Interstellar dimethyl ether gas-phase formation: a quantum chemistry and kinetics study

TL;DR

This study uses quantum chemistry and kinetics calculations to investigate the gas-phase formation pathways of interstellar dimethyl ether, confirming some proposed reactions and providing new rate data, but suggesting the pathway is inefficient in cold environments.

Contribution

The paper provides the first rate coefficient for the reaction (CH$_3$)$_2$OH$^+$ + NH$_3$, and refines the understanding of dimethyl ether formation in interstellar space through quantum calculations.

Findings

Confirms the gas-phase formation route of dimethyl ether in hot cores.

Provides the first rate coefficient for (CH$_3$)$_2$OH$^+$ + NH$_3$ reaction.

Suggests the formation route is ineffective in cold prestellar cores.

Abstract

Dimethyl ether is one of the most abundant interstellar complex organic molecules. Yet its formation route remains elusive. In this work, we have performed electronic structure and kinetics calculations to derive the rate coefficients for two ion-molecule reactions recently proposed as a gas-phase formation route of dimethyl ether in interstellar objects, namely CH$_3$OH + CH$_3$OH$_2^+$ $\rightarrow$ (CH$_3$)$_2$OH$^+$ + H$_2$O followed by (CH$_3$)$_2$OH$^+$ + NH$_3$ $\rightarrow$ CH$_3$OCH$_3$ + NH$_4^+$. A comparison with previous experimental rate coefficients for the reaction CH$_3$OH + CH$_3$OH$_2^+$ sustains the accuracy of the present calculations and allow a more reliable extrapolation at the low temperatures of interest in interstellar objects (10-100 K). The rate coefficient for the reaction (CH$_3$)$_2$OH$^+$ + NH$_3$ is, instead, provided for the first time ever. The rate coefficients derived in this work essentially confirm the prediction by Taquet et al. (2016) concerning dimethyl ether formation in hot cores/corinos. Nevertheless, this formation route cannot be efficient in cold objects (like prestellar cores) where dimethyl ether is also detected, because ammonia has a very low abundance in those environments.