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Complex reaction mechanisms of organic molecules: from ground state reaction to excited state dynamics
Complex reaction mechanisms of organic molecules: from ground state reaction to excited state dynamics
The significance of computational chemistry for a broad field of applications, as for example investigations of mechanisms or dynamics of chemical reactions, is rising. In this thesis various state-of-the-art computational chemistry methods are applied to different topics. This covers complex ground state reactions, excited state dynamics of molecular machines and detailed quantum dynamical studies at conical intersections (CoIns), decisive features for ultrafast reactions. The complex mechanism of triplet oxygen with specifically substituted cyclobutene derivatives with a conjugated π system leading to an oxidative ring contraction under kinetic conditions and an oxidative ring opening reaction under thermodynamic conditions is revealed. The resulting barriers are in agreement with the experimentally observed product distribution under different conditions and the occurrence of a stable intermediate can be explained. Further, the mechanism of the solvent dependent twisted intramolecular charge transfer (TICT) formation of certain hemithioindigo-stilbene (HTI) photoswitches is investigated. The necessary molecular conditions for a possible TICT formation as competing pathway next to the well-known Z → E isomerization are identified. With theoretical IR spectra at essential geomtries along the reaction pathway the dynamics of a marker band observable in the experimental transient IR spectra of TICT formation is explained. After this, the dynamics of the first photochemical step of a HTI based molecular motor is investigated. The significantly different reaction time of two related derivatives only differing in two substituents on the stilbene part is explained by comparing the electronic structure. These findings are strengthened by calculations of a related hypothetical motor with different electron distribution on the stilbene part. The dynamics of the faster derivative is simulated with semi-classical trajectories and by analyzing the molecular motion significant coordinates for the rotational step can be revealed. Thereafter, the dynamics of the photodissociation of Ph2CH−Br with two competing pathways is simulated with grid-based wave packet quantum dynamics on two-dimensional ab-initio potential energy surface with specially adapted reactive coordinates and with full-dimensional ab-initio semi-classical trajectories with surface hopping. Both approaches can be tuned to reproduce the sequential appearance of the two different product pairs obtained by homolytic and heterolytic bond cleavage, respectively. An analysis of the wave packet motion explains the fluctuation in the experimental transient absorption signal only observable for the Ph2 CH − X compound with bromine as leaving group. The last part deals with the waveform control of a reaction in the vicinity of a conical intersection with a few cycle carrier-envelope phase (CEP) laser pulse. The phase-dependent control efficiency is analyzed varying several different pulse parameters (FWHM, t0 , ω0 , Emax ) beside the CEP. Two different underlying control mechanisms are revealed determining the total control efficiency and the influence of the pulse parameters on the underlying mechanism is evaluated. First, the control scheme is applied to a dissociative model system identifying the general features for a possible CEP control. Then the procedure is successfully adopted to the nucleobase uracil and the underlying control mechanism is discussed in detail.
quantum dynamics, mixed quantum-classical dynamics, ultrafast photochemical reactions, molecular machines, control
Schüppel, Franziska
2022
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Schüppel, Franziska (2022): Complex reaction mechanisms of organic molecules: from ground state reaction to excited state dynamics. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

The significance of computational chemistry for a broad field of applications, as for example investigations of mechanisms or dynamics of chemical reactions, is rising. In this thesis various state-of-the-art computational chemistry methods are applied to different topics. This covers complex ground state reactions, excited state dynamics of molecular machines and detailed quantum dynamical studies at conical intersections (CoIns), decisive features for ultrafast reactions. The complex mechanism of triplet oxygen with specifically substituted cyclobutene derivatives with a conjugated π system leading to an oxidative ring contraction under kinetic conditions and an oxidative ring opening reaction under thermodynamic conditions is revealed. The resulting barriers are in agreement with the experimentally observed product distribution under different conditions and the occurrence of a stable intermediate can be explained. Further, the mechanism of the solvent dependent twisted intramolecular charge transfer (TICT) formation of certain hemithioindigo-stilbene (HTI) photoswitches is investigated. The necessary molecular conditions for a possible TICT formation as competing pathway next to the well-known Z → E isomerization are identified. With theoretical IR spectra at essential geomtries along the reaction pathway the dynamics of a marker band observable in the experimental transient IR spectra of TICT formation is explained. After this, the dynamics of the first photochemical step of a HTI based molecular motor is investigated. The significantly different reaction time of two related derivatives only differing in two substituents on the stilbene part is explained by comparing the electronic structure. These findings are strengthened by calculations of a related hypothetical motor with different electron distribution on the stilbene part. The dynamics of the faster derivative is simulated with semi-classical trajectories and by analyzing the molecular motion significant coordinates for the rotational step can be revealed. Thereafter, the dynamics of the photodissociation of Ph2CH−Br with two competing pathways is simulated with grid-based wave packet quantum dynamics on two-dimensional ab-initio potential energy surface with specially adapted reactive coordinates and with full-dimensional ab-initio semi-classical trajectories with surface hopping. Both approaches can be tuned to reproduce the sequential appearance of the two different product pairs obtained by homolytic and heterolytic bond cleavage, respectively. An analysis of the wave packet motion explains the fluctuation in the experimental transient absorption signal only observable for the Ph2 CH − X compound with bromine as leaving group. The last part deals with the waveform control of a reaction in the vicinity of a conical intersection with a few cycle carrier-envelope phase (CEP) laser pulse. The phase-dependent control efficiency is analyzed varying several different pulse parameters (FWHM, t0 , ω0 , Emax ) beside the CEP. Two different underlying control mechanisms are revealed determining the total control efficiency and the influence of the pulse parameters on the underlying mechanism is evaluated. First, the control scheme is applied to a dissociative model system identifying the general features for a possible CEP control. Then the procedure is successfully adopted to the nucleobase uracil and the underlying control mechanism is discussed in detail.