Resonance and aromaticity: an ab initio valence bond approach

Area of Science:

• * Quantum Chemistry
• * Theoretical Chemistry
• * Aromaticity Studies

Background:

• * Resonance energy is a key metric for quantifying aromaticity in molecules.
• * The Valence Bond Self-Consistent Field (VBSCF) method provides a framework for calculating electronic structure and resonance energies.
• * Previous studies have explored various computational approaches to understand aromaticity.

Purpose of the Study:

• * To investigate the impact of different orbital models (local vs. delocalized) on calculated resonance energies.
• * To analyze the contributions of various conjugated circuits to overall resonance energy.
• * To refine empirical parameters used in existing theories of conjugated circuits.

Main Methods:

• * Employed ab initio Valence Bond Self-Consistent Field (VBSCF) calculations.
• * Utilized two types of p-orbitals: strictly atomic (local) and delocalized atomic (delocal).
• * Constructed Valence Bond (VB) wave functions from linear combinations of VB structures resembling Kekulé structures.

Main Results:

• * Delocalized orbitals increased resonance energy calculations compared to local orbitals, particularly for orthogonalized structures.
• * Resonance in 6π electron conjugated circuits showed the most significant contribution to resonance energy.
• * The enhanced stability of phenanthrene over anthracene was attributed to π-system resonance, not H-H interactions.

Conclusions:

• * The choice of orbital model significantly influences calculated resonance energies.
• * Resonance contributions from specific conjugated circuits are crucial for understanding molecular stability.
• * Empirical parameters in conjugated circuit theories require scaling by structure coefficients for accurate quantification.