High pressure ESR-investigations of exchange reactions

In recent years ionic liquids (ILs) have become increasingly popular as solvents in chemistry. They are environmentally friendly alternatives to organic solvents and are already widely used in chemical synthesis and catalysis. Due to their ionic, or "two-component", structure, the properties of an IL may be changed according to the ions used. Future perspectives even indicate that ILs may be tuned towards certain properties, creating a virtually infinite set of "designer solvents". With the proposed research project, the main goal is to provide information on how simple chemical reactions, so- called exchange reactions, take place in ILs and to compare them to reactions in organic solvents. One type of exchange reaction is a simple electron transfer (ET) between the partners of a redox couple and such self-exchange reactions are often described using the Marcus Theory. Additionally, we will study spin exchange reactions, thus leading it information on diffusion processes in the IL. The intended reactions for study involve radical cations and anions as well as neutral radicals. The method of observation will be electron spin resonance (ESR) spectroscopy, which is the method of choice for studies on such reactions. Apart from studying the exchange reactions in ILs at ambient conditions, we intend to perform measurements at variable temperatures and pressures. These will yield the activation parameters of the respective processes, which may then be interpreted within the Marcus Theory. Additionally, we will be looking at solvent dynamic effects, which are known to be present for some ET reactions in organic solvents, where the dielectric relaxation properties of the solvent influence the reaction. Not much is known about the extent of these effects in ILs and we hope to provide information on the matter during the course of the project. A certain parameter, the resonance splitting energy, which plays a central role in the Marcus Theory as well as in related theories can be obtained using near infra-red (NIR) spectroscopy. We intend to do so for the proposed systems in order to complete the energetic picture and so provide the best possible understanding of the processes studied. Moreover, we shall do so at variable temperature and pressure, something which is not often attempted. For both NIR and ESR, especially the pressure dependencies are scarce, but we believe that they are important in order to properly understand the reactions at hand as well as the theories used to describe them. For ESR, our department already possesses a high-pressure facility for the proposed studies and we plan to implement an UV- VIS-NIR high-pressure cell as part of this project. In conclusion, we believe that the proposed work will provide valuable information on ILs and processes that take place therein.

As one of most significant scientific results of this project is the experimental proof that the currently widely used Marcus-Theory of electron transfer reaction in solution (Nobel Prize in Chemistry 1992) must be modified when applied to room-temperature ionic liquids as solvents. Marcus-Theory is well established and gives an excellent description of electron transfer reactions as well as their kinetic and the energetic behavior. All predictions made by the theory have been verified sooner or later in detail. Even the peculiar behavior of the so-called Marcus-Inverted Region was found experimentally. This is a region where rates of electron transfer reactions first increase and finally decrease with decreasing driving forces. Especially, intermolecular electron-self exchange reactions where no chemical bonds are broken or newly formed and the corresponding heterogeneous electrochemical electron transfer reactions on the surface of an electrode have been used to test the theory in respect of the size dependence of the reactants and the solvent dependence. Latter expressed by the solvent properties like dielectric constant and refractive index summarized by the so-called Pekar factor and creating the polarization effects based on the charge transfer reactions. Within this project this polarization concept should be tested experimentally if applicable to room temperature ionic liquids, a new class of solvent beginning to be widely used in industrial and scientific investigations. It is found that the concept of outer-sphere reorganization energy based on the Pekar-factor cannot be applied for ionic liquids. This is shown for various homogeneous and heterogeneous electron-transfer reactions in a variety of different ionic liquids. It seems that the reorientation of the solvent dipole moments contributing to the activated complex is not going to occur, since the dipole moment is an ill-defined quantity for ionic-liquids. In contrast to electron transfer reactions in classical organic solvents, in ionic liquids the experimental activation energies are dominated by the inner-sphere reorganization energies, responsible for the changes in bond lengths under charge transfer, and the temperature dependent of the viscosities of the ionic liquids. The outer-sphere reorganization energy in sense of Marcus Theory does not play any significant role in ionic liquids.Additionally, the high viscosity of the ionic liquids made it necessary to investigate in detail corresponding diffusion coefficients and diffusion behavior. This was done by additional electrochemical investigations resulting in different diffusion coefficients for charged and uncharged species. Such a finding is different from that obtained in classical organic solvents. The results obtained maybe of interest for the industrial use of ionic liquids.