Computational Investigation into the Mechanism of Water Oxidation in Nature

Abstract

Water oxidation into molecular oxygen is not only a key process for our aerobic atmosphere but also of high interest as potential source of green hydrogen. Improvements to this process could benefit plant growth and as such food yields. Though many advancements into the elucidation of its mechanism have been made both experimentally and computationally, there is still much uncertainty around the exact structures and electronic changes involved throughout the catalytic cycle performed by photosystem II. This thesis presents computational work, using broken-symmetry density functional theory primarily, on the nature of the bonding and magnetic exchange in the oxygen evolving complex, the nature of the S3 state, the mechanism of deprotonation in the S3 state and the identity of the S3YZ* state. The exchange pathways between Mn centres are illustrated by the use of intrinsic bonding orbitals and corresponding orbitals, showing that protonation of linking oxygen bridges can interrupt superexchange couplings between the metal centres. It is established that Ca2+ is ionic in nature and acts to modulate the properties of the water oxidising complex as a whole. These illustrative methods are used subsequently to give insights into the S3 state, in which a unique low energy barrier for O2 formation via an [O5O6]3− intermediate was identified. This intermediate is shown to have a two-centre one-electron bond, with the shared oxygen spin strongly stabilised by anti-ferromagnetic interaction with the Mn centres. Inclusion of this intermediate in electron paramagnetic spectroscopy simulations better reproduces experimental observations. In order to form the formal O5-O6 bond a proton must be removed from O6, two possible pathways for O6 deprotonation are presented. While the Glu189 pathway is found unsuitable a promising pathway via W3 is identified and analysed at various O5-O6 distances. Finally the S3YZ* state is investigated and three intermediates on the path to O-O bond formation are analysed. It was found that as the O5-O6 bond is formed long range exchange interaction between the oxygen evolving complex and the nearby YZ increase in strength, encouraging subsequent reduction of YZ. As such a unique mechanism for the formation of molecular oxygen is presented and analysed.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Anderson (Supervisor), Patrick O'Malley (Supervisor) & Sam Hay (Supervisor)