Theory and modeling of ion–molecule radiative association kinetics

Abstract

Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A general formalism for evaluating the effective rate constant for radiative and collisional association is reviewed. The implementation of variable reaction coordinate transition state theory estimates within this formalism is described. A detailed discussion is given of the limiting cases of high and low stabilization efficiency. The basic validity of the algorithm is illustrated through sample calculations for the high efficiency limit. The low efficiency limit allows for the determination of binding energies which are independent of any transition state model. The relation between the predicted and observed temperature dependence in the low efficiency limit is explored. Sample calculations employing the general formalism illustrate the usefulness of this modeling in estimating the binding energy of the complex. Modest levels of quantum chemistry (e.g., MP2/6-31G*) are found to provide satisfactory estimates of the vibrational frequencies and intensities required in the modeling. Overall, the modeling provides estimated binding energies for the protonated acetone dimer, NO+...3-pentanone, and Al+...C6H6 complexes which agree with the available literature values to within 2 kcal/mol.