Abstract
Saturn’s moon Titan is a unique and chemically complex world in the Solar System with a diverse inventory of organic species that make this moon a target of astrobiological interest as an analogue to early Earth. Molecular co-crystals are crystalline solids incorporating more than one chemical species in a repeating unit cell, making these phases relevant to the study of solid-state intermolecular interactions and ice-phase prebiotic radiochemistry. The detection and characterisation of co-crystals relevant to the environment of Titan therefore is an important pursuit for understanding the properties of cryominerals located on this moon. This thesis explores the use of molecular and crystallographic symmetry as a framework for developing analytical tools towards the unification of the spectroscopic and diffraction methods currently employed towards this endeavour, such that one method may be used to reinforce the other in future investigations.
First, the predicted vibrational spectra of the known acetylene/benzene co-crystal system are simulated in silico. Here, symmetry correlation is introduced as a predictive tool to identify diagnostic signatures indicative of co-crystal formation, such as changes in peak shape and intensity attributable to specific changes in crystal structure. These predictions are then validated against experimental formation of the co-crystal via vapour-deposition, a novel method. Here, the fundamental principles of symmetry correlation and its impact on vibrational spectroscopy are introduced for subsequent use in the structural elucidation of novel co-crystal systems.
Next, the binary combination of ethylene with benzene is investigated computationally and spectroscopically, with tentative evidence for co-crystal formation present in experiment. Here, a number of auxiliary analytical tools are introduced for the analysis of this system. The first of these is use of a Duschinsky matrix as (i) a theoretical tool for quantifying the degree of intermolecular vibrational coupling within the co-crystal and (ii) a technique for evaluating whether it is appropriate to apply the same labels for spectral features in the co-crystal as used in the spectra of its constituent components. Secondly, deuteration of reagents is explored as a means to solve issues associated with spectral peak superposition and to untangle observed changes in peak shape due to multiple symmetry-based causes, although this latter method requires further experimentation.
A series of further chemical systems are investigated for the existence of novel co-crystals. First, the benzene/phenylacetylene co-crystal is detected with both infrared and Raman spectroscopy, and x-ray diffraction methods, where structural elucidation from diffraction data is attempted. This analysis is guided by insights in crystal symmetry obtained from analysis of the spectroscopic data, including the application of symmetry principles towards efficient evaluation of multiple structures as suitable solutions. The resulting structural solution fits the data well in terms of unit cell geometry, although further refinement of the molecular positions is needed. Throughout this investigation, it is made apparent by spectroscopic and diffraction methods that for pure phenylacetylene, the relationship between temperature and structure is less well understood than literature suggests.
Next, diacetylene in combination with benzene, and then phenylacetylene, is investigated for the existence of co-crystal phases between each coformer combination. Spectroscopic evidence for the existence of co-crystal phases with both species is evident, although poor understanding of the phase properties of pure phenylacetylene limits the structural information that can be obtained by spectroscopy alone.
Finally, the restrictions molecular symmetry places on the validity of different positions for molecules within the crystal are put into practice as a tool for structural elucidation. Here, returning to the acetylene/benzene co-crystal as an idealised case study, it is demonstrated that molecular positions can be almost totally solved without diffraction intensity data. To validate the use of this new methodology, the analysis is applied to a co-crystal of propane with acetonitrile that was first identified in diffraction experiments, where constraints on the molecular ratio and likely space group were able to be placed on this novel phase.