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

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, CH_{2(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 × 10^{11 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.}}