Refining the VSEPR Model: Limits on the Lone Pair Size as Described by the Electron Localization Function

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Modeling

Background:

• The Valence Shell Electron Pair Repulsion (VSEPR) principle predicts molecular shapes based on electron pair repulsion.
• Exceptions exist where asymmetric lone pairs lead to bond angles smaller than idealized shapes.

Purpose of the Study:

• Investigate exceptions to the VSEPR principle concerning asymmetric lone pairs.
• Clarify the spatial influence of lone pairs on molecular geometry and bond angles.

Main Methods:

• Computational structure optimization of small molecules and polyatomic anions.
• Topological analysis using the electron localization function (ELF).
• Natural Bond Orbital (NBO) analysis.

Main Results:

• Lone pairs occupy more volume around a central atom than single covalent bonds.
• Lone pairs exert greater steric influence in lower oxidation states.
• Larger, multiply bonded, or less electronegative substituents reduce lone pair steric demands and expand bond angles.

Conclusions:

• Understanding lone pair volume is crucial for predicting molecular geometry.
• Ligand close packing and Bent's rule are consistent with observed steric effects.
• Molecular structure is a complex interplay between lone pair repulsion and substituent properties.