Exobiology and Evolutionary Biology

Theoretical Studies of the Extraterrestrial Chemistry of Biogenic Compounds: Projects for 2010-2013

PI: David Woon

OBJECTIVES. We propose to continue our theoretical studies of chemistry occurring in the interstellar medium, comets, or on outer solar system bodies to gain insight into possible chemical pathways to form prebiotic organic compounds of relevance to the origin of life. Our primary emphasis will continue to be on reactions occurring in ices (icy grain mantles), with some studies of gas phase reactions that are either potentially competitive with ice-bound processes or are otherwise of interest. Projects will include studies that exploit our previous results of processes already shown to be efficient at low temperatures such as proton transfer reactions involving organic acids (HNCO, HCN, HCOOH with NH3) or reactions of ammonia with carbonyl species such as formaldehyde, acetaldehyde, and acetone. These processes lead to complexes that may undergo subsequent chemistry leading to more complex species. Another promising direction is direct reaction of cation species with water in icy grain mantles, with examples such as HCO+ and CH3+ reacting with ice to form HCOOH and CH3OH, respectively. Formation of cyano species — which are good candidates for amino acid precursors — is another emphasis, as is the related subject of the chemistry of ice-bound and gas phase CN-. Finally, we will continue our studies of photolysis in ice by focusing on unsaturated species.

Tied to most of the projects we will propose will be the prediction of spectroscopic properties such as infrared vibrational bands of product and intermediate species. This is an important aspect of this work, since these features may either be observed in the laboratory or in astronomical sources and thus provide a critical means to evaluate the impact of various processes.

METHODOLOGY. Quantum chemical calculations at various levels of theory will be used to characterize chemical pathways and spectroscopic properties of product and intermediate compounds. For smaller systems, perturbation theory (MP2) and coupled cluster methods [CCSD] will be used, along with density functional theory (B3LYP or other functionals). Ices will be modeled with large cluster calculations using B3LYP. For clusters of modest size, self-consistent reaction field calculations using polarizable continuum models will be performed to assess the influence of bulk ice on energetics and properties. We are now routinely performing calculations with clusters of 14-18 chemical entities (for example, HCO+ with 17 water molecules).
SIGNIFICANCE. This project falls under the “Planetary Conditions for Life” area of the Astrobiology: Exobiology and Evolutionary Biology program solicitation, specifically: “... the formation of complex organic molecules in space and their delivery to planerary surfaces…” It will contribute to reaching objectives expressed in the Astrobiology Roadmap (Des Marais et al., Astrobiology 6, 715, 2008) such as Goal 3 (“Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental, and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.”)