Abstract
Titan, Saturn’s largest moon, possesses a dense nitrogen-methane atmosphere, extensive hydrocarbon lakes, and a dynamic surface–atmosphere system rich in organic molecules. These features make Titan a prime target for astrochemical and prebiotic investigations. A key aspect of Titan’s atmospheric and surface chemistry lies in the solid-phase behaviour of molecular ices subjected to energetic processing. Among these, co-crystals are hypothesised to serve as reactive media facilitating solid-state synthesis of complex organics. In this context, the present thesis explores the structural and chemical evolution of pyridine-based ices under conditions relevant to Titan’s environment. The formation, phase behaviour, and radiation-driven chemistry of pure and binary ices were systematically investigated using vibrational spectroscopy, powder X-ray and neutron diffraction, temperature-programmed desorption mass spectrometry, and periodic DFT calculations.
Chapter 1 is dedicated to introducing the icy moons in the solar system, emphasising the current knowledge about Titan’s structure and composition. Chapter 2 outlines the theoretical background on molecular crystals and the experimental and computational techniques applied throughout the thesis.
Chapter 3 examines the effects of VUV irradiation on the well-characterised pyridine:acetylene co-crystal, produced here via gas-phase deposition, compared to its amorphous analogue. Energetic processing led to the formation of nitrogen containing polycyclic aromatic hydrocarbons (NPAHs) and precursors such as quinolizinium and ethynylpyridines. The efficiency of these reactions was found to be highly dependent on both temperature and structural arrangement.
Next, the pyridine:diacetylene system was identified as a promising target for co-crystal formation, particularly due to the molecular similarity between acetylene and diacetylene. Towards the study of this new system, Chapter 4 presents the first detailed crystallographic characterisation of solid diacetylene at 5–220 K using powder X-ray and neutron diffraction, revealing a stable orthorhombic phase stabilised predominantly by CH···π interactions. The structure exhibited anisotropic thermal expansion and no phase transitions, suggesting structural stability under Titan-relevant cryogenic conditions.
This foundational characterisation enabled subsequent photochemical experiments, described in Chapter 5. The VUV photolysis of amorphous and crystalline diacetylene ices revealed that solid-state phase and irradiation temperature impact reaction pathways. Crystalline ices irradiated at 70 K exhibited enhanced reactivity towards the formation of higher-order hydrocarbons, with ion counts for species with m/z upto200amudetected during temperature programmed desorption experiments. These findings underscore the capacity of diacetylene ices to act as precursors to complex organic matter on Titan.
Chapter 6 introduces a new crystalline phase involving pyridine and diacetylene, which was experimentally identified and proposed as a novel co-crystal—the first of its kind involving diacetylene—formed via gas co-deposition and stabilised by N···H–C interactions. This co-crystal exhibited distinct spectroscopic and crystallographic signatures and showed thermal stability up to ∼230 K. VUV irradiation of this system yielded smaller unsaturated hydrocarbons (m/z < 100 amu), with product distributions differing from those of pyridine:acetylene co crystals, indicating that structural arrangement and ice phase exert a dominant influence over photochemical outcomes.
Chapter 7 expands the investigation to other Titan-relevant species, examining the interactions of pyridine with ethane and acrylonitrile. Although no true co-crystals were observed, both systems induced an unexpected polymorphic phase transition in pyridine to its orthorhombic phase II, likely triggered by weak intermolecular interactions and thermal cycling within Titan-relevant temperature ranges. Electron irradiation of these mixtures induced chemical reactions, with the detection of new volatile species. Notably, an ion at m/z 102 was exclusively observed in the pyridine–acrylonitrile system, indicating the possibility of unique solid-state reaction pathways arising from their co-condensation.
Finally, Chapter 8 brings a summary of the main findings across the 5 experimental chapters and the concluding remarks for this thesis, along with suggestions for future work.