (semi)local approximations in DFT for describing electron correlation effects.
• Molecular orbitals can be experimentally observed, bridging theory and reality in chemical bonding.