The Hydration Kinetics of Sulfur Oxides

Research project P 14357 The Hydration Kinetics of Sulfur Oxides Klaus R. LIEDL 08.05.2000 We plan to study the reaction rate constants of the hydration of sulfur dioxide (SO2 ) and sulfur trioxide (SO3 )- Calculations will investigate the influence of water molecules forming reactive rings (these "reactive rings); are prototype examples of water-mediated proton transfer; cf. Figure 1) and water molecules outside these reactive rings (i.e., "microsolvation"). We also Want to study the effect of solvation by considering solvent in form of a dielectric continuum. In this way gas-phase description of dynamics will be extended step by step towards the description of condensed phase. To provide accurate predictions of the reaction rates we need to perform benchmark calculations for the reaction barriers within the reactive ring systems. Detailed reaction hypersurfaces will be constructed from density functional theory calculations to estimate the influence of nuclear quantum tunneling within the framework of variational transition state theory and semi-classical tunneling corrections. Primary project aims are the accurate predictions of the stability of H2 SO 3 and its oligomers at atmospheric conditions, the elucidation of the number of water molecules involved in the hydration reaction Of S02 and S03 (reaction chamber studies imply the involvement of water-dimers), the comparison of the hydration rate constants at different atmospheric conditions and finally the implications for the atmospheric sulfur cycle.

Water molecules are able to influence reaction rates in several different ways. They can build a reactive ring of water molecules, they can co-ordinate to the reactive complex outside the reactive ring and they can influence the reaction by their dielectric properties in the bulk phase as solvent. We investigated the effect of water molecules on the transfer of protons and hydrogen atoms respectively. Such proton- and hydrogen transfer reactions are a very common and important reaction type both in inorganic and organic chemistry as well as in biochemistry. These reactions stand out as their reaction rate constants are strongly accelerated due to quantum mechanical tunneling processes of atomic nuclei. Therefore it is necessary to use methods for their description that are able to reproduce tunneling probabilities adequately. For this purpose not only an exact thermodynamic description for educts and the transition state is necessary, as it is the case for transition state theory, but also a proper description of the reactions` hypersurface. On the basis of these data additionally to the classical reaction rates tunneling probabilities can be calculated by semiclassical methods. On the one hand our special interest was directed towards the kinetics of the hydration of sulfur oxides. We were able to suggest a reaction mechanism, that can explain the experimental findings. On the other hand we studied also the hydration of other inorganic oxides, especially the hydration of carbon dioxide under different conditions, starting from the gas-phase, over aqueous solution up to the enzymatic hydration. As a result we could show, why carbonic acid is stabil in its pure form, but rather decays quite fast in aqueous solution. Finally we investigated also some H-transfer reactions of compounds of importance in atmospheric chemistry. Accompanying to these studies we developed methological improvements, that finally led to a new method to predict tunneling splittings. These tunneling splittings are an indispensable tool to analyse hydrogen transfer in various systems of biological and biochemical interest.