Nitrogen-bearing molecules, like ammonia (NH3) or methylamine (CH3NH2) can be precursors of prebiotic molecules in the interstellar medium. In this work, the gas-phase kinetics of the CN reaction with NH3 (reaction 1) and CH3NH2 (reaction 2) was experimentally investigated between 11.7 and 177.5 K. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the temperature dependence of the rate coefficients (k1(T) for NH3 and k2(T) for CH3NH2). It was observed that both rate coefficients increase when temperature decreases. The modified Arrhenius expressions obtained by combining our results with those from the literature for reactions 1 and 2 are: k1(11.7-295 K) = (2.74 +/- 0.09) & times; 10-11 (T/300 K)-1.36 +/- 0.04 exp[-(16.6 +/- 2.2) K/T] cm3 s-1 and k2(49.1-297 K) = (3.10 +/- 0.25) & times; 10-10 (T/300 K)-0.27 +/- 0.07 cm3 s-1. Additionally, k2 was found to be independent on temperature between 11.7 and 49.1 K, (5.23 +/- 0.65) & times; 10-10 cm3 s-1. No pressure dependence of k1(T) and k2(T) was observed in the (3.37-16.7) & times; 1016 cm-3 total gas density range at 22 K. The impact of the determined rate coefficients k1(11.7 K) and k2(11.7 K) in the modeled abundances of the reactants NH3 and CH3NH2 is only moderate, although k2(11.7 K) drives a significant enhancement of the reaction products, especially for CH3NH radical, the abundance of which increases by about 1 order of magnitude.

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.

Nitrogen-bearing molecules, like methylamine (CH3NH2), can be the building blocks of amino acids in the interstellar medium (ISM). At the ultralow temperatures of the ISM, it is important to know its gas-phase reactivity towards interstellar radicals and the products formed. In this work, the kinetics of the OH + CH3NH2 reaction was experimentally and theoretically investigated at low- and high-pressure limits (LPL and HPL) between 10 and 1000 K. Moreover, the CH2NH2 and CH3NH yields were computed in the same temperature range for both pressure regimes. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the rate coefficient, k(T), in the 11.7-177.5 K range. A drastic increase of k(T) when the temperature is lowered was observed in agreement with theoretical calculations, evaluated by the competitive canonical unified statistical (CCUS) theory, below 300 K in the LPL regime. The same trend was observed in the HPL regime below 350 K, but the theoretical k(T) values were higher than the experimental ones. Above 200 K, the calculated rate coefficients are improved with respect to previous computational studies and are in excellent agreement with the experimental literature data. In the LPL, the formation of CH3NH becomes largely dominant below ca. 100 K. Conversely, in the HPL regime, CH2NH2 is the only product below 100 K, whereas CH3NH becomes dominant at 298 K with a branching ratio similar to the one found in the LPL regime (≈70%). At T > 300 K, both reaction channels are competitive independently of the pressure regime.

Interstellar molecules containing N atoms, such as ammonia (NH 3 ) and methylamine (CH 3 NH 2 ), could be potential precursors of amino acids like the simplest one, glycine (NH 2 CH 2 COOH). The gas-phase reactivity of these N-bearing species with OH radicals, ubiquitous in the interstellar medium, is not known at temperatures of cold dark molecular clouds. In this work, we present the first kinetic study of these OH-reactions at around 22 K and different gas densities [(3.4–16.7) × 10 16 cm −3 ] in helium. The obtained rate coefficients, with ± 2σ uncertainties, can be included in pure gas-phase or gas-grain astrochemical models to interpret the observed abundances of NH 3 and CH 3 NH 2 . We observed an increase of k 1 and k 2 with respect to those previously measured by others at the lowest temperatures for which rate coefficients are presently available: 230 and 299 K, respectively. This increase is about 380 times for NH 3 and 20 times for CH 3 NH 2 . Although the OH + NH 3 reaction is included in astrochemical kinetic databases, the recommended temperature dependence for k 1 is based on kinetic studies at temperatures above 200 K. However, the OH + CH 3 NH 2 reaction is not included in astrochemical networks. The observed increase in k 1 at ca. 22 K does not significantly change the abundance of NH 3 in a typical cold dark interstellar cloud. However, the inclusion of k 2 at ca. 22 K, not considered in astrochemical networks, indicates that the contribution of this destruction route for CH 3 NH 2 is not negligible, accounting for 1/3 of the assumed main depletion route (reaction with HCO + ) in this IS environment. k1(OH+NH3)=(2.7±0.1)×10−11cm3s-1 k2(OH+CH3NH2)=(3.9±0.1)×10−10cm3s-1

The role of water vapor (H2O) and its hydrogen-bonded complexes in the gas-phase reactivity of organic compounds with hydroxyl (OH) radicals has been the subject of many recent studies. Contradictory effects have been reported at temperatures between 200 and 400 K. For the OH + acetaldehyde reaction, a slight catalytic effect of H2O was previously reported at temperatures between 60 and 118 K. In this work, we used Laval nozzle expansions to reinvestigate the impact of H2O on the OH-reactivity with acetaldehyde between 21.7 and 135.0 K. The results of this comprehensive study demonstrate that water, instead, slows down the reaction by factors of ∼3 (21.7 K) and ∼2 (36.2-89.5 K), and almost no effect of added H2O was observed at 135.0 K.

Gas-phase reactions in the interstellar medium (ISM) are a source of molecules in this environment. The knowledge of the rate coefficient for neutral-neutral reactions as a function of temperature, k(T), is essential to improve astrochemical models. In this work, we have experimentally measured k(T) for the reaction between the OH radical and acetaldehyde, both present in many sources of the ISM. Laser techniques coupled to a CRESU system were used to perform the kinetic measurements. The obtained modified Arrhenius equation is k(T = 11.7-177.5 K) = (1.2 ± 0.2) × 10-11 (T/300 K)-(1.8±0.1) exp-{(28.7 ± 2.5)/T} cm3 molecule-1 s-1. The k(T) value of the title reaction has been measured for the first time below 60 K. No pressure dependence of k(T) was observed at ca. 21, 50, 64 and 106 K. Finally, a pure gas-phase model indicates that the title reaction could become the main CH3CO formation pathway in dark molecular clouds, assuming that CH3CO is the main reaction product at 10 K.

An experimental product study of the reactions of furfural with the main tropospheric oxidants (Cl, OH and NO3) has been carried out using a Fourier Transform Infrared spectrophotometer (FTIR) and a gas chromatograph–mass spectrometer with a time of flight detector (GC–TOFMS). The main gas-phase products detected were 5-chloro-2(5H)-furanone, maleic anhydride, 2-nitrofuran and CO. Molar yields were quantified for the detected products in these reactions, thus suggesting the existence of nongaseous products that could not be observed with the analytical techniques employed. The formation of Secondary Organic Aerosol (SOA) from the oxidation of furfural with Cl atoms, OH, NO3 and ozone was investigated in a smog chamber in the absence of inorganic seed aerosols. The experimental results show the formation of ultrafine particles (less than 1 µm in diameter) for all of the studied reactions except for the nitrate radical. Given their small size, these ultrafine particles (<1 µm) can easily penetrate into the respiratory tract and reach the alveolar region. These particles, therefore, have the potential to cause severe damage to the respiratory system. The aerosol yield obtained, Y, was low (<0.04) in all cases, which means that the aerosols generated from furfural, under atmospheric conditions, could have little impact.

Abstract The rate coefficients, k(T= 11.7 – 64.4 K), for the gas-phase reaction between OH radicals and acetone, CH3C(O) CH3, have been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique, the most suitable one to cool down gases below the freezing point without gas condensation. The experimental k(T) was found to increase as temperature was lowered and is several orders of magnitude higher for low temperature than k(300 K). No pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density is twice higher than the average values found at lower gas densities. The obtained values of k(11.7 K) and k(21.1 K) were 2.45’10-10 and 1.39’10-10 cm3 molecule-1 s-1, respectively.

The reactivity of methanol (CH3OH) toward the hydroxyl (OH) radical was investigated in the temperature range 11.7-177.5 K using the CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique. In the present study, the temperature dependence of the rate coefficient for the OH + CH3OH reaction, k(T), has been revisited and additional experimental and computational data are reported. New kinetic measurements were performed to fill the existing gaps (<22 K, 22-42 K and 88-123 K), reporting k(T < 20 K) for the first time. The lowest temperature ever achieved by a pulsed CRESU has been obtained in this work (11.7 K). k(T) abruptly increases by almost 2 orders of magnitude from 177.5 K to around 100 K. At T < 100 K, this increase is less pronounced, reaching the capture limit at temperatures below 22 K. The pressure dependence of k(T) has been investigated for selected temperatures and gas densities (1.5 × 1016 to 4.3 × 1017 cm-3), combining our results with those previously reported. No dependence was observed within the experimental uncertainties below 110 K. The high- and low-pressure rate coefficients, kHPL(T) and kLPL(T), were also studied in detail using high-level quantum chemical and theoretical kinetic methodologies, closely reproducing the experimental data between 20 and 400 K. The results suggest that the experimental data are near the high pressure limit at the lowest temperatures, but that the reaction remains a fast and effective source of CH2OH and CH3O at the low pressures and temperatures prevalent in the interstellar medium.

AbstractMethanol (CH3OH) and hydroxyl (OH) radicals are two species abundant in cold and dense molecular clouds which are important for the chemistry of the interstellar medium (ISM). CH3OH is a well-known starting point for the formation of more complex organic molecules (COMs) in these molecular clouds. Thus, the reactivity of CH3OH in the gas-phase with OH may play a crucial role in the formation of species as complex as prebiotic molecules in the ISM and reliable rate coefficients should be used in astrochemical models describing low temperature reaction networks.

The rate coefficient, k(T), for the gas-phase reaction between OH radicals and acetone CH3C(O)CH3, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique (T = 11.7-64.4 K). The temperature dependence of k(T = 10-300 K) has also been computed using a RRKM-Master equation analysis after partial revision of the potential energy surface. In agreement with previous studies we found that the reaction proceeds via initial formation of two pre-reactive complexes both leading to H2O + CH3C(O)CH2 by H-abstraction tunneling. The experimental k(T) was found to increase as temperature was lowered. The measured values have been found to be several orders of magnitude higher than k(300 K). This trend is reproduced by calculations, with a special good agreement with experiments below 25 K. The effect of total gas density on k(T) has been explored. Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47×1017 cm-3 is twice higher than the average values found at lower densities. The computed k(T) is also reported for 103 cm-3 of He (representative of the interstellar medium). The predicted rate coefficients at 10 K surround the experimental value which appears to be very close to the low pressure regime prevailing in the interstellar medium. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8×10-10 cm3 molecule-1 s-1 at 10 K and the reaction products are water and CH3C(O)CH2 radicals.

The article Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature proposes a dimer mediated mechanism in order to explain the large low temperature rate coefficients for the OH + methanol reaction measured by several groups. It is demonstrated here theoretically that under the conditions of these low temperature experiments, there are insufficient dimers formed for the proposed new mechanism to apply. Experimental evidence is also presented to show that dimerization of the methanol reagent does not influence the rate coefficients reported under the conditions of methanol concentration used for the kinetics studies. It is also emphasised that the low temperature experiments have been performed using both the Laval nozzle expansion and flow-tube methods, with good agreement found for the rate coefficients measured using these two distinct techniques.

The relative-rate technique has been used to determine the rate coefficient for the reaction of CxF2x+1CHCH2 (x = 1, 2, 3, 4 and 6) with ozone at (298 ± 2) K and (720 ± 5) Torr of air by FTIR (Fourier Transform Infrared Spectroscopy) and by GC-MS/SPME (Gas Chromatography-Mass Spectroscopy with Solid Phase Micro Extraction) in two different atmospheric simulation chambers. The following rate coefficients, in units of 10-19 cm3 molecule-1 s-1, were obtained: (3.01 ± 0.10) for CF3CHCH2, (2.11 ± 0.35) for C2F5CHCH2, (2.34 ± 0.42) for C3F7CHCH2, (2.05 ± 0.31) for C4F9CHCH2 and (2.07 ± 0.39) for C6F13CHCH2, where uncertainties represent ±2σ statistical error. The atmospheric lifetime of CxF2x+1CHCH2 due to reaction with ozone was estimated from the reported rate coefficients. Additionally, the gaseous products formed in these reactions were investigated in the presence of synthetic air simulating a clean atmosphere. Perfluoroaldehydes, CxF2x+1C(O)H (PFALs), formaldehyde, formic acid and CF2O were identified as reaction products in the investigated reactions. The identified products made possible to propose a reaction mechanism that justifies the observed products. The atmospheric implications of these results are discussed in terms of the potential contribution of the atmospheric degradation of these species to PFAL and PFCA burden.

Rate coefficients for the OH-reaction with ethanol, ubiquitous in the interstellar medium, has been determined at ultra-cold temperatures by using the pulsed and continuous CRESU technique.

We report here the rate coefficients for the OH reactions (kOH) with E-CF3CH═CHF and Z-CF3CH═CHF, potential substitutes of HFC-134a, as a function of temperature (263-358 K) and pressure (45-300 Torr) by pulsed laser photolysis coupled to laser-induced fluorescence techniques. For the E-isomer, the existing discrepancy among previous results on the T dependence of kOH needs to be elucidated. For the Z-isomer, this work constitutes the first absolute determination of kOH. No pressure dependence of kOH was observed, while kOH exhibits a non-Arrhenius behavior: kOH(E) = [Formula: see text] and kOH(Z) = [Formula: see text] cm3 molecule-1 s-1, where uncertainties are 2σ. UV absorption cross sections, σλ, are reported for the first time. From σλ and considering a photolysis quantum yield of 1, an upper limit for the photolysis rate coefficients and lifetimes due to this process in the troposphere are estimated: 3 × 10-8 s-1 and >1 year for the E-isomer and 2 × 10-7 s-1 and >2 months for Z-CF3CH═CHF, respectively. Under these conditions, the overall estimated tropospheric lifetimes are 15 days (for the E-isomer) and 8 days (for the Z-isomer), the major degradation pathway being the OH reaction, with a contribution of the photolytic pathway of less than 3% (for E) and 13% (for Z). IR absorption cross sections were determined both experimentally (500-4000 cm-1) and theoretically (0-2000 cm-1). From the theoretical IR measurements, it is concluded that the contribution of the 0-500 cm-1 region to the total integrated cross sections is appreciable for the E-isomer (9%) but almost negligible for the Z-isomer (0.5%). Nevertheless, the impact on their radiative efficiency and global warming potential is negligible.

The reaction of 4-hydroxy-4-methyl-2-pentanone (4H4M2P) with CI atoms was studied for the first time experimentally and theoretically. Relative kinetic measurements were carried out at room temperature and 1 bar of synthetic air/N-2 in two different environmental chambers: a 300 L Teflon bag and a 16 L borosilicate glass cell. Reactants, reference compounds and products were monitored either by IR absorption or by GC-FID. Theoretical calculations were performed using the density functional theory method at BH&HLYP level of theory for twelve hydrogen abstraction pathways. The individual rate coefficients for the most favorable H-abstraction pathways were calculated by canonical variational theory using small curvature tunneling method at 298 K. An average experimental rate coefficient of (7.4 +/- 0.6) x 10(-11) cm(3) molecule(-1) s(-1) was obtained at 298 K, in good agreement with the theoretical rate coefficient. The branching ratios for each reaction channel were evaluated theoretically from the individual rate coefficients of the identified channels. The H-atom abstracted on the-CH2 group appeared to be the dominant channel with a small barrier height. Formaldehyde, acetic acid, HCI, CO2 and CO were identified by IR as the major primary products. The obtained results are presented and discussed in terms of structure-reactivity relationships. A mechanism is suggested for the formation of the observed products. The atmospheric implications of the studied reaction are presented and more particularly, the lifetime of 4H4M2P towards Cl atoms is evaluated to be about 3 days. (C) 2017 Elsevier Ltd. All rights reserved.