A systematic study into the influence of aromatic stacking interactions and fluorine substituent effects on molecular organic crystal assembly

Abstract

This thesis reports the aromatic stacking interactions observed within a system of related fluorinated organic compounds, fluorinated benzylideneanilines (or fluoroanilines). A comprehensive library of fluoroaniline structures was assembled to systematically study the effect of fluorine substitution primarily on the aromatic stacking interactions, but also the complementarity with any other non-covalent intermolecular interactions. Through controlled, iterative changes to the number and position of fluorine substituents in the molecule, it is possible to examine the shifting preference for different aromatic interaction configurations. The entire library of structures was structurally characterised through single crystal X-ray diffraction analysis. The structures are categorised into groups based on their crystal assembly, intermolecular geometry and spatial packing similarity. A subset of these structures was selected for quantitative analysis of the intermolecular interactions by high resolution data collection and quantum crystallographic methods. Topological analysis of the experimental electron density field gradient using Quantum Theory of Atoms in Molecules and complementary theoretical calculations using the interaction energy approach reveal the nature of these interactions. These analyses compute energies for intermolecular interactions from which the observed crystal structure assembly can be rationalised. The primary results of this study characterise the influence of fluorine substituents on the molecular packing in the crystal. Significantly, these findings show that contrary to popular belief, this influence is not necessarily due to atom-atom intermolecular interactions, but rather to dipole-dipole direct substituent interactions between stacked aromatic rings. This study also generated many more results which characterise the nature of these interactions and when combined with the categorisation of packing motifs this provides a framework within which molecular assembly in this system can be rationalised. From this it is possible to identify the nature of interactions which are clearly dominant and provide a means for characterising other interactions which are not structure-directing.

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ORCID for Simon Coles: orcid.org/0000-0001-8414-9272

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