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Gas phase kinetics of the OH + CH3CH2OH reaction at temperatures of the interstellar medium (T = 21-107 K).

ABSTRACT: Ethanol, CH3CH2OH, has been unveiled in the interstellar medium (ISM) by radioastronomy and it is thought to be released into the gas phase after the warm-up phase of the grain surface, where it is formed. Once in the gas phase, it can be destroyed by different reactions with atomic and radical species, such as hydroxyl (OH) radicals. The knowledge of the rate coefficients of all these processes at temperatures of the ISM is essential in the accurate interpretation of the observed abundances. In this work, we have determined the rate coefficient for the reaction of OH with CH3CH2OH (k(T)) between 21 and 107 K by employing the pulsed and continuous CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, which means Reaction Kinetics in a Uniform Supersonic Flow) technique. The pulsed laser photolysis technique was used for generating OH radicals, whose time evolution was monitored by laser induced fluorescence. An increase of approximately 4 times was observed for k(21 K) with respect to k(107 K). With respect to k(300 K), the OH-reactivity at 21 K is enhanced by two orders of magnitude. The obtained T-expression in the investigated temperature range is k(T) = (2.1 ± 0.5) × 10-11 (T/300 K)-(0.71±0.10) cm3 molecule-1 s-1. In addition, the pressure dependence of k(T) has been investigated at several temperatures between 21 K and 90 K. No pressure dependence of k(T) was observed in the investigated ranges. This may imply that this reaction is purely bimolecular or that the high-pressure limit is reached at the lowest total pressure experimentally accessible in our system. From our results, k(T) at usual IS temperatures (?10-100 K) is confirmed to be very fast. Typical rate coefficients can be considered to range within about 4 × 10-11 cm3 molecule-1 s-1 at 100 K and around 1 × 10-10 cm3 molecule-1 s-1 at 20 K. The extrapolation of k at the lowest temperatures of the dense molecular clouds of ISM is also discussed in this paper.

SUBMITTER: Ocana AJ

PROVIDER: S-EPMC5975950 | biostudies-literature | 2018 Feb

REPOSITORIES: biostudies-literature

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Gas phase kinetics of the OH + CH3CH2OH reaction at temperatures of the interstellar medium (T = 21-107 K).

Ocaña A J AJ Blázquez S S Ballesteros B B Canosa A A Antiñolo M M Albaladejo J J Jiménez E E

Physical chemistry chemical physics : PCCP 20180201 8

Ethanol, CH3CH2OH, has been unveiled in the interstellar medium (ISM) by radioastronomy and it is thought to be released into the gas phase after the warm-up phase of the grain surface, where it is formed. Once in the gas phase, it can be destroyed by different reactions with atomic and radical species, such as hydroxyl (OH) radicals. The knowledge of the rate coefficients of all these processes at temperatures of the ISM is essential in the accurate interpretation of the o ...[more]

PMID: 29417104

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Project description:Isocyanic acid, HNCO, mainly emitted by combustion processes, is doubted to be detrimental to human health if its concentration surpasses ∼1 ppbv. Very little information has been found regarding the HNCO loss in the gas phase. This study aims to close this knowledge gap by performing a theoretical kinetic study on the reaction of HNCO with the propargyl radical. The potential energy surface of the HNCO + C3H3 reaction was characterized utilizing high-level CCSD(T)/CBS(TQ5)//B3LYP/6-311++G(3df,2p) quantum-chemical approaches, followed by TST and RRKM/ME kinetic computations. The obtained results reveal that the reaction can proceed via H-abstraction, leading to the C3H4 + NCO bimolecular products with energy barriers of 23-25 kcal/mol, and/or addition, resulting in C4H4NO intermediates with 23-26 kcal/mol barrier heights. The C4H4NO adducts when formed can decompose to products and/or return to HNCO + C3H3 in which the reverse decompositions are found to be dominant with a branching ratio that accounts for nearly 100% at 300 K and 760 Torr. The calculated P-independent rate coefficients indicate that at low temperatures, the H-abstraction channels are insignificant. However, at high temperatures (T > 1500 K), the H-abstraction path leading to H3CCCH + NCO prevails with a branching ratio of ∼50-53% in the descending 1800-1500 K temperature range at 760 Torr, while the H-abstraction leading to H2CCCH2 + NCO is favorable at T > 1800 K, with the yield reaching above 50% at 760 Torr. In contrast to the H-abstraction rate constants, the calculated values for the additions and the C4H4NO decompositions show a positive pressure dependence. Both the total rate constants for the reactions HNCO + C3H3 → products and C4H4NO → products, which are, respectively, k _total_bimo(T) = 3.53 × 10-23 T 3.27 exp[(-21.35 ± 0.06 kcal/mol)/RT] cm3 molecule-1 s-1 and k _total_uni(T) = 1.13 × 1025 T -4.02 exp[(-11.77 ± 0.16 kcal/mol)/RT] s-1, increase with the increasing temperature in the 300-2000 K range at 760 Torr. The rate constant of HNCO + C3H3 → products is about 8 orders of magnitude smaller than the value of HCHO + C3H3 → products, showing that HCHO is more reactive toward the C3H3 free radicals than HNCO. The computed heats of formation for several species agree well with the available literature data with the deviation less than 1.0 kcal/mol, indicating that the methods used in this study are extremely reliable. With the given results, it is vigorously suggested that the predicted rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling HNCO-related systems under atmospheric and combustion conditions.

Dataset Information

Gas phase kinetics of the OH + CH3CH2OH reaction at temperatures of the interstellar medium (T = 21-107 K).

Publications

Gas phase kinetics of the OH + CH<sub>3</sub>CH<sub>2</sub>OH reaction at temperatures of the interstellar medium (T = 21-107 K).

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