Ion–polar molecule collisions. Conservation of angular momentum in the average dipole orientation theory. The AADO theory

Abstract

The average dipole orientation (ADO) theory for capture collisions between ions and dipolar molecules is modified to include conservation of angular momentum approximately; termed AADO theory. The AADO theory is compared to experiment for proton transfer reactions from various positive ions to NH₃, HCN, and CH₃CN substrates and for several negative ions to HCN. The agreement between theory and experiment is quite good in all cases. The AADO rate constants are larger than the ADO rate constants in all cases. For the systems studied the average increases in AADO rate constants over ADO values were 12% (NH₃), 20% (HCN), and 31% (CH₃CN). For most polar molecules, AADO predictions should be about 10%–30% larger than ADO values. The theory has been parameterized in the form k_AADO= (2πq/μ^1/2)[α^1/2+Cμ_D (2/πkT)^1/2+(Zμ_DI^1/2/α^1/4)], where the first two terms are simply k_ADO and the last term comprises the correction due to angular momentum conservation. The constant C is the ADO dipole locking constant. The constant Z is a function of temperature only and a table of Z values for a wide temperature range is given. For molecules with moments of inertia I?1×10⁻³⁹g cm² a somewhat different parameterized equation must be used. As part of the development of the AADO theory, a comparison is made between ADO theory and a recent treatment of Barker and Ridge. A number of recommendations are made for modifying the Barker–Ridge approach. Implementation of these modifications brings essential agreement between the Barker–Ridge approach and the ADO theory.