Author(s): , , ,
Institution(s): 1. NASA Ames Research Center, 2. UC Berkeley
Large polyatomic carbonaceous molecules, known as polycyclic aromatic hydrocarbons, are known to exist in the outflows of carbon stars. How these large polyatomic molecules are synthesized in such exotic conditions is, thus far, unknown. Molecular ions, including positive and negative ions, are in relative abundance in the high radiation fields present under such conditions. Hence, barrierless ion-molecule interactions may play a major role in guiding molecules towards each other and initiating reactions. We study these condensation pathways to determine whether they are a viable means of forming large pure hydrocarbon molecules, and nitrogen-containing carbonaceous chains, stacks, and even cyclic compounds. By employing accurate quantum chemical methods we have investigated the processes of growth, structures, nature of bonding, mechanisms, and spectroscopic properties of the ensuing ionic products after pairing small carbon, hydrogen, and nitrogen-containing molecules. We have also studied the ion-neutral association pathways involving pure-carbon molecules e.g. acetylene, ethylene and other hydrocarbons, and their dissociation fragments in a plasma discharge as well as how nitrogen atoms are incorporated into the carbon ring during growth. Specifically, we explored the mechanisms by which the synthesis of pyrimidine will be feasible in the gas phase in conjunction with ion-mobility experiments. We have used accurate ab initio coupled cluster theory, Møller-Plesset and Z-averaged perturbation theories, density functional theory, and coupled cluster theory quantum chemical methods together with large correlation consistent basis sets in these investigations. We found that a series of hydrocarbons with a specific stoichiometric composition prefers cyclic molecule formation rather than chains. Some of the association products we investigated have large oscillator strengths for charge-transfer type electronic excitations in the near infrared and visible regions of the electromagnetic spectrum.
P. P. Bera, Roberto Peverati, M. Head-Gordon, and Timothy J. Lee, Phys. Chem. Chem. Phys. 17, 1859-1869 (2015)