"In Situ" Orbital Correlations

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Chemical Bonding Theory

Background:

• Traditional orbital correlation diagrams use isolated reactant orbital energies, failing to account for physical interactions.
• Orbital energy levels are significantly affected by external fields and neighboring molecules, leading to reshuffling.
• Existing models struggle to explain reactions where frontier molecular orbitals (MOs) alone are insufficient.

Purpose of the Study:

• To introduce and demonstrate the application of a novel concept: "in situ" orbital correlation.
• To account for physical effects (electrostatic, Pauli repulsion, van der Waals) on orbital energies during chemical interactions.
• To provide a more accurate method for understanding chemical reactions, electron transfer, and molecular bonding.

Main Methods:

• Utilized the block-localized wave function (BLW) method, a variant of ab initio valence bond (VB) theory.
• BLW self-consistently derives orbital energies in the presence of other species or external fields.
• Applied "in situ" orbital correlation to analyze CO activation by diboryne, NCCL- anions, and Al-Mg bonding.

Main Results:

• Demonstrated HOMO-LUMO swaps in B2(NHCR)2 upon CO approach due to Pauli repulsion, enabling orbital compatibility.
• Revealed orbital swaps in NCC- fragments upon ligand approach, confirming C(0) theory for NCCL- anions.
• Showed the Al-Mg bond is ionic, not dative covalent, by observing significant HOMO energy decrease and band gap extension upon Mg compound approach.

Conclusions:

• "In situ" orbital correlation accurately models orbital energy shifts caused by physical interactions.
• This method provides deeper insights into reaction mechanisms, electron transfer pathways, and bonding nature.
• The concept fundamentally enriches the understanding of chemical phenomena beyond traditional frontier molecular orbital theory.