Are isomers of the vinyl cyanide ion missing links for interstellarpyrimidine formation?

摘要

In the interstellar medium (ISM) there are many regions where the formation of molecules iskinetically driven rather than thermochemically, which can lead to the formation of many isomerseven though some may be fairly higher in energy relative to the molecular global minimum. Recentlaboratory experiments where noble gas cations are reacted with pyrimidine favored the formation of C_3H_3N~+, but the molecular structure(s) of this fragment was not determined. Microscopicreversibility means that pyrimidine could form under interstellar conditions should the required C_3H_3N~+reactant be detected in the ISM.Hence could a strong candidate forinvolvement in the formation of heterocyclic biomolecules such as pyrimidine in the ISM. In this study, we have investigated the low energy isomers of the acrylonitrile ion (C_3H_3N~+) using densityfunctional theory as well as high levels of ab initio theory, namely, the singles and doublescoupled-cluster theory that includes a perturbational correction for connected triple excitations,denoted as CCSD(T). An automated stochastic search procedure, Kick, has been employed to find isomers on the ground state doublet potential energy surface. Several new structures, along with allthe previously reported minima, have been found. The global minimum H_2CCCNH~+is energetically much lower than either H_2CC(H)CN~+, the acrylonitrile ion, or HCC(NCH~+, most likelyintermediate of the reaction between HCCH~+and HCN. These isomers are connected to the global minimum via several transition states and intermediates. The results indicate that not only the globalminimum but also several higher energy isomers of the C_3H_3N~+ion could be important in interstellar pyrimidine formation. The isomeric molecules have the necessary CCNC backboneneeded for the reaction with HCN to form the cyclic pyrimidine framework. The structural and rotational parameters of all the isomers studied in this work have been predicted at the CCSD(T)level of theory with the anticipation that it will expedite their laboratory as well as astronomicalidentification.