Photoinitiated processes in functionally diverse organic molecules elucidated by theoretical methods

Roos, Matthias K. (2018): Photoinitiated processes in functionally diverse organic molecules elucidated by theoretical methods. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

In this thesis reaction mechanisms of organic compounds with applications in different areas, such as kinetic studies, labeling, and battery electrolytes, are investigated with theoretical methods from quantum chemistry, quantum dynamics, and molecular dynamics. The variety of the investigated molecules ranges from polycyclic hydrocarbons, dyes, electrolytes to precursors of reactive species. The work was performed either in close collaboration with experimentalists or based on experimental results and in this way allows an in-depth look at the occurring chemistry. In the first part the concept of adapted reactive coordinates for reduced dimensional quantum dynamics is presented necessary for the studies in the following part. It relies on the Wilson G-matrix method as formulation of the kinetic part of the Hamiltonian and allows to include the relaxation of background coordinates besides the identified main reactive coordinates without optimizations for each grid point. The concept is shown for a photodissociation involving complex structural changes and the G-matrix elements and their influence on the dynamics are discussed. In the second part the photoinitated bond cleavage reaction for diphenylmethyl chloride and diphenylmethyl bromide is studied. Based on the reactive coordinate system presented before, quantum dynamical simulations enlighten the path of the wave packet, which passes through two consecutive conical intersections—a three-state and a two-state one—as decisive elements for the product splitting. In the case of chlorine, the experimental signal is modeled from the simulated data to further prove the mechanism. For the bromine case, additionally non-adiabatic mixed quantum-classical dynamics is used to clarify the role of vibrations during the bond cleavage, which are responsible for small amplitude oscillations of the experimental signals. Throughout this part the “our own n-layered integrated molecular orbital and molecular mechanics” (ONIOM) method is used to reduce the involved computational cost. The third part is dedicated to the photophysics of elongated π systems in organic molecules discussing two examples. The first one is the polycyclic hydrocarbon pyrene, the second one covalently linked constructs of DNA and the dye Cyanine 3. For pyrene, the ultrafast transition from the photo-accessible S2 state to the fluorescent S1 is simulated for the first time using two complementary dynamical methods. It is shown that both methods yield comparable results and demonstrate the strong coupling between the two states. The constructs in the second example are investigated experimentally and theoretically. Simulated spectra for a model system help attributing an occurring blue-shift to dimerization. Circular dichroism measurements and molecular dynamics simulations further characterize the formed dimers. The last part comprises a joint experimental and theoretical study concerning the chemical stability of two electrolytes commonly used in lithium-ion batteries towards singlet oxygen. It is shown that singlet oxygen is reactive towards the electrolyte ethylene carbonate. Ab initio calculations suggest a concerted double hydrogen abstraction by the singlet oxygen as mechanism, which is not possible for the second electrolyte dimethyl carbonate. It is an example for the unusual reaction of an alkyl group with singlet oxygen and yields hydrogen peroxide. Ground state mixed quantum-classical dynamics verify the further decay of the reaction intermediate vinylene carbonate to carbon dioxide, which is found experimentally. The theoretically predicted intermediate formation of hydrogen peroxide is detected colorimetrically proving the reaction mechanism and its detrimental effect is investigated experimentally.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: quantum chemistry, quantum dynamics, mixed quantum-classical dynamics, conical intersections, ultrafast chemical reactions
• 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: 14. November 2018
• 1. Bericht­erstatter:in: Vivie-Riedle, Regina de