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Atom-Tunneling in Chemistry

Periodic Reporting for period 4 - TUNNELCHEM (Atom-Tunneling in Chemistry)

Reporting period: 2020-01-01 to 2020-06-30

Atom tunneling is a quantum phenomenon that accelerates chemical reactions. We investigated its effect on chemical reactions in very diverse areas of chemistry. We have found significant tunneling effects in astrochemistry, biochemical processes and catalysis. Because of its quantum nature, atom tunneling is difficult to treat computationally. We developed algorithms to calculate the tunneling probability more efficiently and more accurately than previously possible. To that end, we extended instanton theory in a simple, yet efficient approximation to beyond the crossover temperature, we derived and implemented a formulation of microcanonical instanton theory and found improved treatments for bimolecular reactions.
We investigated the phenomenon of atom tunneling in chemical reactions using several approaches. We developed a method to simulate tunneling rate constants with higher accuracy and lower computational demand than previously possible. This was achieved by deriving a microcanonical formulation of instanton theory along with an extension of instanton theory to higher temperatures. This allows the calculation of bimolecular rate constants over the whole temperature range over a large range of pressures. The computational efficiency was increased by a dual-level scheme for instanton theory, as well as by employing machine-learning techniques for the approximation of potential energy surfaces. To treat surface reactions, especially in astrochemistry, we found an implicit surface treatment, which allows us to calculate surface rate constants at the computational cost of gas-phase reactions.
We applied it to many reactions relevant to the interstellar medium, gas-phase reactions as well as surface reactions. The resulting rate constants were subsequently used in astrochemical models and resulted in an improved agreement with observed abundances. The tunnel effect is also important in biochemical systems (enzymes). We investigated the enzyme Taurine/alpha-ketoglutarate Dioxygenase (TauD). Moreover, we have written a review paper on atom tunneling in chemistry and edited a textbook on the subject to be published by the RSC to serve as a general source of reference for atom tunneling in chemistry.
Our tunneling rate constants for astrochemical reactions significantly increase our understanding of interstellar chemical processes. To achieve that, microcanonical tunneling rate calculations were necessary. We developed an algorithm to calculate these accurately and efficiently. This and our other algorithmic developments are available to the scientific community within our open-source library DL-FIND.
Tunneling paths at different temperatures
Tunneling path (=instanton) of the reaction H2 + OH --> H2O + H
Ice sufrace, different molecules and a dust particle in front of a molecular clowd in space