Theoretical insights into the coupled nuclear and electron dynamics in molecules of biological relevance

Bäuml, Lena (2025): Theoretical insights into the coupled nuclear and electron dynamics in molecules of biological relevance. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Some of the fundamental natural processes like photosynthesis or the photostability of nucleobases depend on ultrafast phenomena. Here, especially the relaxation of a photoexcited molecule via conical intersections (CoIns) is an important phenomenon. The evolving field of attosecond spectroscopy and X-ray absorption spectra (XAS) in particular offer the crucial temporal resolution to observe nuclear and electron dynamics on their natural time scale and even follow the relaxation through CoIns. To interpret and predict the complex spectra the development of theoretical workflows that describe ever more realistic systems is necessary. In this regard this thesis represents an important contribution towards the simulation of time-resolved XAS that include the often neglected electronic coherence. In the first part the NEMol-grid method is introduced, that allows for the calculation of the coupled nuclear and electron dynamics in molecules. With the grid-based version of NEMol the previously existing ansatz was extended to treat also very delocalized nuclear wavepackets, which often occur in higher dimensional molecular systems. In the first chapter the NEMol-grid approach was applied to the nucleobase uracil to investigate the excitation process and the ensuing relaxation through a CoIn. The second chapter treats the excited state dynamics of the natural pigment chlorophyll a. The coupling between the first two excited states Qy and Qx, as well as that of the higher lying Bx and By were investigated together with the B → Q relaxation. The obtained results help to better understand these processes, which are crucial for photosynthetic light-harvesting. In the last part of this thesis a workflow is derived, that combines the coupled dynamics from NEMol with static XAS to obtain time-resolved spectra that include the electronic coherence. The calculations are performed at the XMS-CASPT2 level of theory, capturing the multireference character of the excitations. With XAS it is possible to track the population dynamics and the relaxation through a CoIn. The workflow was first derived to follow the excited states dynamics of uracil and then extended to the much larger chlorophyll molecule. For the latter time-resolved XAS offer the possibility to confirm the coupling between Qy and Qx.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: photochemistry, quantum dynamics, photosynthesis, theoretical spectroscopy, conical intersections
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 5. Juni 2025
• 1. Bericht­erstatter:in: Ochsenfeld, Christian