Reaction Mechanisms to Enhance Chemical Complexity in Cold, Dark Interstellar Clouds

The cold, dark, interstellar cloud TMC-1 is home to more than 148 known molecules, and possibly many more. To understand the formation and destruction of these molecules, we present detailed theoretical work combined with a vast chemical kinetic model, Nautilus. Along with laboratory experimental work and observational data, these methods form the toolkit for understanding the chemistry of interstellar
environments. Here, three different theoretical frameworks are used to understand the chemical reaction rates of first, cosmic rays hitting interstellar dust grains; second, neutral radicals in the gas-phase; and finally, cyanides and isocyanides also in the gas-phase. Cosmic ray radiolysis chemistry is shown to significantly enhance the abundances of HC2O and HCOOCH3 by energizing their grain-surface precursors. Radiative association between the neutral radicals CH3 and CH3O is shown to be rapid but unable to account for the observed abundance of CH3OCH3 by phase-space rate calculations conserving energy and angular momentum. On the other hand, radiative association is shown to overproduce the molecules that lead to CH3CN, CH3NC, H2CNC, and, to a lesser extent, H2CCN, perhaps because its efficiency is overestimated. Together, these investigations represent our effort to understand the ways complex organic molecules may be formed and destroyed in the cold, dark interstellar medium. We discuss possible reasons for these differences in observed and modeled
abundances, and provide ideas for future directions.