Theoretical and experimental studies of the structure and reactivity of doubly-charged ions in the gas phase
The purpose of the research described in this thesis is to investigate certain aspects of the reactivity of doubly-charged cations (dications) with neutral molecules in the gas phase, specifically, the mechanisms governing the bond-forming reactivity of dications with neutrals as well as the degree of state-selectivity involved in their electron transfer reactivity. Computational investigations of the mechanisms governing the bond-forming reactivity in the following systems have been performed: CF22+ + H2O Ar2+ + NH 3 and CF32+ + H2. All of these systems are found to possess a well in their potential energy surfaces corresponding to an associated complex between the dication and the neutral. For the systems involving molecular dications (CF22+ and CF32+), the mechanisms that emerge from the quantum chemical calculations indicate a pattern of reactivity that is in support of computational and experimental studies previous to and concurrent with the work presented in this thesis. This mechanistic pattern involves three main steps: bond formation, rearrangement and fragmentation, indicating a degree of complexity in bond-forming dication-molecule reactions that had not been previously expected. The mechanisms calculated quantum chemically were compared with experimental studies on the same system, either performed previously by others or as part of the work for this thesis, and found to be in good agreement. Crossed-beam collision experiments employing time-of-flight mass spectrometry analysis of the products were performed on two dication-neutral systems: CF22+ + D20 and SF2+ + H 2O. The results of experiments on CF22+ + D20 were compared with previous experimental studies on CF22+ + H2O in order to determine the possible existence of an intermolecular isotope effect. No isotope effect was observed within the experimental error, in agreement with our predictions from theoretical rate constant calculations. Experiments on the SF2+ + H2O system were carried out in order to determine the relative degree of dissociative and non-dissociative electron transfer. The ratio of the product ions corresponding to these two processes, S+/SF+, was then compared with a ratio determined from theoretical calculations. These calculations involved the determination of Landau-Zener probabilities for forming products in a given electronic and vibrational state, as well as Franck-Condon factors for the electron capture of SF2+, in order to determine the degree of state-selectivity in the formation of electron transfer products. Good agreement was found between the experimental and theoretical S+/SF+ ratios and it was also found that only excited states contributed to the formation of electron transfer products.