Photolysis of acetonitrile in a water-rich ice as a source of complex organic molecules: CH₃CN and H₂O:CH₃CN ices

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bulak@strw.leidenuniv.nl
² Research Laboratory for Astrochemistry, Ural Federal University, Kuibysheva St. 48, 620026 Ekaterinburg, Russia

Received: 16 October 2020
Accepted: 8 January 2021

Abstract

Context. Many C-, O-, and H-containing complex organic molecules (COMs) have been observed in the interstellar medium (ISM) and their formation has been investigated in laboratory experiments. An increasing number of recent detections of large N-bearing COMs motivates our experimental investigation of their chemical origin.

Aims. We investigate the potential role of acetonitrile (CH₃CN) as a parent molecule to N-bearing COMs, motivated by its omnipresence in the ISM and structural similarity to another well-known precursor species, CH₃OH. The aim of the present work is to characterize the chemical complexity that can result from vacuum UV photolysis of a pure CH₃CN ice and a more realistic mixture of H₂O:CH₃CN.

Methods. The CH₃CN ice and H₂O:CH₃CN ice mixtures were UV irradiated at 20 K. Laser desorption post ionization time-of-flight mass spectrometry was used to detect the newly formed COMs in situ. We examined the role of water in the chemistry of interstellar ices through an analysis of two different ratios of H₂O:CH₃CN (1:1 and 20:1).

Results. We find that CH₃CN is an excellent precursor to the formation of larger nitrogen-containing COMs, including CH₃CH₂CN, NCCN/CNCN, and NCCH₂CH₂CN. During the UV photolysis of H₂O:CH₃CN ice, the water derivatives play a key role in the formation of molecules with functional groups of: imines, amines, amides, large nitriles, carboxylic acids, and alcohols. We discuss possible formation pathways for molecules recently detected in the ISM.