Kinetic isotope effects of &lt;sup&gt;12&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;D  + OH and &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;D  + OH from 278 to 313 K

Abstract. Methane is the second most important long-lived greenhouse gas and plays a central role in the chemistry of the Earth's atmosphere. Nonetheless there are significant uncertainties in its source budget. Analysis of the isotopic composition of atmospheric methane, including the doubly substituted species 13CH3D, offers new insight into the methane budget as the sources and sinks have distinct isotopic signatures. The most important sink of atmospheric methane is oxidation by OH in the troposphere, which accounts for around 84 % of all methane removal. Here we present experimentally derived methane + OH kinetic isotope effects and their temperature dependence over the range of 278 to 313 K for CH3D and 13CH3D; the latter is reported here for the first time. We find kCH4/kCH3D = 1.31 ± 0.01 and kCH4/k13CH3D = 1.34 ± 0.03 at room temperature, implying that the methane + OH kinetic isotope effect is multiplicative such that (kCH4/k13CH4)(kCH4/kCH3D) = kCH4/k13CH3D, within the experimental uncertainty, given the literature value of kCH4/k13CH4 = 1.0039 ± 0.0002. In addition, the kinetic isotope effects were characterized using transition state theory with tunneling corrections. Good agreement between the experimental, quantum chemical, and available literature values was obtained. Based on the results we conclude that the OH reaction (the main sink of methane) at steady state can produce an atmospheric clumped isotope signal (Δ(13CH3D) = ln([CH4][13CH3D]/[13CH4][CH3D])) of 0.02 ± 0.02. This implies that the bulk tropospheric Δ(13CH3D) reflects the source signal with relatively small adjustment due to the sink signal (i.e., mainly OH oxidation).

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Development of a new methane tracer: kinetic isotope effect of 13CH3D + OH from 278 to 313 K

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-27853-2015 ◽

2015 ◽

Vol 15 (19) ◽

pp. 27853-27875 ◽

Cited By ~ 2

Author(s):

L. M. T. Joelsson ◽

J. A. Schmidt ◽

E. J. K. Nilsson ◽

T. Blunier ◽

D. W. T. Griffith ◽

...

Keyword(s):

Isotope Effect ◽

Kinetic Isotope Effect ◽

Theoretical Calculations ◽

Isotope Effects ◽

Oxidative Capacity ◽

State Theory ◽

Atmospheric Methane ◽

Isotopic Signatures ◽

Budget Analysis ◽

Kinetic Isotope

Abstract. Methane is the second most important long lived greenhouse gas and impacts the oxidative capacity of the Earth's atmosphere. Nontheless there are significant uncertainties in its source budget. Analysis of the isotopic composition of atmospheric methane, including doubly substituted species (e.g. 13CH3D), offers new constraints on the methane source budget as the sources and sinks have distinct isotopic signatures. The most important sink of atmospheric methane is oxidation by OH which accounts for around 90 % of methane removal in the troposphere. Here we present experimentally derived methane + OH kinetic isotope effects and their temperature dependence over the range of 278 to 313 K for CH3D and 13CH3D; the latter is reported here for the first time. We find kCH4/kCH3D=1.31 ± 0.01 and kCH4/k13CH3D = 1.34 ± 0.03 at room temperature, implying that the methane + OH kinetic isotope effect is multiplicative such that (kCH4/k13CH4)(kCH4/kCH3D) = kCH4/k13CH3D to within the experimental uncertainty. In addition the kinetic isotope effect were characterized using transition state theory with tunneling correction. Good agreement between the experimental, quantum chemical and available literature values was obtained. The theoretical calculations show that 13CH3D isotope effects is the product of D- and 13C-isotope effect. Based on the results we conclude that the OH reaction at steady-state can produce an atmospheric clumped isotope signal (Δ(13CH3D) = ln([CH4][13CH3D]/[13CH4][CH3D])) of 0.02 ± 0.02.

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Quantum tunneling observed without its characteristic large kinetic isotope effects

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1501328112 ◽

2015 ◽

Vol 112 (24) ◽

pp. 7438-7443 ◽

Cited By ~ 13

Author(s):

Tetsuya Hama ◽

Hirokazu Ueta ◽

Akira Kouchi ◽

Naoki Watanabe

Keyword(s):

Chemical Kinetics ◽

Quantum Tunneling ◽

Interstellar Dust ◽

Isotope Effects ◽

Kinetic Isotope Effects ◽

State Theory ◽

Dust Particles ◽

Interplanetary Dust Particles ◽

Chemical Systems ◽

Kinetic Isotope

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.

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