Extracting covalent and ionic structures from usual delocalized wave functions: the electron-expansion methodology

Area of Science:

• Quantum Chemistry
• Computational Chemistry

Background:

• Chemical bonds are typically described using delocalized wave functions.
• Understanding the local covalent and ionic character of bonds is crucial for predicting molecular properties.
• Existing methods may not fully capture the nuances of electron distribution in different bond types.

Purpose of the Study:

• To develop a method for calculating local covalent and ionic bond weights from standard delocalized wave functions.
• To investigate the role of electronic populations, including three-electron populations, in determining bond localization and character.
• To analyze the specific behavior of Charge-Shift bonds and their covalent-ionic interactions.

Main Methods:

• Utilizing the electron-expansion methodology to expand hole conditions in terms of electrons.
• Deriving relations applicable to both Hartree-Fock (HF) and correlated levels of theory.
• Expressing covalency/ionicity and bond localization using electronic populations.

Main Results:

• Developed programmable expansions for calculating local covalent and ionic bond weights.
• Established that three-electron populations are key to bond localization.
• Demonstrated that for two-electron/two-center (2e/2c) bonds, including Charge-Shift bonds, three-electron populations are not critical for covalency/ionicity.
• Applied the method to butadiene, hexatriene, and pyrrole using natural atomic orbitals (NAOs) and pre-NAOs.

Conclusions:

• The electron-expansion methodology provides a robust framework for analyzing chemical bond character.
• The study offers new insights into the electronic basis of covalent and ionic bonding, with implications for understanding molecular structure and reactivity.
• The findings are applicable to various molecules and computational levels, enhancing the general utility of the approach.