  Hydrogen cyanide (HCN) is a critical reactive source of nitrogen for building
key biomolecules relevant for the origin of life. Still, many HCN reactions
remain uncharacterized by experiments and theory, and the complete picture of
HCN production in planetary atmospheres is not fully understood. To improve
this situation, we develop a novel technique making use of computational
quantum chemistry, experimental data, and atmospheric numerical simulations.
First, we use quantum chemistry simulations to explore the entire field of
possible reactions for a list of primary species in N2-, CH4-, and H2-dominated
atmospheres. In this process, we discover 33 new reactions with no previously
known rate coefficients. From here, we develop a consistent reduced atmospheric
hybrid chemical network (CRAHCN) containing experimental values when available,
and our calculated rate coefficients otherwise. Next, we couple CRAHCN to a 1D
chemical kinetic model (ChemKM) to compute the HCN abundance as a function of
atmospheric depth on Titan. Our simulated atmospheric HCN profile agrees very
well with the Cassini observations. CRAHCN contains 104 reactions however
nearly all of the simulated atmospheric HCN profile can be obtained using a
scaled down network of only 19 dominant reactions. From here, we form a
complete picture of HCN chemistry in Titan's atmosphere, from the dissociation
of the main atmospheric species, down to the direct production of HCN along 4
major channels. One of these channels was first discovered and characterized in
Pearce et al. (2019) and this work.
