These projects aims to use computational chemistry to better understand reaction mechanisms in organic chemistry. By the use of density functional theory calculation, a better understanding of the structure and reactivity of reactants, intermediates, and product leads to insight in the development of new reactions in organic chemistry. The reactions that will be studied include radical species in photoredox catalysed reactions. In the first project, we will utilise DFT calculation to investigate the potential energy surface of photoredox-based chemistry. In photoredox catalysis, light is used to generate radical species that react in a selective manner. In order to understand the reactivity of the elusive radicals (that are transient in there nature and are difficult to detect experimentally), we will optimise the radical species and transition states using DFT and from these results correlate with the reactivity that we observe experimentally. Since the molecules we will investigate are open-shell radical species it is important to verify that the metods we use are consistent, which usually take a lot of effort and computational demand. Examples of reaction that will be investigated is photoredox catalysed functionalization of carbohydrates and photoredox catalysed syntheses of spiro-compounds. In the second project, we will investigate new methodologies for C(sp3)–H bond activation by the means of energy transfer catalysis that will provide strategic alternatives to today’s ineffective multistep processes. The use of low-energy visible light as the source of activation of the photocatalysts has the potential to control the reactivity of the generated O- and N-centered radicals from oxime carbonates more precisely and thereby delivering more sustainable access to complex organic molecules via energy transfer, N–O bond cleavage, hydrogen atom transfer and radical-radical coupling. The theoretical part of this project is to investigate the excited states of different photocatalyst and how they can be used to participate in Dexter energy transfers with organic substrates in order to generate radicals that participate in cascade reactions. The calculation will be performed at high level of excited photocatalyst and substrates and the open-shell triplet states are in general difficult to calculate without high-level ab initial calculations.