Potential of mean force and reaction rates for proton transfer in acetylacetone

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Abstract
The intramolecular proton transfer in the enol form of acetylacetone is investigated at various temperatures both classically and quantum-mechanically using computer simulations. The potential energy surface is modeled using the empirical valence bond (EVB) approach of Warshel and fitted to the results of ab initio calculations. Quantum-statistical results are obtained via discretized Feynman path integral simulations. The classical and centroid potential of mean force for the reaction coordinate is obtained using umbrella sampling. The proton transfer rate is calculated based on classical and on Feynman path integral quantum transition state theory. For the classical system, the transmission coefficient is obtained from activated dynamics. Two different reaction coordinates are compared, the first one involving explicitly the transferring proton and the second one involving only heavy atoms in the molecules. The influence of isotopic substitutions is investigated by considering a fully deuterated version of acetylacetone. It is observed that there are significant differences between classical and quantum-mechanical calculations caused mainly by the lack of tunneling effects in the former. The quantum fluctuations of heavy atoms are found to have a considerable influence on the magnitude of the proton transfer rate.

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