Journal Article

Quantum tunneling observed without its characteristic large kinetic isotope effects

Naoki Watanabe, Hirokazu Ueta, Akira Kouchi, Hama, Tetsuya

2015

Overview

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. Significance Quantum tunneling, an important phenomenon in many surface and interfacial chemical processes, is strongly dependent on the isotope of the tunneling atom. However, surface tunneling during the hydrogenation/deuteration of solid benzene at 15–25 K is accompanied by an almost semiclassical kinetic isotope effect (KIE) of 1–1.5, which is much lower than that intrinsic to tunneling (≳100), because isotopically insensitive surface diffusion of the adsorbed atoms controls the chemical kinetics. Our results suggest that tunneling has been unrecognized in studies of the chemistry of condensed phases, and small-KIE tunneling may account for the unexplained fast reactions of hydrogen and deuterium observed in surface/interface chemical systems such as aerosols, enzymes, and interstellar dust grains.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: aerosols / Aliphatic hydrocarbons / Aromatic hydrocarbons / astrochemistry / Benzene / Chemical kinetics / Chemical reactions / Deuterium / Dust / Effects / enzymes / grains / heterogeneous reactions / hydrogenation / Isotopes / kinetic isotope effect / Physical Sciences / quantum tunneling / reaction dynamics / Reaction kinetics