Surfaces Hirshfeld analysis

Dean et al. report [203] an analysis of the crystal structures of a series of iV-alkyl-3-benzylimidazolium iodides, in which the alkyl group, R, is methyl, ethyl, propyl, butyl, 1-methylpropyl and pentyl. The methyl and 1-methylpropyl salts melt at 154 and 135°C, respectively, significandy higher than the others in the series 86 (R = Et), 71 (Pr), 61 (Bu) and 61°C (Pent). No classical hydrogen bonding is seen. Hirshfeld surface analysis is used to decompose the fraction of the surface associated with a particular type of interionic interaction in the four non-disordered salts. The analysis shows the clear dominance of C—H... 

C—C and gauche (g ) at N—C—Cp —Cy —C for the butyl derivative and gg and tt in the two pentyl cations in [(bz)CiC5im]I. The cation-anion interactions have been discussed in terms of Hirshfeld surface analysis summarised in Section 11.2.6. 

Dean, P.M., Pringle, J.M., Forsyth, C.M., Scott, J.L. and MacFarlane, D.R., Interactions in bisamide ionic liquids-insights from a Hirshfeld surface analysis of their crystalline states. New J. Chem. 32 (12), 2121-2126 (2008). 

McKinnon, J.J., Fabbiani, F.P.A. and Spackman, M.A., Conparison of polymorphic molecular crystal structures through Hirshfeld surface analysis, Cryst. Growth Des. 7 (4), 755-769 (2007). 

Kastbjerg, S., Uvarov, C. A., Kauzlarich, S. M., Nishibori, E., Spackman, M. A., Iversen, B. B. (2011). Multi-temperature synchrotron powder x-ray diffraction study and hirshfeld surface analysis of chemical bonding in the thermoelectric zintl phase Ybi4MnSbn. Chemistry of Materials, 23, 3723-3730. 

Given the correlation between bond valence and electron density, it appears tempting to compare also what electron density maps and maps of the BVSE predict as ion transport pathways. Hirshfeld surface analysis has been explored to characterize intermolecular interactions in molecular crystals [47,48]. This analysis is based on the procrystal, which is obtained from superposition of spherical atomic electron densities placed at the crystal structure positions, a quantity that can readily be calculated from the structure using software tools such as CrystalExplorer [49]. The approach was also explored as a tool to map out voids in porous crystals such as metal organic framework materials and zeolites [50]. 

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