Simulation of Ligand Exchange Reactions

The development of intermolecular potentials including 3-body terms based on Hartree-Fock ab initio calculations and of a mixed quantum mechanical / molecular mechanical approach to molecular dynamics simulations for a larger number of ions has shown that it is possible through such simulations, to obtain very detailed information about the mechanism of very fast ligand exchange reactions in complexes. Numerous models have been proposed for such reactions, but due to the time-scale in the picosecond range, they can hardly be verified by experimental methods. The present project intends to systematically investigate such reactions by means of the aforementioned simulation technique, in order to obtain a solid basis for the interpretation of experimental data, and for the mechanism of these reactions. The systems envisaged for these investigations are solvate complexes of alkaline, alkaline earth and transition metal ions, plus a few selected monoatomic anions. Water, ammonia and complex-forming anions will be considered as ligands involved in the exchange reactions. In the case of transition metal ions with very stable first hydration shell, exchange reactions in the second solvation sphere and replacement of first-shell water by ligands of affinity higher than water will be investigated. Apart from " classical" publications, the results of these investigations will be presented in the form of video clips, with the help of a proprietary software (MOLVISION) recently developed by the applicant and coworkers. These video clips will be made available via the Internet and provide a most illustrative description of the chemical reactions investigated.

As most of the relevant chemical reactions proceed in the liquid state and almost all biochemical processes in aqueous medium, the detailed understanding of solvation is crucial for the understanding of chemistry and related disciplines. This is particularly true for solvated ions, which play manifold roles in chemistry and biology. Experimental investigations of structure and dynamics of liquid systems are highly complicated and expensive, and for numerous processes suitable experimental settings are not even available yet. Classical theoretical simulation methods for liquids on the other hand are mostly not accurate enough to correctly reveal all details of solvated ions and molecules. The target of the present project was, therefore, to develop further the methodology of hybrid quantum mechanical - molecular mechanical molecular dynamics simulations, which provide the required accuracy, albeit at the price of an almost hundred-fold computational effort. With this methodology a large number of chemically and biologically important ions was studied, focussing on the dynamics of ligand exchange, which is crucial for the reactivity of these ions in the liquid state and in the interaction with other compounds than the solvent itself. The 25 metal ions investigated in this project are representative for the most important parts of the periodic system, ranging from extremely stable solvates to the most labile structure-breakers. Besides water, ammonia and aqueous ammonia were included as solvents, and particular attention was paid to the influence of heteroligands in metal complexes on the dynamics of ligand exchange reactions. In addition, two halide anions were included in the studies, and an extensive investigation of liquid water by various levels of quantum mechanical simulation was performed, which proved the accuracy of the employed method by its full agreement with the most recent and sophisticated experimental techniques concerning hydrogen bonding and the ultrafast exchange rates. In the progress of the project work, ideas for a new, strongly inproved quantum mechanical ab initio simulation methodology (later on termed Quantum Mechanical Charge Field Molecular Dynamics, QMCF MD) were developed and successfully translated into a new program code. This new method provides the access to simulations of composite solutes without the need of analytical potentials and further improves the accuracy of the simulations for systems with strong charge-transfer and polarisation effects. The project resulted in 55 peer-reviewed publications, including 3 invited book contributions and 2 invited review articles. 3 of the other publications were also solicited articles. A number of invited lectures at international conferences document the broad interest in the project results.