Understanding and Manipulating The Reactivity of Nitroxides and Other Stable Free Radicals

Abstract

In this thesis, computational quantum chemistry has been used to investigate the factors influencing the reactivity of nitroxides and other stable free radicals, and their relevance to the important biochemical processes and useful practical applications. At all stages, accuracy and appropriateness of the chosen theoretical procedures was validated via benchmarking against higher levels of theory and/or experimentally measured values. Results of the theoretical modelling were analysed in the context of available literature data and, where relevant, in the context of numerical kinetic models, and the following key discoveries were made. A new type of electrostatic effect on radical stability, that is potentially general and non-directional and arises in the enhanced polarisability of highly delocalised electrons, has been discovered and investigated. This through-space stabilising interaction between remote negative charges and delocalised radicals is associated with an unusual non-aufbau orbital configuration with a potential in molecular electronics, and impacts significantly upon the energetics of chemical reactions, in which the extent of electron delocalisation changes. This finding has a huge range of practical applications, and is shown to have significant implications for enzyme catalysis and biological autooxidation. Furthermore, the basic scheme of autooxidative degradation of organic molecules under elevated temperatures and/or UV light was revised so as to reflect the low reactivity of peroxyl radicals towards the hydrogen abstraction and explain the surprising antioxidant effect of oxygen. On this basis, a number of stabilisation strategies were suggested, according to the chemical structure of the degrading substrate. One such strategy is the use of hindered amine light stabilisers (HALSs), and the true mechanisms behind their remarkable efficiency have been clarified in this thesis for the first time. The corresponding new nitroxide chemistry was also found relevant to the side-reactions disrupting nitroxide mediated polymerization (NMP), and the strategies for their minimisation have been proposed. Combined with the discovered pH switching, this knowledge was utilised in the design of switchable control agents for room-temperature NMP. Finally, the reversible redox chemistry of nitroxides was employed to design novel redox mediators for dye-sensitised solar cells (DSSCs), one of which has been subsequently shown to double the cell’s energy conversion efficiency.