Letter to the Editor

The rich interstellar reservoir of dinitriles: Detection of malononitrile and maleonitrile in TMC-1^⋆

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es
² Departamento de Química Física y Química Inorgánica, Facultad de Ciencias – I. U. CINQUIMA, Universidad de Valladolid, Valladolid 47011, Spain
³ Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan
⁴ Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, 28014 Madrid, Spain
⁵ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁶ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, 35000 Rennes, France

Received: 16 July 2024
Accepted: 2 August 2024

Abstract

While the nitrile group is by far the most prevalent one among interstellar molecules, the existence of interstellar dinitriles (molecules containing two −CN groups) has recently been proven. Here we report the discovery of two new dinitriles in the cold dense cloud TMC-1. These newly identified species are malononitrile, CH₂(CN)₂, and maleonitrile, the Z isomer of NC−CH=CH−CN, which can be seen as the result of substituting two H atoms with two −CN groups in methane and ethylene, respectively. These two molecules were detected using data from the ongoing QUIJOTE line survey of TMC-1 that is being carried out with the Yebes 40 m telescope. We derive column densities of 1.8 × 10¹¹ cm⁻² and 5.1 × 10¹⁰ cm⁻² for malononitrile and maleonitrile, respectively. This means that they are eight and three times less abundant than HCC−CH₂−CN and (E)-HCC−CH=CH−CN, respectively, which are analog molecules detected in TMC-1 in which one −CN group is converted into a −CCH group. This is in line with previous findings in which −CCH derivatives are more abundant than the −CN counterparts in TMC-1. We examined the potential chemical pathways to these two dinitriles, and we find that while maleonitrile can be efficiently formed through the reaction of CN with CH₂CHCN, the formation of malononitrile is not clear because the neutral-neutral reactions that could potentially form it are not feasible under the physical conditions of TMC-1.