Unusual Low-Temperature Reactivity of Water: The CH + H₂O Reaction as a Source of Interstellar Formaldehyde?

Abstract

Water is an important reservoir species for oxygen in interstellar space and plays a key role in the physics of star formation through cooling by far-infrared emission. Whilst water vapour is present at high abundances in the outflows of protostars, its contribution to the chemical evolution of these regions is a minor one due to its limited low temperature reactivity in the gasphase. Here, we performed kinetic experiments on the barrierless CH + H 2 O reaction in a supersonic flow reactor down to 50 K. The measured rate increases rapidly below room temperature, confirming and extending the predictions of earlier statistical calculations. The open Corresponding Author

THE C(3P) + NH3REACTION IN INTERSTELLAR CHEMISTRY. II. LOW TEMPERATURE RATE CONSTANTS AND MODELING OF NH, NH2, AND NH3ABUNDANCES IN DENSE INTERSTELLAR CLOUDS

“…The exploitable ranges of NH 3 concentrations at 50 K and 75 K were limited by deviations in the observed k 1st values from linearity at high NH 3 concentrations. As with our earlier studies of the CH + H 2 O (Hickson et al 2013) and C + CH 3 OH reactions at low temperatures (Shannon et al 2014), this effect is almost certainly due to the onset of cluster formation (NH 3 -Ar). Only experiments conducted in the linear regime were used in the final analysis.…”

The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol

“…This effect was interpreted as the onset of cluster formation leading to a reduction in the free gas-phase CH 3 OH concentrations, in a similar manner to our previous experiments on the reactivity of H 2 O at low temperature. 29 Consequently, only experiments conducted in the linear regime were used in the final analysis.…”

“…It can be seen from Figure 4 that a small enhancement of the rate constant (a 50% increase of the rate constant with respect to the calculated trend for Argon) is observed in this case although the magnitude of the effect is lower than the corresponding effect for the CH + H 2 O reaction at the same temperature. 29 In order to explore the difference between the experimental results using Ar and SF 6 carrier gases, calculations were performed using SF 6 as a bath gas. Given the relative sizes of the two bath gases, collision energy transfer is expected to be more efficient for SF 6 compared to Ar.…”

“…In particular, species such as methanol which are in the liquid phase at room temperature can be introduced into the cold supersonic flow at concentrations well above their saturation limit. A detailed description of the method has been provided elsewhere 28,29 so only experimental details specific to the present investigation will be outlined here. Five different Laval nozzles were employed in the present study giving access to six different temperatures and densities through the use of different carrier gases.…”

Uniform Supersonic Chemical Reactors: 30 Years of Astrochemical History and Future Challenges

“…O( 3 P) 39 [44] OH 56 [45] CN 56 [46] CH 56 [47] 1 C 2 ( 1 S g + )57 [48] C 2 N(X 2 P i )54 [49] O 2 13 [11,43] 13 [50] 15 [51] 145 [52] 15 [53] 23 [54] 15 [55] 24 [56] NO 13 [50] 53 [57] 24 [52] 24 [52] 15 [53b, c] 15 [55b] 48 [58] H 2 50 [59] 6 [9] 11 [60] D 2 13 [61] H 2 O50 [62] 50 [63]…”