Tafadar, Nurun Nabi; (2001) Reactions of molecular dications in the gas-phase. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents the results from a series of experiments investigating the reactivity of gas phase molecular dications with neutral collision partners, at collision energies between 3 and 13 eV in the laboratory frame using a crossed-beam apparatus. The experiments involve measurement of product ion intensities, which are determined by means of time of flight mass spectrometry. The experimental methodology, together with relevant theory is described in the thesis. The relative intensities of product ions formed are a powerful probe of the reaction mechanism. Where appropriate, the reactions are examined for isotope effects by using the isotopic analogue of the neutral collision partner. Our investigation of the CF32+/Ar collision system shows neutral loss and electron transfer dominating the product ion yield. The variation of the neutral loss ion yield with collision energy provides a first estimate of the bond energy of the weak CF22+ -F bond. Ab initio calculations indicate the ground state of adopts a C2v equilibrium geometry. We further conclude that at least two electronic states of CF32+ are present in the dication beam. Intramolecular isotope effects in the reactions of CO22+ and CF32+ with HD indicates the operation of an intramolecular isotope effect, favouring the formation of the deuterated products DCF2+ and DCO+. However, for the CF32+/HD system our data reveals no isotope effect for the formation of HF+ and the DF+ within our experimental uncertainty. Statistical effects have been suggested as an alternative to the orientational model previously used to explain these effects. In our investigation of the CF32+/H2/D2 and CO22+/H2/D2 collision systems, experiments indicate that no intermolecular effects are in operation and the observed collision energy dependence is symptomatic of the absence of a barrier to reaction. In the CF32+/H2/D2 system we observe the formation of the XF+ product ion; a reaction channel that has not been observed before.

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