Collision cross section calculations for polyatomic ions considering rotating diatomic/linear gas molecules

Area of Science:

• Physical Chemistry
• Analytical Chemistry
• Computational Chemistry

Background:

• Ion collision cross sections (CCS) are crucial for gas-phase ion structural characterization using techniques like ion mobility spectrometry.
• Existing CCS prediction methods often model gas molecules as spherical or non-rotating, which is inadequate for diatomic gases like nitrogen and air.
• Accurate comparison between experimental CCS measurements and theoretical predictions necessitates refined models for gas-ion interactions.

Purpose of the Study:

• To adapt a momentum transfer-based CCS calculation approach for rotating diatomic gas molecules colliding with polyatomic ions.
• To compare CCS predictions using diatomic gas models with those using spherical gas models for various ion types.
• To investigate the influence of different collision rules (specular-elastic, diffuse-inelastic) and ion-induced dipole potential on CCS calculations.

Main Methods:

• Adapted a momentum transfer-based CCS calculation method to incorporate rotating diatomic gas molecules.
• Performed CCS calculations for spherical ions, tetra-alkylammonium ions, and polyethylene glycol ions.
• Utilized specular-elastic and diffuse-inelastic collision rules to model energy exchange between ions and gas molecules.
• Examined the effect of the ion-induced dipole potential on CCS predictions.

Main Results:

• CCS calculations using diatomic gas models showed distinct trends compared to spherical gas models, with CCS decreasing under specular-elastic rules and increasing under diffuse-inelastic rules.
• The study highlights the coupled nature of ion and gas molecule structural models, potential energy fields, and kinetic energy exchange in CCS calculations.
• Discrepancies between predicted and measured CCS are minimized when considering the rotational dynamics of diatomic gas molecules and appropriate collision models.

Conclusions:

• Accurate CCS prediction and interpretation of experimental data require explicit consideration of the structural model of both the ion and the gas molecule.
• The degree of kinetic energy exchange during ion-molecule collisions significantly impacts CCS predictions, necessitating careful selection of collision rules.
• This refined computational approach enhances the structural characterization of ions in gas-phase experiments utilizing diatomic buffer gases.