Redondo Marey, Carmen-Maria; (2002) Femtodynamics of double proton transfer reactions. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The aim of this work is to develop a method to study excited-state double proton transfer reactions in biomolecules, looking mainly at their time scale. We focused our attention on the double proton transfer reaction in the 7-azaindole dimer. This reaction has been experimentally studied by Zewail and co-workers and Castleman and co-workers using femtospectroscopic techniques (resolution 10-15 s). In addition, the reaction is a model for that occurring in DNA base pairs. The 7-azaindole dimer is a bifunctional molecule that contains two hydrogen-atom donor groups and two acceptor groups in close proximity. These groups form two hydrogen bonds between the two moieties of the molecule. During the reaction, the two hydrogen atoms move from one moiety of the molecule to the other. The goal is to predict the time scale of this reaction (the two proton transfer times). A quantum tunnelling in a dissipative environment approach is used to study this process. This method considers the system composed of a tunnelling particle (the H atom) moving in a potential field, V, coupled to a bath that represents the rest of the molecule. Our first approach was to use only two degrees of freedom to describe the process: the donor-atom hydrogen-atom distance and the donor-atom acceptor-atom distance (or intermolecular distance). The results were compared with the experimental ones. To improve our results we introduced another degree of freedom in our calculations. It represents the other modes of the molecule, which had not been considered in the first approach. This improves agreement with the experiments. Calculations considering different initial vibrationally excited states and different isotopic isomers (which show the importance of quantum tunnelling in this process) were also carried out and compared with the experiments.

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