Champkin, Phillip; (2000) Computational studies of the properties of molecular dications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Computational methods have been used to investigate properties of the doubly charged molecules (dications), SiF32+, O32+ and BCl2+. Ab Initio electronic structure calculations have been performed on all systems to provide information on equilibrium geometries and ground and excited state energies. Other computational techniques have also been used dependent on the system under study. For SiF32+, relative product ion intensities following electron-transfer reactions between SiF32+ and the rare gases neon, argon, krypton and xenon have been rationalised using a combination of ah initio electronic structure techniques and Landau-Zener reaction window theory. The calculations show that the experimentally observed products derived from the dications (SiF3+, SiF2+ and SiF+) require the ions in the dication beam to be present in three different electronic states. The predicted and experimental product ion distributions, given this energy distribution, are in very close agreement. The combined computational approach adopted in this study is valuable for large molecular systems where the reactant molecules have several degrees of freedom and adopt markedly different equilibrium geometries depending on the degree of ionisation. The theoretical study of the ozone dication O32+was prompted by experimental studies into the double ionisation of neutral ozone. No stable O32+ ions were detected in the experiments and this study determines a possible mechanism for the rapid dissociation of the ozone dication upon formation via double ionisation of neutral O3. The dication ground singlet state is found to have a linear equilibrium geometry that is stable with respect to dissociation to O+ and O2+. However at the Franck-Condon zone for formation of O32+ from the neutral molecule the singlet potential energy surface intersects with a dissociative triplet state. It is proposed that crossing to this dissociative triplet state can account for the absence of any long lived O32+ ions in the electron impact mass spectrum of ozone. For BCl2+ state averaged complete active space, SA-CAS, calculations have been used to determine the stabilities of the ground electronic state and 34 excited states. Vibrational level energies and tunnelling lifetimes have been determined for those states found to be quasibound. The majority of the states are found to be unbound, and of the bound states the ground Σ state has the largest barrier to dissociation at 2.57 eV. Nine other excited states are found to be quasibound, although crossing to unbound states through strong spin orbit coupling is likely to reduce the lifetime of these states. The long lifetime of the ground state and the presence of other low lying quasibound states suggest that BCl2+ could be made and studied successfully in electron ionisation experiments.

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