journal menu
abstracts
Hirshfeld surface analysis [1] has very quickly become a routine tool for rationalising and visualising intermolecular interactions in crystals. The serendipitous discovery of an intriguing and novel way to identify the space `belonging' to a molecule in a crystal has led to the development of a suite of computational tools that facilitate a deeper understanding of how molecules pack in crystals and why it makes sense that a particular crystal packing occurs [2]. We have previously used the Hirshfeld surface as a vehicle for mapping inherent shape and curvature, surface-mediated distances between closest atoms, as well as quantum mechanical properties such as molecular orbital density, electron density and electrostatic potential. Combining visualisation tools like these with quantum mechanical wavefunctions - and hence properties derived from these wavefunctions - offers a powerful and unique opportunity to investigate intuitive concepts like `electrostatic complementarity' [3]. With this in mind we have been investigating ways to subdivide Hirshfeld surfaces into discrete patches that can be identified with specific pairs of molecules in close contact in crystals, and testing different expressions to quantify our ideas on electrostatic complementarity. Coupled with this appealing visual approach we also compute the electrostatic energy of interaction between the respective molecular wavefunctions. This combination of approaches within an easy to use software package will be powerful enough to not only routinely explore and visualise the patterns of interaction exhibited by molecules in crystals, but also provide meaningful energies of interaction between relevant pairs of molecules. In this way we can readily attach some real significance - energetics - to what are more usually classified as close contacts of various kinds.
abstracts
The effect of an electric field on the vibrational spectra, the Vibrational Stark Effect (VSE), has been utilized extensively to probe the local electric field in the active sites of enzymes [1, 2]. For this reason, the electric field and consequent polarization effects induced by a supramolecular host system upon its guest molecules attain special interest due to the implications for various biological processes. Although the host-guest chemistry of crown ether complexes and clathrates is of fundamental importance in supramolecular chemistry, many of these multicomponent systems have yet to be explored in detail using modern techniques [3]. In this direction, the electrostatic features associated with the host-guest interactions in the inclusion complexes of halogenated acetonitriles and formamide with 18-crown-6 host molecules have been analyzed in terms of their experimental charge density distribution. The charge density models provide estimates of the molecular dipole moment enhancements which correlate with the simulated values of dipole moments under electric field. The accurate electron density mapping using the multipole formalism also enable the estimation of the electric field experienced by the guest molecules. The electric field vectors thus obtained were utilized to estimate the vibrational stark effect in the nitrile (-C≡N) and carbonyl (C=O) stretching frequencies of the guest molecules via quantum chemical calculations in gas phase. The results of these calculations indicate remarkable elongation of C≡N and C=O bonds due to the electric fields. The electronic polarization in these covalent bonds induced by the field manifests as notable red shifts in their characteristic vibrational frequencies. These results derived from the charge densities are further supported by FT-IR experiments and thus establish the significance of a phenomenon that could be termed as the "supramolecular Stark effect" in crystal environment.
abstracts
Single crystal X-ray diffraction data from several Hydroquinone clathrate systems, with various small guest molecules (e.g. HCOOH, MeOH), have been obtained up to a pressure of 10 GPa, using a diamond anvil cell (DAC). Hydroquinone clathrates are key examples of supramolecular aggregates, having a diverse structural chemistry controlled, to a large extent, by the detailed intermolecular interactions between the host and the guest molecules. Although supramolecular chemistry is the foundation for the design and development of advanced materials (e.g. for catalysis, targeted drug delivery, chemical separation and nonlinear optics) the basic understanding leading to such complex systems are often lacking. High pressure (HP) crystallography is an excellent method of systematically increasing host-guest interactions by forcing the molecules closer together, often leading to interesting and unexpected results. At ambient pressure smaller guest molecules are often disordered inside the clathrate cavities. As the external pressure increases the cavities shrink, and it seems likely that guest molecules will order inside the cavity breaking the host symmetry. Guest ordering transitions are also found upon cooling. In this work, results from HP studies of the hydroquinone - formic acid system reveal that the structure is stable up to 10 GPa, at which pressure the guest cavity volume is reduced by more than 50 % without ordering of the guest atoms. Earlier studies have shown that the empty Hydroquinone clathrate undergoes a phase transition into a nonporous structure already at 0.4 GPa. [1] This indicates that formic acid stabilizes the host framework through strong intermolecular host-guest interactions, but without lowering the host symmetry.
