Atomic Charges in Describing Properties of Aromatic Molecules

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Organic Chemistry

Background:

• Accurate atomic charges are vital for understanding substituent effects in aromatic systems.
• Several population analysis methods exist, but their performance varies.
• Experimental data provides a benchmark for evaluating theoretical methods.

Purpose of the Study:

• To assess the performance of four population analysis methods: Mulliken, NPA, Hirshfeld, and QTAIM.
• To compare calculated atomic charges with experimental substituent constants (σ ⁰ _m and σ ⁰ _p).
• To determine the most reliable method for describing electron density distribution and substituent effects in monosubstituted benzenes.

Main Methods:

• Calculated atomic charges using B3LYP, ωB97X-D DFT, and MP2 MO methods with the 6-311++G(3df,2pd) basis set.
• Investigated benzene and 18 monosubstituted derivatives.
• Correlated atomic charges with experimental σ ⁰ constants.

Main Results:

• Established a quantitative correspondence between Hirshfeld charges and σ ⁰ constants.
• MP2 calculations were essential for realistic electron density distribution.
• NPA and QTAIM charges generally provided satisfactory descriptions of substituent effects.
• Assessed net charge transfer between substituents and the aromatic ring.

Conclusions:

• The Hirshfeld method offers a reliable approach for quantifying substituent effects in benzene derivatives.
• MP2 calculations are recommended for accurate electron density analysis in such systems.
• NPA and QTAIM methods are suitable alternatives when MP2 is not feasible, providing a good description of substituent influences.