Atomic Tunneling in Organic Reactions

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For elucidating the mechanism of inter- and intra-molecular tunneling of hydrogen atoms, hydrogen-atom abstraction, addition and exchange in organic systems have been studied between the temperature range of 10 K to 290 K by means of ESR, IR, mass spectroscopy, and product analyses. The results are summarized as follows ;1) Quantum-mechanical tunneling of hydrogen atoms is the measure process of chemical reaction at low temperatures, since the probability of the thermal activation of reaction systems is negligible with respect to the tunneling probability.2) The rate of tunneling depends exponentially on the area of tunneling barriers and the effective mass of tunneling particles. The rate strongly depends on the viscosity of the surroundings, if the deformation of, not only a tunneling particle, but also the other atoms in a reaction system are necessary during the optimum tunneling process. The surroundings with high viscosity prevent the deformation of the reaction-system for the op … More timum tunneling so that the tunneling barrier increases.3) The tunneling of hydrogen atoms plays a significant role in cosmic chemistry. The rate of the tunneling generally increases in the order of abstraction, addition to double bonds, and addition to free radicals. Ethane is only the product by hydrogen atom spray to the cryogenic thin film of ethylene, since the ethyl radicals generated by hydrogen-atom addition successively react with hydrogen atoms. The efficiency of the hydrogen-atom addition increases with decreasing temperature, since the slowing down of the diffusion of hydrogen atoms causes the increase of the reaction time for the tunneling.4) The shapes of the tunneling barriers for the intramolecular transfer of hydrogen atoms in carbene and nitrene strongly depend on the electronic configurations and the energy differences between the singlet and the triplet states.5) Hydrogen-atom tunneling does not play a significant role at ambient temperature in asymmetric syntheses by BINAP/Ru-catalyzed hydrogenation. Less