Abstract
The research described in this thesis concerns an investigation of the kinetics and mechanisms of two gas phase chemical reactions. It is thus convenient to divide the work into these two main parts. The first part deals with the oxidation of carbon monoxide by nitrous oxide. Many gas phase oxidation processes have been studied. Often these have involved molecular oxygen as the oxidant, and the results have been confused by the occurrence of branched chain reactions leading to explosions and results that are highly dependent on the nature of reaction vessel surface and the presence of trace impurities. Nitrous oxide was used in this work as the oxidant because it was expected that no branching of the chains resulting in the observation of explosion limits, would occur. On reaction with a univalent radical, nitrous oxide is likely to give nitrogen and one further univalent radical : R + N2O → N2 + OR (R is any free radical) which is a chain propagating step. The analogous reaction with molecular oxygen is a chain branching process : R + O2 → OR + O. It was thus hoped that by studying oxidation by nitrous oxide rather than oxygen more tractable data on combustion processes would be obtained, and also that some information on the reactions of oxygen atoms (produced by thermal decomposition of N2O) might be forthcoming. The oxidation of carbon monoxide is known to be an important final stage in the burning of hydrocarbon fuels. Since Dixon's discovery in 1880 that water vapour had a marked influence on the combustion of carbon monoxide [37], the CO/O2 reaction has been extensively investigated. However, the mechanism is not completely understood, and the role of water is still obscure. On the other hand, since the rate constants of most of the elementary steps that may be involved in the carbon monoxide/nitrous oxide reaction are known to some extent, a semiquantitative evaluation of postulated reaction mechanisms is feasible. So that theory may be closely compared with experimental results. The second major section of this thesis also concerns water, but its environment was more unusual, Ion-molecule reactions were investigated in the ion-source of a mass spectrometers Ion-molecule reactions in water, giving rise to such species as H3O+ are of obvious importance in solution chemistry, and the application of gas phase methods to the study of solvated species is arousing a great deal of interest [100]. A number of theories have been developed to predict and explain reactions between gas phase ions and molecules. One of the more successful of these was first developed by Gioumousis and Stevenson [101] in which they considered the path of the ion in the potential field set up between the ion and the ion-induced-dipole of the neutral molecule. They treated both the ion and the molecule as hard spheres and predicted rate constants entirely dependent on physical parameters such as the charge, mass, velocity of the primary ion, and mass and polarizability of the neutral species, rather than on any specific chemical properties of the reactants. The ion-induced dipole model proposed by Gioumousis and Stevenson is clearly an excellent approximation to the truth for many reactions between ions and non-polar neutral molecules. Comparatively little work has been carried out on reactions between ions and polar molecules, so it was decided to investigate an important reaction of this type.