Reaction of OH radicals with CH3NH2 in the gas phase: experimental (11.7-177.5 K) and computed rate coefficients (10-1000 K)

Insights into the role of the H-abstraction reaction kinetics of amines in understanding their degeneration fates under atmospheric and combustion conditions

Amines, a class of prototypical volatile organic compounds, have garnered considerable interest within the context of atmospheric and combustion chemistry due to their substantial contributions to the formation of hazardous pollutants in the atmosphere. In the current energy landscape, the implementation of carbon-neutral energy and strategic initiatives leads to generation of new amine sources that cannot be overlooked in terms of the emission scale. To reduce the emission level of amines from their sources and mitigate their impact on the formation of harmful substances, a comprehensive understanding of the fundamental reaction kinetics during the degeneration process of amines is imperative. This perspective article first presents an overview of both traditional amine sources and emerging amine sources within the context of carbon peaking and carbon neutrality and then highlights the importance of H-abstraction reactions in understanding the atmospheric and combustion chemistry of amines from the perspective of reaction kinetics. Subsequently, the current experimental and theoretical techniques for investigating the H-abstraction reactions of amines are introduced, and a concise summary of research endeavors made in this field over the past few decades is provided. In order to provide accurate kinetic parameters of the H-abstraction reactions of amines, advanced kinetic calculations are performed using the multi-path canonical variational theory combined with the small-curvature tunneling and specific-reaction parameter methods. By comparing with the literature data, current kinetic calculations are comprehensively evaluated, and these validated data are valuable for the development of the reaction mechanism of amines.

Unveiling the chemical kinetics of aminomethanol (NH2CH2OH): insights into O . H and O2 photo-oxidation reactions and formamide dominance

Aminomethanol is released into the atmosphere through various sources, including biomass burning. In this study, we have expounded the chemical kinetics of aminomethanol in the reaction pathways initiated by the hydroxyl radical (O ˙ H) with the aid of ab initio//density functional theory (DFT) i.e., coupled-cluster theory (CCSD(T))//hybrid-DFT (M06-2X/6-311++G (3df, 3pd). We have explored various possible directions of theO ˙ H radical on aminomethanol, as well as the formation of distinct pre-reactive complexes. Our computational findings reveal that the H transfer necessitates activation energies ranging from 4.1 to 6.5 kcal/mol from the -CH2 group, 3.5-6.5 kcal/mol from the -NH2 group and 7-9.3 kcal/mol from the -OH group of three rotational conformers. The H transfer from -CH2, -NH2 and -OH exhibits an estimated total rate constant (k OH) of approximately 1.97 × 10-11 cm3 molecule-1 s-1 at 300 K. The branching fraction analysis indicates a pronounced dominance of C-centered NH2C ˙ HOH radicals with a favorability of 77%, surpassing the N-centeredN ˙ HCH2OH (20%) and O-centered NH2CH2O ˙ (3%) radicals. Moreover, our investigation delves into the oxidation of the prominently favored carbon-centered NH2C ˙ HOH radical through its interaction with atmospheric oxygen molecules. Intriguingly, our findings reveal that formamide (NH2CHO) emerges as the predominant product in the NH2C ˙ HOH + 3O2 reaction, eclipsing alternative outcomes such as amino formic acid (NH2COOH) and formimidic acid (HN = C(H)-OH). At atmospheric conditions pertinent to the troposphere, the branching fraction value for the formation of formamide is about 99%, coupled with a rate constant of 5.5 × 10-12 cm3 molecule-1 s-1. Finally, we have scrutinized the detrimental impact of formamide on the atmosphere. Interaction of formamide with atmospheric hydroxyl radicals could give rise to the production of potentially perilous compounds such as HNCO. Further, unreactedN ˙ HCH2OH radicals may initiate the formation of carcinogenic nitrosamines when reacting with trace N-oxides (namely, NO and NO2). This, in turn, escalates the environmental risk factors.

Effect of temperature on the gas-phase reaction of CH3CN with OH radicals: experimental (T = 11.7-177.5 K) and computational (T = 10-400 K) kinetic study

Acetonitrile (CH3CN) is present in the interstellar medium (ISM) in a variety of environments. However, at the ultracold temperatures of the ISM, radical-molecule reactions are not widely investigated because of the experimental handicap of getting organic molecules in the gas phase by conventional techniques. The CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique solves this problem. For this reason, we present in this work the kinetic study of the gas-phase reaction of CH3CN with one of the most ubiquitous radicals, the hydroxyl (OH) radical, as a function of temperature (11.7-177.5 K). The kinetic technique employed to investigate the CH3CN + OH reaction was the pulsed laser photolysis-laser induced fluorescence. The rate coefficient for this reaction k(T) has been observed to drastically increase from 177.5 K to 107.0 K (about 2 orders of magnitude), while the increase in k(T) from 107.0 K to 11.7 K was milder (around 4 times). The temperature dependent expressions for k(T) are provided in the two distinct T-ranges, excluding the upper limit obtained for k(177.5 K): In addition, the rate coefficients estimated by the canonical competitive unified statistical (CCUS) theory show a similar behaviour to the experimental results, when evaluated within the high-pressure limit. This is consistent with the experimentally observed independence of k(T) with total gas density at selected temperatures. Astrochemical networks, such as the KIDA database or UMIST, do not include the CH3CN + OH reaction as a potential depletion process for acetonitrile in the ISM because the current studies predict very low rate coefficients at IS temperatures. According to the model (T = 10 K), the impact of the titled reaction on the abundances of CH3CN appears to be negligible in dark molecular clouds of the ISM (∼1% of the total depletion reactions included in UMIST network). With respect to the potential formation of the CH2CN radical in those environments, even in the most favourable scenario, where this radical could be formed in a 100% yield from the CH3CN + OH reaction, this route would only contribute around 2% to the current assumed formation routes by the UMIST network.

Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of •CH2NH2 + O2 (+NH3/H2O)

The aminomethyl (•CH2NH2) radical is generated from the photo-oxidation of methylamine in the troposphere and is an important precursor for new particle formation. The effect of ammonia and water on the gas-phase formation of methanimine (CH2NH) from the •CH2NH2 + O2 reaction is not known. Therefore, in this study, the potential energy surfaces for •CH2NH2 + O2 (+NH3/H2O) were constructed using ab initio//DFT, i.e., coupled-cluster theory (CCSD(T))//hybrid-density functional theory, i.e., M06-2X with the 6-311++G (3df, 3pd) basis set. The Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation with Eckart's asymmetric tunneling was used to calculate the rate coefficients and branching fractions relevant to the troposphere. The results show 40% formation of CH2NH at the low-pressure (<1 bar) and 100% formation of CH2NH2OO• at the high-pressure limit (HPL) condition. When an ammonia molecule is introduced into the reaction, there is a slight increase in the formation of CH2NH; however, when a water molecule is introduced into the reaction, the increase in the formation of CH2NH was from 40% to ∼80%. The calculated rate coefficient for •CH2NH2 + O2 (+NH3) [1.9 × 10-23 cm3 molecule-1 s-1] and for CH2NH2 + O2 (+H2O) [3.3 × 10-17 cm3 molecule-1 s-1] is at least twelve and six order magnitudes smaller than those for free •CH2NH2 + O2 (2 × 10-11 cm3 molecule-1 s-1 at 298 K) reactions, respectively. Our result is consistent with that of previous experimental and theoretical analysis and in good agreement with its isoelectronic analogous reaction. The work also provides a clear understanding of the formation of tropospheric carcinogenic compounds, i.e., hydrogen cyanide (HCN).

Gas-phase formation of glycolonitrile in the interstellar medium

Our automated reaction discovery program, AutoMeKin, has been utilized to investigate the formation of glycolonitrile (HOCH2CN) in the gas phase under the low temperatures of the interstellar medium (ISM). The feasibility of a proposed pathway depends on the absence of barriers above the energy of reactants and the availability of the suggested precursors in the ISM. Based on these criteria, several radical-radical reactions and a radical-molecule reaction have been identified as viable formation routes in the ISM. Among the radical-radical reactions, OH + CH2CN appears to be the most relevant, considering the energy of the radicals and its ability to produce glycolonitrile in a single step. However, our analysis reveals that this reaction produces hydrogen isocyanide (HNC) and formaldehyde (CH2O), with rate coefficients ranging from (7.3-11.5) × 10-10 cm3 molecule-1 s-1 across the temperature range of 10-150 K. Furthermore, the identification of this remarkably efficient pathway for HNC elimination from glycolonitrile significantly broadens the possibilities for any radical-radical mechanism proposed in our research to be considered as a feasible pathway for the formation of HNC in the ISM. This finding is particularly interesing given the persistently unexplained overabundance of hydrogen isocyanide in the ISM. Among the radical-molecule reactions investigated, the most promising one is OH + CH2CHNH, which forms glycolonitrile and atomic hydrogen with rate coefficients in the range (0.3-6.6) × 10-10 cm3 molecule-1 s-1 within the 10-150 K temperature range. Our calculations indicate that the formation of both hydrogen isocyanide and glycolonitrile is efficient under the harsh conditions of the ISM.