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Thallmair, Sebastian (2015): Photogeneration of reactive intermediates: From initial quantum dynamics to chemical yields in solution. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



Bond cleavage and formation are key steps in chemistry and biochemistry. The present work investigates the generation of diphenylmethyl cations (Ph2CH+) via photoinduced bond cleavage of diphenylmethyl derivatives with a cationic or neutral leaving group. The resulting Ph2CH+ cations and its numerous derivatives serve as reference electrophiles for one of the most extensive reactivity scales covering 40 orders of magnitude. In chapter 1, the focus is on the initial bond cleavage of diphenylmethyltriphenylphosphonium ions (Ph2CH−PPh3+) exhibiting a cationic leaving group. With the help of state-of-the-art quantum chemical and quantum dynamical methods, the reaction mechanism of the bond cleavage is revealed. Using a reduced model system, the potential energy surfaces can be calculated at the ONIOM level of theory along specially designed reactive coordinates. Two competing reaction channels emerge: a homolytic one in the S1 state and a heterolytic one in the ground state. They are connected via an energetically accessible conical intersection which makes an efficient generation of the observed Ph2CH+ cations feasible. In contradiction with the experiment in polar or moderately polar solvents, quantum dynamical calculations for the isolated molecule reveal the formation of Ph2CH• radicals. While electrostatic solvent effects are negligible in this system, dynamic solvent effects emerge as being essential to explain the molecular mechanism. Two methods with increasing complexity to describe the dynamic impact of the solvent environment are developed. The first approach, the dynamic continuum ansatz, treats the environment implicitly. It uses Stokes’ law and the dynamic viscosity of the solvent in combination with quantum chemically and dynamically evaluated quantities to obtain the decelerating force exerted on the dissociating fragments. The ansatz does not require any fitting of parameters. The second method, the QD/MD approach, is based on an explicit treatment of the solvent surrounding. It combines molecular dynamics (MD) simulations of the reactant in a box of solvent molecules with quantum dynamics (QD) calculations of the reactant’s dynamics. In this way, a more detailed microscopic picture of the molecular process can be derived taking into account individual arrangements of the solvent. Both methods unveil the crucial impact of the solvent cage on the bond cleavage mechanism. It hinders the free dissociation in the S1 state and guides the molecular system to the conical intersection. QD simulations including the non-adiabatic coupling around the conical intersection show the formation of Ph2CH+ within ∼400 fs which compares well with the initial rise of the cation absorption in the experiment. Chapter 2 deals with the position of the counterion X– in the ion pairs Ph2CH−PPh3+ X–, PhCH2−PPh3+ X–, and (p-CF3-C6H4)CH2−PPh3+ X– in solution with X– being Cl–, Br–, BF4–, and SbF6–. These structures are essential to clarify the role of oxidizable counterions like e.g. Cl– during the initial bond cleavage in dichloromethane. The structures determined quantum chemically in dichloromethane show a similar counterion position than in the crystal. They are confirmed by the good accordance of the calculated and measured 1H NMR shifts. The C(α)–H···X– hydrogen bonds account for the pronounced counterion-dependent 1H NMR shifts of the C(α)–H in CD2Cl2. The strong downfield shift of the signals increases according to SbF6– < BF4– << Br– < Cl–. The last part (chapter 3) focuses on the secondary processes within a few picoseconds to several nanoseconds after the C-Cl bond cleavage in diphenylmethylchloride in solution. Initially, the neutral leaving group Cl leads mainly to the formation of radical pairs; only a minor fraction of ion pairs is generated in the beginning. A combined Marcus-Smoluchowski model is used to simulate the interplay between geminate recombination, diffusional separation, and electron transfer of the radical and ion pair populations. The distance-dependent rates of the three processes together with broad distance-dependent population distributions faithfully reproduce the spectroscopically observed dynamics. The majority of Ph2CH+ cations is generated via electron transfer from the radical pairs. The detailed understanding of the secondary processes shows that a high Ph2CH+ cation yield can be expected if the radicals within a pair stay nearby for a long time to achieve an efficient electron transfer and if the resulting ions are separated fast to prevent geminate recombination.