Molecular orbital treatment of gas-phase ion molecule collision rates: Reactive and nonreactive collisions
✍ Scribed by Ralph C. Dougherty
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 196 KB
- Volume
- 20
- Category
- Article
- ISSN
- 0277-7037
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✦ Synopsis
Abstract
| I. | Introduction | 142 |
| II. | Reaction Efficiency | 144 |
| III. | Ion Polar‐Molecule Collisions | 145 |
| IV. | Momentum‐Transfer Ion‐Molecule Collisions | 146 |
| V. | Summary | 151 |
| Acknowledgments | 151 |
| References | 151 |
This paper reviews some applications of quantum mechanical models to ion‐molecule collision processes, and presents a new theoretical approach to ion mobilities of isobaric ions. Reactive ion‐molecule collisions have been successfully modeled, using perturbation molecular orbital (PMO) theory, and ab initio quantum mechanical models. In ion polar‐molecule collisions, the Stark effect exerts a controlling influence on the collision rate. The average dipole orientation for an ion polar‐molecule collision is exactly zero. The instantaneous dipole orientation to the ion‐molecule axis in these collisions is not zero. The instantaneous angle between the dipole and the ion‐molecule axis, Θ, increases as the distance between the ion and molecule, r, decreases. The Stark effect couples collisional angular momentum of the ion‐molecule complex and rotational angular momentum of the molecule. Conversion of collisional angular momentum to rotational angular momentum causes the rotation of the molecule to increase as the ion and molecule approach each other. Changes in Θ directly follow the changes in rotational angular momentum for the molecule. In momentum transfer ion‐molecule collisions in gases, the energy difference between the frontier orbitals of the collision pair is such that a first‐order perturbation model is not appropriate. An approximate second‐order PMO equation for the reduced mobility of ions in non‐polar gases, K~o PMO~, is presented and used to analyze data from the literature. Ion mobility is linearly related to K~o PMO~, a function of the ion‐molecule electronic potential, Δ__E__~IM~, and the reduced mass, μ~IM~, for the ion bath‐gas interaction. The electronic states of first‐row transition metal ions separated by ion‐mobility in helium have been assigned on the basis of their reduced mobilities. © 2001 John Wiley & Sons, Inc., Mass Spec Rev 20:142–152, 2001