Ab initio molar volumes and Gaussian radii

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Understanding ion-molecule interactions is key in various chemical and biological processes.
• The Hofmeister series describes specific ion effects, but their underlying mechanisms require precise atomic-level data.
• Accurate radii are fundamental for modeling electrostatic and dispersion forces.

Purpose of the Study:

• To develop a method for calculating ion and neutral molecule radii using ab initio molar volumes and a Gaussian electron distribution model.
• To derive radii applicable to calculating nonelectrostatic ion-ion dispersion forces, explaining Hofmeister effects.
• To investigate the impact of hydration shells and electron overlap on electrostatic calculations for different ion types.

Main Methods:

• Ab initio calculation of molar volumes.
• Application of a spatially diffuse electron distribution model with Gaussian spread to derive radii.
• Derivation of equivalent hard-sphere radii and comparison with crystalline ionic radii.
• Calculation of Born electrostatic self-energy for the Gaussian model.
• Determination of spherical and nonisotropic ellipsoidal radii using static polarizability tensors.

Main Results:

• Gaussian radii were obtained and shown to be applicable for computing ion-ion dispersion forces underlying Hofmeister effects.
• Equivalent hard-sphere radii showed good agreement with established crystalline ionic radii.
• The study provides guidelines for incorporating hydration shells and electron overlap in electrostatic calculations based on ion hydration strength (cosmotropic vs. chaotropic).
• Both spherical and nonisotropic ellipsoidal radii were calculated for various ions.

Conclusions:

• The Gaussian electron distribution model offers a robust method for deriving fundamental radii for ions and molecules.
• These radii are essential for accurate modeling of inter-ionic forces and electrostatic interactions, particularly in aqueous solutions.
• The findings provide critical insights into the factors influencing ion behavior in solution, advancing the understanding of phenomena like the Hofmeister effect.