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Theoretical Studies of the Extraterrestrial Chemistry of Biogenic Elements and Compounds
PI: Woon, David
We propose to continue to use theoretical quantum chemistry to study reactions occurring on or within ices that lead to the formation of compounds relevant to Astrobiology, providing insight into processes that can operate in protostellar nebulae and form prebiotic molecules or their precursors. Icy grain mantles consist of water, ammonia, and simple carbon-bearing species such CO, CO 2 , CH 3 OH, and HCN. There are various ways in which chemistry can occur even at very cold temperatures to produce more complex species. For example, these and other species may encounter radicals deposited from the gas phase or produced by photolysis in the ice itself and may include H, OH, CN, NH 2 , or others. Alternatively, photoionization may generate charged species and these may exhibit different chemical behavior than the neutrals. Under some circumstances, charge transfer processes may occur, such as proton transfer between an acid and a base.
We propose to employ quantum chemistry to evaluate chemical pathways and to make predictions of corresponding spectroscopic properties. To achieve this, we will use density functional cluster calculations, supported as necessary with continuum salvation or ONIOM hydrid calculations. We will also develop an extended model that incorporates a mineral substrate, forsterite (Mg 2 SiO 4 ), which may be a component of interstellar dust and was observed in Comet Hale-Bopp.
In the next three-year funding cycle, we propose to continue to focus on potential pathways occurring within icy grain mantles leading to the formation of critical biochemical precursor species, including amino acids or their precursors and nitrogencontaining heterocyclic compounds or their precursors. We will examine paths involving both neutral radicals formed through photolysis and charged intermediates generated via photoionization. We will characterize radical intermediates and estimate the fractions that do not go to product or revert to reactants. Specific reactions will include CN, NH 2 , and OH generated by photolysis of HCN, NH 3 , and H 2 O, respectively, reacting with CO, CO 2 , HCN, C 2 H 2 , CH 3 C 2 H, and HC 3 N. We will also characterize cyanide ion (CN-) formed in astrophysical ices via proton transfer from HCN and HNC.
These studies will contribute to meeting the Astrobiology programs goal to chararacterize “… the formation of complex molecules in space…”
