The National Astronomical Observatory of Japan discovers the formation path of complex organic molecules in molecular clouds at cryogenic temperatures

In the high-temperature areas of star-forming regions (with temperatures ≥ 100 kelvins), where massive and small stars are born, various complex organic molecules have been detected to date. Among these complex organic molecules, dimethyl ether and methyl formate are typically observed in the cores of molecular clouds, where stars are formed. These molecules are hypothesized to be created through chemical reactions in the hot (≥100 kelvins) gas phase after star formation or through radical chemical reactions on warm dust surfaces (≥20 kelvins). Recently, however, these molecules have been observed even in molecular cloud cores at cryogenic temperatures of approximately 10 kelvins, where star formation is yet to be reported, making it necessary to reexamine the molecular formation mechanism.

Researcher Yu Komatsu of the Astrobiology Center and Project Assistant Professor Kenji Furuya of the National Astronomical Observatory of Japan of the National Institutes of Natural Sciences examined the formation processes of dimethyl ether and methyl formate, which are typical complex organic molecules detected in interstellar space, using an automated reaction path search method based on quantum chemistry. Consequently, they discovered a path for each molecule, through which the reaction could take place in a molecular cloud at cryogenic temperatures (below 10 kelvins). The corresponding results have been published in the online edition of ACS Earth and Space Chemistry.

To clarify the formation processes of dimethyl ether and methyl formate at cryogenic temperatures, the research group used an automated chemical path search method based on the transition state theory of quantum chemistry to investigate the paths through which these molecules could be energetically produced in their electronic ground states. In this method, for each target molecule, the energy profiles of the possible structures were considered. When a target molecule split into two molecules, the method was reversed to capture the formation path to the target molecule and extract the path requiring less external energy.

The calculations revealed a formation path proceeding through an exothermic reaction in the gas phase with no reaction barriers for either molecule. For dimethyl ether, the obtained reaction network revealed a formation path from CH₃O and CH₃. This was partially inferred from previous studies, and a more comprehensive path consistent with these reports was obtained. In contrast, a more complex formation path was estimated for methyl formate.

A route without reaction barriers was also identified; however, the main products were carbon dioxide and methane, with methyl formate as a byproduct. Such theoretical chemical predictions of paths through which these complex organic molecules can be formed, even at cryogenic temperatures, can provide a fundamental guideline for clarifying the complete picture of the formation of complex organic molecules.

Journal Information
Publication: ACS Earth and Space Chemistry
Title: The Automated Reaction Pathway Search Reveals the Energetically Favorable Synthesis of Interstellar CH₃OCH₃ and HCOOCH₃
DOI: 10.1021/acsearthspacechem.3c00117