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Abstract

This thesis presents the results of investigations on chemical structures and reaction mechanisms. All investigations reported here employed theoretical methods especially semi-empirical and Density Functional Theory methods.
The semi-empirical OM2/MRCI method was used to study the origin of photostability of the nucleobase guanine. It is known from experiment that nucleobases in general strongly absorb in the UV/VIS range but there exist decay mechanisms that prevent damage to the molecule. The decay of excited guanine in water was simulated through surface hopping dynamics and the geometrical changes along the decay pathway were monitored. The use of a QM/MM computational setup allowed a detailed study of solvent effects in aqueous solution. Comparing the results with those obtained in the gas phase reveals that the preferred decay channels change and that the decay is faster in aqueous solution, while the optimized geometries of the ground and first excited state and of the relevant conical intersections are similar in the gas phase and in solution. The results are in good agreement with static ab initio computations and with the available experimental studies, which validates the application of the semi-empirical OM2/MRCI method. The time evolution of the computed excited-state ionization potentials is consistent with recent experimental results from ultrafast photoelectron spectroscopy on guanosine in water.
Density Functional Theory methods were used to answer questions on chemical structure and reaction mechanisms raised by experimental investigations. Concerning structures the question was how to describe the bonding interactions of bimetallic gold-chromium complexes and whether these complexes are carbenes. Starting from the crystal structure the geometries of the complexes were optimized and then these structures were examined by using NBO analysis, fragment analysis, and the AIM method. It was found that these complexes are best described as chromium-carbenes with a gold unit coordinated to the C-Cr bond in an η² mode.
On the mechanistic side, an organocatalytic reaction and the reaction of an organoferrate complex were studied. In the first case the issue was whether the ring closing reaction of an unsaturated hydrazone could be regarded as a pericyclic electrocyclization or not. The analysis showed that it is better classified as an intramolecular nucleophilic addition (structurally) or a pseudopericyclic reaction (electronically); it is not pericyclic as confirmed by comparison with the cyclization of the pentadienyl anion which is known to cyclize in a pericyclic manner. In the second case study, methylation reactions mediated by an organoferrate complex were found to proceed via a substitution mechanism. In agreement with experiment up to four methyl groups can be transferred per ferrate complex, the first three in a facile manner and the fourth one with some more activation. This demonstrates the ability of ferrate complexes to alkylate, and hence their potential to act as intermediates in iron cross coupling reactions.