Quantum/classical simulation of molecular excited state dynamics and spectroscopy

Peschel, Martin Thomas (2023): Quantum/classical simulation of molecular excited state dynamics and spectroscopy: from iodine in krypton to Enone-Lewis acid complexes in an explicit solvent. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

The ability of modern quantum chemistry to answer ever more complex questions rises steadily. In this thesis, a comprehensive exploration of molecular photochemistry using high-level electronic structure methods for quantum-classical dynamics is presented. The first chapter introduces theoretical methods for simulating photodynamical processes, focussing on the relaxation of molecules in explicit atomistic environments. These approaches include nuclear wavepacket dynamics embedded within classical molecular dynamics. The presented Ehrenfest and multi-configurational Ehrenfest approaches are applied to small molecules surrounded by noble gas atoms. Furthermore, trajectory surface hopping is discussed, as, in later chapters, the program SHARC is used to perform such simulations. During this thesis, adaptive time-stepping and two new interfaces to electronic structure codes were implemented. These methods facilitate efficient and accurate dynamics calculations on a variety of photochemically relevant systems ranging from simulations in the gas phase with high-level XMS-CASPT2 electronic structure (including spin-orbit couplings) to QM/MM simulations in the condensed phase. The second chapter focuses on the energy transfer between an infrared laser and solvated molecules, combining the traditional harmonic approximation to calculate infrared spectra with methods based on \textit{ab initio} molecular dynamics. This methodology is used to elucidate the coherent energy transfer dynamics from the field to the molecule in field-resolved spectroscopic measurements. The third chapter of this thesis surveys the intricate world of 2-enone photochemistry. By exploring $\pi\pi^*$ and $n\pi^*$ reactivity using high-level electronic structure methods, insights are gained into the \textit{Z}/\textit{E} isomerization of cyclohept-2-enone and the photoinduced rearrangement of 5,5-dimethylcyclopent-2-enone to a ketene. In the final chapter, mechanistic investigations are extended to Lewis acid\hyp coordinated enones, uncovering the impact of coordination on the electronic states, UV-Vis spectra, and reactivity. Trajectory surface hopping calculations are used in combination with ultrafast transient absorption spectroscopy to uncover the dynamics of the relaxation of cyclohex-2-enone-BF$_3$ to the reactive triplet states and the photo-induced B\textendash Cl bond dissociation in benzaldehyde-BCl$_3$. Collectively, this work exemplifies the potent synergy of computational and spectroscopic techniques in unraveling photochemical mechanisms. From explicit solvent relaxation to multi-step organic reactions and from spectroscopic signatures to intricate electronic transitions, this thesis advances our understanding of photochemical processes across a spectrum of molecular examples. The findings have implications for the design and understanding of photochemical reactions and spectroscopic studies in complex environments.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: quantum chemistry, ultrafast chemical reactions, enones, molecular dynamics, excited states
• 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: 20. Oktober 2023
• 1. Bericht­erstatter:in: Vivie-Riedle, Regina de