Compactness aromaticity of atoms in molecules

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Chemical Reactivity Theory

Background:

• Aromaticity is a fundamental concept in chemistry, crucial for understanding molecular stability and reactivity.
• Existing aromaticity definitions often rely on electronic delocalization or energetic criteria, leading to varying interpretations.
• A need exists for a robust, universally applicable definition of aromaticity grounded in observable properties.

Purpose of the Study:

• To introduce a novel definition of aromaticity based on molecular compactness.
• To establish a benchmark aromaticity scale using the geometrical reactivity index of polarizability.
• To develop and validate new aromaticity scales using energetic reactivity indices like electronegativity and chemical hardness.

Main Methods:

• Formulation of aromaticity as a ratio of atoms-in-molecule to orbital molecular facets.
• Development of a new Hydrogenic polarizability quantum formula.
• Computation and analysis of aromaticity scales using electronegativity and chemical hardness via semi-empirical and ab initio quantum chemical methods.
• Introduction of five referential aromatic rules (Aroma 1 to 5 Rules).

Main Results:

• A polarizability-based aromaticity scale was established, serving as a benchmark.
• Chemical hardness-based aromaticity demonstrated better agreement with the polarizability scale compared to electronegativity-based aromaticity.
• Semi-empirical quantum chemical methods were found most suitable for chemical hardness-based aromaticity, while ab initio methods were preferred for electronegativity-based aromaticity.

Conclusions:

• The proposed compactness formulation offers a new perspective on defining aromaticity.
• Polarizability serves as a reliable observable for benchmarking aromaticity scales.
• Chemical hardness is a more consistent predictor of aromaticity than electronegativity within the tested computational frameworks.