The many flavours of halogen bonds – message from experimental electron density and Raman spectroscopy

Abstract

Experimental electron-density studies based on high-resolution diffraction experiments allow halogen bonds between heavy halogens to be classified. The topological properties of the electron density in Cl...Cl contacts vary smoothly as a function of the interaction distance. The situation is less straightforward for halogen bonds between iodine and small electronegative nucleophiles, such as nitrogen or oxygen, where the electron density in the bond critical point does not simply increase for shorter distances. The number of successful charge–density studies involving iodine is small, but at least individual examples for three cases have been observed. (a) Very short halogen bonds between electron-rich nucleophiles and heavy halogen atoms resemble three-centre–four-electron bonds, with a rather symmetric heavy halogen and without an appreciable σ hole. (b) For a narrow intermediate range of halogen bonds, the asymmetric electronic situation for the heavy halogen with a pronounced σ hole leads to rather low electron density in the (3,−1) critical point of the halogen bond; the properties of this bond critical point cannot fully describe the nature of the associated interaction. (c) For longer and presumably weaker contacts, the electron density in the halogen bond critical point is only to a minor extent reduced by the presence of the σ hole and hence may be higher than in the aforementioned case. In addition to the electron density and its derived properties, the halogen–carbon bond distance opposite to the σ hole and the Raman frequency for the associated vibration emerge as alternative criteria to gauge the halogen-bond strength. We find exceptionally long C—I distances for tetrafluorodiiodobenzene molecules in cocrystals with short halogen bonds and a significant red shift for their Raman vibrations.

Bonding features in Appel's salt from the orbital-free quantum crystallographic perspective

“…The other studies covered aspects of bonding in biologically important molecules such as the drug metronidazole with a diazole heterocycle in its structure (Kalaiarasi et al, 2019), polymorphs of famotidine based on a thiazole ring (Overgaard & Hibbs, 2004) and (2-oxo-1,3benzoxazol-3(2H)-yl) acetic acid (Wang et al, 2016). The electron density point of view on halogen bonds has been thoroughly presented in the recent works dealing with the NÁ Á ÁI bond (Wang et al, 2018), interactions in tetrafluorodiiodobenzene (Wang et al, 2019) and short ClÁ Á ÁCl bonding (Wang et al, 2012(Wang et al, , 2017. The bonding in 3-alkoxy-4-methylthiazole-2(3H)-thione (Vé nosová et al, 2020), which is one of the most frequently used progenitors of oxygen radicals, is discussed with the main focus on the features of the N-O bond involved in radical generation.…”

Hydrosulfide (HS−) Recognition and Sensing in Water by Halogen Bonding Hosts

“…Furthermore, the j V(r) j /G(r) ratio gives insights into the nature of the XB interactions. [34] Thus, the XB interactions of XB1 Et and XB2 Et with I À are highly electrostatic, with ratios of ca. 0.93, while the interactions with HS À , with ratios around 1.04, indicate a slight increase of the covalency degree on the associations with this more nucleophilic anion (see Table S9-4).…”

Hydrogen-Bonded and Halogen-Bonded: Orthogonal Interactions for the Chloride Anion of a Pyrazolium Salt

“…More recently, halogen bonds and related interactions have gained increasing attention from theory [ 11 , 12 , 13 , 14 ], and the -hole model has also been supported by experimental charge density studies [ 15 , 16 , 17 , 18 , 19 ]. In view of their occurrence in various fields, including macromolecular and supramolecular chemistry, halogen bonds have also been applied in the design of extended solids, i.e., in crystal engineering [ 20 ].…”