Quantum tunneling observed without its characteristic large kinetic isotope effects

Abstract

Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (≳100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system. quantum tunneling | kinetic isotope effect | heterogeneous reactions | reaction dynamics | astrochemistry

Large Hydrogen Isotope Fractionation Distinguishes Nitrogenase-Derived Methane from Other Methane Sources

“…7 ) ( 73 ). It is also possible that proton tunneling, which is generally thought to have a very large KIE (but also see references 96 and 97 ) and has been proposed to occur in nitrogenase ( 79 ), could be contributing to the KIE observed here, although we note that the temperature effect observed here is opposite the predicted effect for tunneling ( 80 , 81 ). Computational models, which can distinguish the rates of hydrogenation based on 1 H and 2 H, and might be able to shed light on the mechanism responsible for the observed fractionation and whether the currently proposed, multistep mechanisms of hydrogenation by nitrogenase ( 82 – 85 ) are compatible with the measured KIE of 2.1.…”

Atom Tunneling in Chemistry

“…[297] In this case, tunneling contributes to the reaction rate of the first hydrogen chemisorption, while the addition of the second hydrogen atom is barrierless. [297][298][299] Amorphous solid water is among the most common surfaces in the interstellar medium, as most dust grains are covered by water. It was found experimentally that water can be formed from H 2 and OH on water surfaces even at 10 K, [33] even though the gas-phase reaction exhibits ab arrier of 17.5 kJ mol À1 .F or further reactions on water surfaces enhanced by atom tunneling,w er efer to Ref.…”

“…One model to simulate H 2 formation on carbonaceous dust grains is the hydrogenation of benzene, which was studied by quantum chemical methods . In this case, tunneling contributes to the reaction rate of the first hydrogen chemisorption, while the addition of the second hydrogen atom is barrierless . Amorphous solid water is among the most common surfaces in the interstellar medium, as most dust grains are covered by water.…”

Der Tunneleffekt von Atomen in der Chemie

“…Eine Modellreaktion für die H 2 ‐Bildung auf kohlenstoffhaltigen Staubteilchen ist die Wasserstoffaddition an Benzol, die mit quantenchemischen Methoden untersucht wurde . In diesem Fall trägt Tunneln zur Reaktiongeschwindigkeit der ersten Wasserstoffchemisorption bei, während die Addition des zweiten Wasserstoffatoms barrierelos abläuft . Amorphes Eis ist vermutlich die häufigste Oberfläche im interstellaren Medium, da Wasser die meisten Staubkörner bedeckt.…”