On the General Mechanism for the Gas-phase Reaction of Methanimine with a Radical Species in the Interstellar Medium: Some Failures and an Important Success

The gas-phase reactions of methanimine (CH2NH) with small radicals, such as CN, CP, CCH, and OH, have been extensively studied theoretically in the literature, and the presence of a common, general reaction mechanism has been postulated. Since methanimine is considered the main precursor of complex imines in the interstellar medium (ISM), the present study extends the investigation of its reaction with other small radicals that have already been detected in the ISM. These are SiN, SH, NO, NS, HCO, HCS, and C3N. The corresponding products are easily formulated on the basis of the aforementioned general mechanism, and to understand whether they can be formed in the ISM, a preliminary thermochemical study has been carried out. The only exothermic addition reaction is that occurring between CH2NH and the C3N radical. This reaction has been further investigated in order to accurately characterize its reactive potential energy surface, which has then been employed in ab initio transition state theory calculations to derive global rate coefficients. The products of the CH2NH + C3N reaction are new potential interstellar species, namely, the Z and E isomers of HNCHCCCN and CH2NCCCN. For the first time, their structural characterization has been reported. In addition, this work investigates the possibility of H-abstraction processes for each radical species considered, and re-examines the CH2NH + CP reaction to derive the corresponding rate constants, that were still missing in the literature.