Hydrogen Cyanide at the Onset of Prebiotic Chemical Reactivity

Doctoral thesis, 2026

Hydrogen cyanide (HCN) is a unique molecule. It is ubiquitous in the universe, highly reactive, and capable of forming a diverse range of life's building blocks through self-reaction. Its solid phase is equally intriguing, as it has been observed to glow and exhibit electric discharges, likely driven by its strong polarity. While investigating these exceptional properties is crucial for astrochemistry and the origin of life, HCN's high toxicity and tendency to form intractable polymers have left its rich chemistry relatively unexplored. In this thesis, I employ quantum chemical methods to unravel the reactivity and solid-state properties of HCN.
 
The first part of this thesis focuses on the reactivity of HCN. An exploration of the thermodynamic landscape of its self-reaction reveals 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. The kinetics of adenine formation is further investigated by comparing proposed base-catalyzed pathways in an HCN-rich environment. These results provide a kinetic explanation for the low adenine yields typically observed in experiments.
 
The second part concerns the HCN crystal, with a particular focus on Titan, where HCN ice is abundant. In this part of the thesis, crystal morphology and polar surface properties are investigated. The results suggest a high anisotropy of the crystal, making it grows into needle-shaped structures, whose tips can exert strong electric fields. These electric fields arise from the uncompensated electrostatic potential at the polar surfaces and are suggested to facilitate chemical transformations. 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. This thesis provides insights into the exceptional properties of HCN in the context of prebiotic chemistry and astrochemistry.