Destruction of dimethyl ether and methyl formate by collisions with He⁺

Abstract

Context. To correctly model the abundances of interstellar complex organic molecules (iCOMs) in different environments, both formation and destruction routes should be appropriately accounted for. While several scenarios have been explored for the formation of iCOMs via grain and gas-phase processes, much less work has been devoted to understanding the relevant destruction pathways, with special reference to (dissociative) charge exchange or proton transfer reactions with abundant atomic and molecular ions such as He + , H 3 + and HCO + . Aims. By using a combined experimental and theoretical methodology we provide new values for the rate coefficients and branching ratios (BRs) of the reactions of He + ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate (MF). We also review the destruction routes of DME and MF by other two abundant ions, namely H 3 + and HCO + . Methods. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He + reactions over a wide collision energy range (Cernuto et al. 2017, Phys. Chem. Chem. Phys. 19, 19554; 2018 ChemPhysChem 19, 51), we extend our theoretical insights on the selectivity of the microscopic dynamics to calculate the rate coefficients k(T ) in the temperature range from 10 to 298 K. We implement these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected. Results. Due to stereodynamical effects present at low collision energies, the rate coefficients, BRs and temperature dependences here proposed differ substantially from those reported in KIDA and UDfA, two of the most widely used astrochemical databases. These revised rates impact the predicted abundances of DME and MF, with variations up to 40% in cold gases and physical conditions similar to those present in prestellar cores. Conclusions. This work demonstrates that the accuracy of astrochemical models can be improved by a thorough characterization of the destruction routes of iCOMs. The details of the chemical systems, indeed, can strongly affect their efficiency and significant deviations with respect to the commonly used Langevin model estimates are possible.

Stereodynamical Effects by Anisotropic Intermolecular Forces

“…Using such a combined experimental and theoretical methodology we have been able to provide new values for the temperature dependent rate coefficients and branching ratios of the reactions of He + with the two important COM, dimethyl ether and methyl formate (MF). Our results will be relevant for a correct modeling of the chemical kinetics in various regions of the interstellar space, such as prestellar cores and hot corinos (Ascenzi et al, 2019).…”

“…A recent investigation from our laboratories focused on the study of some ion-molecule reactions of relevance to assess the competition and balance of phenomena occurring in many gaseous and plasma environments, ranging from the ionospheres of planets and the interstellar medium (Balucani et al, 2015) (low temperatures) to laboratory plasmas for technological applications (much higher temperatures). In particular, collisions with He + are an important pathway for the decomposition of “complex organic molecules” (COMs, i.e., molecules containing at least six atoms) in various astronomical environments (Balucani et al, 2015; Ascenzi et al, 2019). Since dimethyl ether (DME) and methyl formate (MF) are among the most abundant COMs, experiments have been performed on the reactivity of He + ions with such neutrals, using a Guided Ion Beam Mass Spectrometer, which allows measurements of reactive cross sections and branching ratio (BR) as a function of the collision energy (Cernuto et al, 2017, 2018).…”

Organic chemistry in the protosolar analogue HOPS-108: Environment matters

“…To clarify this point, we need new theoretical and experimental chemical calculations on these pathways. For example, by studying new experimental and theoretical calculations on the destruction of MF and DME by He + , Ascenzi et al (2019) obtained reaction rate coefficients that are very different from those given by KIDA and UDfA databases, showing that this could impact the predicted abundances by 40% or even more depending on the physical conditions. This shows that the inclusion of cor- (Jørgensen et al 2016(Jørgensen et al , 2018, HOPS-108 in OMC-2 FIR 4 (this work), and the sources from the PEACHES survey (Yang et al 2021).…”

Modelling cosmic masers in C-type shock waves -- the coexistence of Class I CH3OH and 1720 MHz OH masers

“…As additional chemical network updates, the gas-phase chemistry is supplemented by the list of neutral-neutral reactions published by Palau et al (2017); Shingledecker et al (2018). Destruction rates of dimethyl ether and methyl formate by collisions with ions are taken from Ascenzi et al (2019). An update is done of the rate coefficients of the reactions involving carbon-chain species (Chabot et al 2013).…”