Reactivity index based on orbital energies.

Area of Science:

• Quantum Chemistry
• Chemical Kinetics

Background:

• Chemical reactivity is fundamentally governed by electronic structure.
• Understanding reaction pathways and rates is crucial in chemistry.

Purpose of the Study:

• To investigate the relationship between orbital energy gaps and chemical reactivity.
• To develop a predictive tool for reaction pathways based on electronic properties.

Main Methods:

• Calculation of orbital energy gap gradients (OEGG) for various reactions.
• Construction of a normalized reaction diagram plotting OEGG against the intrinsic reaction coordinate.
• Analysis of 43 fundamental reactions using the developed diagram.

Main Results:

• Forward reactions with minimal structural change exhibit low OEGGs, indicating charge transfer dominance.
• Backward reactions often show high OEGGs, correlating with significant structural transformations and slower rates.
• Anti-activation-energy reactions predominantly display high OEGGs in their backward pathways.
• Certain reactions (SN2, symmetric, methyl radical) show high OEGGs inconsistent with rate constants, suggesting alternative pathways.

Conclusions:

• Orbital energy gap gradients are key indicators of reaction mechanisms and rates.
• The normalized reaction diagram provides insights into reaction dynamics.
• Reactions can deviate from optimal pathways, especially when charge transfer is not spontaneous.