Hydrogen Cyanide at the Onset of Prebiotic Chemical Reactivity

Hydrogen Cyanide (HCN) is a central molecule in prebiotic chemistry, serving as a key precursor for the synthesis of essential building blocks of life. This simple yet highly energetic nitrile is commonly found in the universe, from the interstellar medium and cometary coma, to the atmospheres of several planets and moons. One notable setting is Saturn’s moon Titan, where \HCN is also found in the solid state, forming clouds. However, the very reactivity that makes HCN such an interesting prebiotic molecule also renders its self-reaction chemistry difficult to unravel, as it is prone to yielding intractable polymers. Furthermore, its peculiar solid-state properties remain largely undiscovered, warranting further investigation. In this thesis, I use quantum chemical methods to investigate HCN's complex reactivity and the unique behavior of its solid phase.The first part of this thesis focuses on HCN reactivity. Exploration of the thermodynamic landscape derived from its self reaction reveal that while most products are thermodynamically favorable, several proposed polymerization pathways are endergonic. Among the most favored products are highly-conjugated polymers and the nucleobase adenine. In a subsequent study, I perform a thorough investigation of proposed base-catalyzed pathways to adenine in a HCN-rich environment, proposing a new pathway and clarifying key missing steps. This work helps explain the kinetic bottlenecks that limit adenine yields in experiments.In the second part, I study the HCN crystal structure and surface properties in cryogenic environments, with particular focus on Titan. I find that HCN forms needle-shaped crystals, whose tips comprise of high-energy polar surfaces that are predicted to exert strong electric fields. These surfaces might assist chemical transformations at low temperatures, such as the isomerization of HCN to HNC. Furthermore, the electronic structure of HCN polar surfaces shows the emergence of localized metallic surface states. This metallicity is transferred to chemisorbed water molecules, suggesting enhanced reactivity at the interface.