Jerosimić Stanka V, Wester Roland, Gianturco Franco A
Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, PAC 105305, 11158 Belgrade, Serbia.
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria.
Phys Chem Chem Phys. 2019 Jun 7;21(21):11405-11415. doi: 10.1039/c9cp00877b. Epub 2019 May 21.
We have selected two neutral C-rich linear molecules, HCN and HCN, which are very abundant in the interstellar medium (ISM) to computationally investigate the stability of their anions and their possible existence in outer space, for which thus far there is no available evidence. Our present, ab initio structural studies strongly indicate that the neutral cyanopolyynes can have two types of stable anions by either forming slightly distorted non-linear structures of anionic Valence Bound States (VBSs), or linear more weakly bound dipole-bound states (DBSs) produced directly from the initial neutral linear configurations. Our present calculations further show that the stability of these anions increases with the number of C atoms in the chains. We carefully analyze possible physical causes for their lack of observation thus far: from the increased complexity of their rotational spectra with respect to those already observed, to their possible losses due to them entering the formation network of the CN and CN carbonitrile anions, which have been observed in the circumstellar envelopes and in the Titan atmosphere. We specifically suggest that de-hydrogenation reactions of the initial anions could take place by following either of two possible, well-known mechanisms: the fragmentation path usually ascribed to Dissociative Electron Attachment (DEA) processes or chemical recombination active in exothermic reactions with the H, N and O atoms present in the circumstellar envelopes. The results from such processes would be the loss of the stable anions of cyanopolyynes and the additional formation of their anionic carbonitriles.
我们选择了两种富含碳的中性线性分子，HCN和HCN，它们在星际介质（ISM）中含量非常丰富，通过计算来研究它们阴离子的稳定性以及它们在外层空间中可能的存在情况，目前尚无相关证据。我们目前的从头算结构研究强烈表明，中性氰基多炔可以通过形成阴离子价束缚态（VBS）的轻微扭曲的非线性结构，或者直接从初始中性线性构型产生的线性较弱束缚的偶极束缚态（DBS），拥有两种类型的稳定阴离子。我们目前的计算进一步表明，这些阴离子的稳定性随着链中碳原子数量的增加而增强。我们仔细分析了迄今为止它们未被观测到的可能物理原因：从其旋转光谱相对于已观测到的光谱的复杂性增加，到它们可能因进入在恒星包层和泰坦大气中已观测到的CN和CN腈阴离子的形成网络而损失。我们特别指出，初始阴离子的脱氢反应可能通过两种可能的、众所周知的机制之一发生：通常归因于解离电子附着（DEA）过程的碎片化路径，或在与恒星包层中存在的H、N和O原子的放热反应中活跃的化学重组。这些过程的结果将是氰基多炔稳定阴离子的损失以及它们腈阴离子的额外形成。
