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"Tracking the short life of highly reactive carbocations:
From the ultrafast bimodal photogeneration
to bimolecular reactions without diffusion" Christian Frank SailerDoktorarbeit am LS für BioMolekulare Optik, LMU München Abstract: Carbocations belong to the most important reactive intermediates in organic chemistry. Understanding their generation and the course of chemical reactions where carbocations are involved is hereby of central interest. The knowledge about the reactions of carbocations gained during the last decades is based on the evaluation of empiric investigations. They do not allow, however, for a direct observation of the formation and the reaction mechanism. In the framework of this thesis, a deep insight into these processes was gained with the aid of ultrafast transient absorption spectroscopy. The measurements furthermore revealed the importance of the concomitantly formed radical pairs for the generation of carbocations. The direct observation of the bond cleavage after UV excitation of benzhydryl chloride was possible with the aid of a setup with highest temporal resolution below 50 fs. The photolytically generated radical and ion pairs are delayed by about 80 fs and 125 fs with respect to the excitation. The delay is caused by the inertial motion of the excited wavepacket towards two successive conical intersections leading to radicals and cations. The exact position of the conical intersections and the path of the wavepacket therefore determines the partitioning into the two product channels. After the ultrafast bond cleavage, the two fragments are still in a geometry close to that of the precursor. The subsequent relaxation in the next hundreds of femtoseconds is hereby associated with strong changes of the electronic states. This leads to the observed rise of the optical absorption signal for the cation as well as for the radical band. Further carbocations can be formed by an electron transfer within the radical pair which is eventually terminated by diffusional separation of the radical pairs. The observed yields and dynamics of the interconversion can be understood with the aid of simulations involving distance dependent radical and ion pair populations which are subject to diffusion and distance-dependent reactions. The simulations furthermore reveal that the absence of carbocations on the nanosecond time scale in solvents such as dichloromethane is caused by an extremely efficient geminate recombination of the ion pairs. This disadvantage can be circumvented by using the neutral triphenylphosphine instead of the charged chloride ion as leaving group. It allows for the generation of carbocations with a reasonable yield even in solvents of moderate polarity. Attention has to be paid, however, to the choice of the counterion. Depending on the experimental conditions, the counterion can open up further reaction channels which deteriorate the cation yield. The observation of ultrafast, bimolecular reactions requires the immediate vicinity of reaction partners. This was implemented on the one hand by the photolytical generation of two fragments in a reverse micelle. Here, the geminate recombination between both fragments is not influenced by diffusion. The measured rate therefore corresponds to the intrinsic reaction process. A general possibility, which was established within this work, is the ultrafast, bimolecular reaction of a photolytically generated carbocation with the solvent. Depending on the choice of the generated cation and the solvent, rate constants from (388 ps)-1 up to (2.6 ps)-1 were found. Besides the reactivity of the cation, the size of the solvent molecule is the factor which influences the rate the most. Comparisons with reactions on slower time scales suggest that the reaction rates on the picosecond scale are limited by molecular motions such as rotations. BMO authors (in alphabetic order): Christian Sailer  WWW-Version |
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Letzte Änderung: 2016-09-14 13:34
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