Importance of tunneling in H-abstraction reactions by OH radicals

Lamberts, Thanja; Fedoseev, G.; Kästner, Johannes; Ioppolo, Sergio; Linnartz, H.

doi:10.1051/0004-6361/201629845

Cited by 15 publications

(17 citation statements)

References 81 publications

(85 reference statements)

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“…The OH radicals are formed by H-atom addition to O 2 ) and the CH 3 radicals are formed by OHmediated H-abstraction of CH 4 (i.e., radicals are formed in situ). A series of control experiments performed by Lamberts et al (2017) showed that the produced OH radicals are solely respon- Table 1. Experiments performed in this study.…”

Section: Experimental Setup and Methodsmentioning

confidence: 99%

“…(♦) "(MWAS)" denotes species that were placed in the MWAS chamber. See also Section 3.2. sible for the H-abstraction of CH 4 (i.e., formation of CH 3 radicals directly through reaction with H-atoms is not efficient, as proved in Lamberts et al (2017)). We note that the formation of OH radicals from O 2 is not necessarily representative of the formation of OH radicals in low A V (low density) environments, where the O + H route dominates, as well as photodissociation of H 2 O.…”

Section: Experimental Setup and Methodsmentioning

confidence: 99%

“…Recent theoretical and experimental efforts (Lamberts et al 2017) have provided results that have sparked the idea of H 2 O and CH 3 OH coexisting in a CO-poor and H 2 O-rich interstellar ice. Such ices are thought to form before the "heavy" CO freezeout stage, which starts to occur at a cloud extinction (A V ) of A V > 3 and dust temperatures < 20 K, and that is followed by the "catastrophic" CO freeze-out stage at A V > 9 and dust temperatures also < 20 K. Prior to the heavy CO freeze-out stage, a CO:H 2 O ice ratio of < 5% is expected due to some CO freezing out as well as atom-addition reactions (C, O, H, etc.).…”

Section: Introductionmentioning

confidence: 99%

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Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

Qasim

Chuang

Fedoseev

et al. 2018

A&A

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Context. The formation of methanol (CH 3 OH) on icy grain mantles during the star formation cycle is mainly associated with the CO freeze-out stage. Yet there are reasons to believe that CH 3 OH also can form at an earlier period of interstellar ice evolution in CO-poor and H 2 O-rich ices. Aims. This work focuses on CH 3 OH formation in a H 2 O-rich interstellar ice environment following the OH-mediated H-abstraction in the reaction, CH 4 + OH. Experimental conditions are systematically varied to constrain the CH 3 OH formation yield at astronomically relevant temperatures. Methods. CH 4 , O 2 , and hydrogen atoms are co-deposited in an ultrahigh vacuum chamber at 10-20 K. OH radicals are generated by the H + O 2 surface reaction. Temperature programmed desorption -quadrupole mass spectrometry (TPD-QMS) is used to characterize CH 3 OH formation, and is complemented with reflection absorption infrared spectroscopy (RAIRS) for CH 3 OH characterization and quantitation. Results. CH 3 OH formation is shown to be possible by the sequential surface reaction chain, CH 4 + OH → CH 3 + H 2 O and CH 3 + OH → CH 3 OH at 10-20 K. This reaction is enhanced by tunneling, as noted in a recent theoretical investigation (Lamberts et al. 2017). The CH 3 OH formation yield via the CH 4 + OH route versus the CO + H route is approximately 20 times smaller for the laboratory settings studied. The astronomical relevance of the new formation channel investigated here is discussed.

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Section: Experimental Setup and Methodsmentioning

confidence: 99%

Section: Experimental Setup and Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

Qasim

Chuang

Fedoseev

et al. 2018

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“…We compare our instanton rate constants to one‐dimensional tunneling approximations and to rate constants obtained without tunneling. Even though these approaches have been used in our group's previous work, they have not yet been described in detail. In harmonic transition state theory, the potential energy surface around the reactant and the transition structure is approximated harmonically.…”

Section: Theorymentioning

confidence: 99%

Instanton rate constant calculations close to and above the crossover temperature

McConnell

Kästner

2017

J Comput Chem

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Canonical instanton theory is known to overestimate the rate constant close to a system-dependent crossover temperature and is inapplicable above that temperature. We compare the accuracy of the reaction rate constants calculated using recent semi-classical rate expressions to those from canonical instanton theory. We show that rate constants calculated purely from solving the stability matrix for the action in degrees of freedom orthogonal to the instanton path is not applicable at arbitrarily low temperatures and use two methods to overcome this. Furthermore, as a by-product of the developed methods, we derive a simple correction to canonical instanton theory that can alleviate this known overestimation of rate constants close to the crossover temperature. The combined methods accurately reproduce the rate constants of the canonical theory along the whole temperature range without the spurious overestimation near the crossover temperature. We calculate and compare rate constants on three different reactions: H in the Müller-Brown potential, methylhydroxycarbene → acetaldehyde and H + OH → H + H O. © 2017 Wiley Periodicals, Inc.

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“…However, in these regions, the water abundance is very low ( 10 −8 ) and the higher rate constants do not improve the efficiency of water formation above abundances of 10 −8...−9 (typical values reached by photodesorption in the outer disk). Lamberts et al (2017) and Beyer et al (2016), respectively. When using the fits to these rate constants in the T Tauri disk model, we did not find significant differences in the species abundances.…”

Section: Use Of Correct Reaction Rate Constantsmentioning

confidence: 99%

The role of atom tunneling in gas-phase reactions in planet-forming disks

Meisner

Kamp

Thi

et al. 2019

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Context. Gas-phase chemical reactions of simple molecules have been recently revised to include atom tunneling at very low temperatures. This paper investigates the impact of the increased reaction rate constant due to tunneling effects on planet-forming disks. Aims. Our aim is to quantify the astrophysical implications of atom tunneling for simple molecules that are frequently used to infer disk structure information or to define the initial conditions for planet (atmosphere) formation. Methods. We quantify the tunneling effect on reaction rate constants by using H 2 + OH → H 2 O + H as a scholarly example in comparison to previous UMIST2012 rate constants. In a chemical network with 1299 reactions, we identify all chemical reactions that could show tunneling effects. We devise a simple formulation of reaction rate constants that overestimates tunneling and screen a standard T Tauri disk model for changes in species abundances. For those reactions found to be relevant, we find values of the most recent literature for the rate constants including tunneling and compare the resulting disk chemistry to the standard disk model(s), a T Tauri and a Herbig disk. Results. The rate constants in the UMIST2012 database in many cases already capture tunneling effects implicitly, as seen in the curvature of the Arrhenius plots of some reactions at low temperature. A rigorous screening procedure identified three neutral-neutral reactions where atom tunneling could change simple molecule abundances. However, by adopting recent values of the rate constants of these reactions and due to the layered structure of planet-forming disks, the effects are limited to a small region between the ion-molecule dominated regime and the ice reservoirs where cold (< 250 K) neutral-neutral chemistry dominates. Abundances of water close to the midplane snowline can increase by a factor of two at most compared to previous results with UMIST2012 rates. Observables from the disk surface, such as high excitation (> 500 K) water line fluxes, decrease by 60% at most when tunneling effects are explicitly excluded. On the other hand, disk midplane quantities relevant for planet formation such as the C-to-O ratio and also the ice-to-rock ratio are clearly affected by these gas-phase tunneling effects.

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Importance of tunneling in H-abstraction reactions by OH radicals

Cited by 15 publications

References 81 publications

Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

Instanton rate constant calculations close to and above the crossover temperature

The role of atom tunneling in gas-phase reactions in planet-forming disks

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