A sequential acid-base mechanism in the interstellar medium: The emergence of cis-formic acid in dark molecular clouds

Abstract

Context. The different abundance ratios between isomers of an organic molecule observed in the interstellar medium (ISM) provide valuable information about the chemistry and physics of the gas and the history of molecular clouds. In this context, the origin of an abundance of cis-formic acid (c-HCOOH) of only 6% the trans isomer (t-HCOOH) abundance in cold cores remains unknown. Aims. In this work, we aim to explain the presence of c-HCOOH in dark molecular clouds through the destruction and back formation of c-HCOOH and t-HCOOH in a cyclic process that involves HCOOH and highly abundant molecules such as HCO+ and NH3. Methods. We used high-level ab initio methods to compute the potential energy profiles for the cyclic destruction and formation routes of c-HCOOH and t-HCOOH. Accurate global rate constants and branching ratios are calculated based on the transition state theory and the master equation formalism under the typical conditions of the ISM. Results. The destruction of HCOOH by reaction with HCO+ in the gas phase leads to three isomers of the cation HC(OH)2+. The most abundant cation can react in a second step with other abundant molecules of the ISM such as NH3 to form back c-HCOOH and t-HCOOH. This mechanism explains the formation of c-HCOOH in dark molecular clouds. Considering this mechanism, the fraction of c-HCOOH with respect t-HCOOH is 25.7%. To explain the 6% reported by the observations, we propose that further destruction mechanisms of the cations of HCOOH by collisions with abundant molecules or interconversion reactions on dust grains should be taken into account. Conclusions. The sequential acid-base (SAB) mechanism proposed in this work involves fast processes with very abundant molecules in the ISM. Thus, HCOOH very likely suffers our proposed transformations in the conditions of dark molecular clouds such as B5 and L483. This is a new approach in the framework of the isomerism of organic molecules in the ISM, which has the potential to explain the ratio between isomers of organic molecules detected in the ISM.