The coupling of quantum chemistry and transition-state theory in the modeling of ion-molecule reaction kinetics

The coupling of transition state theory and quantum chemical calculations is applied to the exploration of the kinetics for ion-molecule reactions in this dissertation. The validity and usefulness of variable reaction coordinate-transition state theory (VRC-TST) is investigated in a number of these studies.;A potential energy surface based on the fitting of ab initio data is generated for the benzene cation dissociating to the phenyl cation and H atom. The energy and angular momentum resolved transition state number of states obtained by employing the ab initio potential are compared with those obtained for an empirically based model potential. The kinetic isotope effects of ;The application of density functional theory (DFT) to the prediction of infrared spectra for polycyclic aromatic hydrocarbons is investigated for naphthalene neutral and cation. The absolute IR absorption intensities from DFT are compared with those obtained from the experiments and from quantum chemical calculations at the Hartree-Fock (HF) level. Also, simulated cooling curves based on different sets of intensities are generated and compared to those observed in experimental measurements.;At low pressure, such as occurs in interstellar space, the stabilization of the energetic complex for ion-molecule associations is dominated by the emission of a photon. A modeling procedure combining VRC-TST with quantum chemical estimates is employed to study the kinetics of radiative association. In this modeling, the binding energy is the only adjustable parameter, which is determined via the fitting of the theoretically predicted radiative association rate constant to that observed experimentally. The modeling based binding energies are compared with those obtained from quantum chemical calculations and other experimental studies. This method is applied to the associations of protonated acetone with acetone and silver cation with various neutrals (i.e., acetaldehyde, acetone, benzene, isoprene, and 2-pentene).;An important component of the modeling is the quantum chemical estimation of the geometries, frequencies, and IR intensities. The moderate-level ab initio methods such as HF and second-order Moller Plesset perturbation theory are employed here. The analysis of the rate constants for each process is compared with the results from an alternative experimental modeling scheme, termed McMahon analysis.