Molecular Orbital Theoretical Exploration of Possible Formation of Interstellar Molecules by Homogeneous Catalysis

点击翻译按钮获取中文摘要

It has been believed that molecular hydrogen in space is formed on the surface of dust particles. Our previous molecular orbital (MO) calculations revealed that polycyclic aromatic hydrocarbon (PAH) cation radicals possibly mediate the formation of molecular hydrogen (H_2) in interstellar space. With the aid of typical PAH cations, such as molecular cations of naphthalene, anthracene, pyrene, and coronene, two H atoms combined to form H_2 with little activation energy. PAH cations are also predicted to possibly mediate the formation of many interstellar molecules even at extremely low temperatures. Among these interstellar molecules are methane, acetylene, ammonia, and hydrogen isocyanide. Reaction mechanisms derived from MO calculations are as follows. First, an H atom attaches to one of the carbon atoms in a PAH cation. Then, a precursor of an interstellar molecule approaches to the H atom to form a loosely bound complex, a kind of reaction intermediate. It crosses a low-energy trans … More ition state and finally forms the interstellar molecule. For example, the naphthalene cation attracts an H atom at the 1-position and then attracts a methyl radical to the H atom, which finally splits into a methane molecule and the starting naphthalenium ion. Our preliminary MO calculations strongly suggest that fullerenes and polyatomic interstellar molecules, such as cyanopolyynes, might also catalyze the formation of H_2 and other interstellar molecules with little activation energies. In MO calculations, the B3LYP functional was found to appreciably underestimates the activation energy of an H-transfer reaction and so the combined B3LYP-PMP2 method and the KMLYP functional were employed. In summary, we found the possibility that many interstellar molecules are formed not only by ion-molecule reactions but also by homogeneous catalysis. In order to firmly establish the occurrence of this chemical process in extremely cold interstellar space, it might be necessary to determine more accurately the activation energies in individual reactions concerned. Less