Gas-Phase Reactivity of OH Radicals With Ammonia (NH3) and Methylamine (CH3NH2) at Around 22 K

Abstract

Interstellar molecules containing N atoms, such as ammonia (NH3) and methylamine (CH3NH2), could be potential precursors of amino acids like the simplest one, glycine (NH2CH2COOH). 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) × 1016 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 NH3 and CH3NH2. We observed an increase of k1 and k2 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 NH3 and 20 times for CH3NH2. Although the OH + NH3 reaction is included in astrochemical kinetic databases, the recommended temperature dependence for k1 is based on kinetic studies at temperatures above 200 K. However, the OH + CH3NH2 reaction is not included in astrochemical networks. The observed increase in k1 at ca. 22 K does not significantly change the abundance of NH3 in a typical cold dark interstellar cloud. However, the inclusion of k2 at ca. 22 K, not considered in astrochemical networks, indicates that the contribution of this destruction route for CH3NH2 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-1k2(OH+CH3NH2)=(3.9±0.1)×10−10cm3s-1

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

“…14,15,32,33 The total uncertainty in k ( T ) has been calculated as the square root of the sum of the ±2 σ statistical error and the systematic error squared. As it can be seen, k ( T ) is continuously increasing while lowering the gas temperature from 177.5 K to 11.7 K. The published k ( T ) at 22 K 26 agrees with the observed trend in k ( T ). However, although the observed increase of k ( T ) is remarkable (more than one order of magnitude from k (177.5 K) to k (11.7 K)), it seems to slow down at T < 22 K. This levelling out of k ( T ) has already been observed more clearly in some other OH-reactions studied in our research group.…”

“…6 (black circles) along with the k (22 K) previously reported data (red circle). 26 The total uncertainties in k ( T ) shown in the Fig. 6 and in the Table 2 include the statistical errors (±2 σ ) derived from the slope of the versus [CH 3 NH 2 ] and the systematic errors, which has previously been estimated to be no more than 10% of the measured k ( T ).…”

“…Hence, varying [CH 3 NH 2 ] allows the individual k ( T ) to be obtained from the slope of versus [CH 3 NH 2 ] plots as described elsewhere. 14–16,26–33 However, at high [CH 3 NH 2 ], these plots exhibit a downward curvature (see Fig. S5 of the ESI†), which could be attributed to the formation of methylamine dimers, (CH 3 NH 2 ) 2 , especially at very low temperatures.…”

“…The CRESU apparatus and the experimental set-up have been described in detail in numerous previous studies. [14][15][16][26][27][28][29][30][31][32][33] A set of seven Laval nozzles, after an appropriate aerodynamic characterization previously reported, [14][15][16]28,29,[32][33][34] have been used to cover the 11.7-177.5 K temperature range. As shown in Table S1 of the ESI, † all experiments, except for (64.2 AE 1.7) K, were performed using the pulsed mode, where the gas is pulsed at 10 Hz for T 4 11.7 K using a rotary disk with two symmetrical apertures.…”

“…Concerning the gas-phase reactivity of CH 3 NH 2 towards OH radicals, it has already been studied up to 600 K (experimental) 21,22,24,25 and 3000 K (theoretical calculations). 18,21 Although a recent work investigated the kinetics of this reaction at 22 K, there are no experimental kinetic data between 22 K 26 and 295 K. 21,22,24,25 Between 22 and 3000 K, a smoking pipe-shape in the Arrhenius plot can be inferred, indicating a change in the reaction mechanism. The minimum k ( T ) is presently observed at around 550 K. 18 To obtain a better insight into the temperature evolution of the rate coefficient, we present in this work a combined experimental and theoretical study on the temperature dependence of k ( T ) in the 11.7–177.5 K and 10–1000 K ranges, respectively.…”