![]() While an HB-donor only has σ-symmetric orbitals on hydrogen, an XB-donor also has orbitals perpendicular to the σ-bond axis that are ideally oriented for π-symmetric orbital overlap. While both of these models have advantages and disadvantages, and an expansive view of these interactions will consider both as contributing factors 7, 21, 24, 30, 31, 32, 33, they both describe hydrogen bonding and halogen bonding as the same type of interaction.ĭespite these strong parallels between hydrogen bonding and halogen bonding, this analogy overlooks the possibility that halogen bonds can mediate not just σ-symmetric charge transfer, but also π-symmetric charge transfer (Fig. Other descriptions focus on the covalent charge-transfer component, where the lone pair of the nucleophile donates electron density into the hydrogen or halogen σ* antibonding orbital of the HB or XB-donor 4, 8, 21, 22, 23, 24, 25, 26, 27, 28, 29. The most commonly cited models of both interactions focus on the electrostatic attraction between the partial negative charge of a nucleophile with either the partial positive charge of a hydrogen atom or a small region of partial positive charge on the terminus of a halogen atom known as a σ-hole (Fig. Halogen bonding is often described in the same terms as hydrogen bonding. Halogen bonding is widely used in many of the same applications as hydrogen bonding, including organic synthesis 11, crystal engineering 7, 12, polymer engineering 13, and medicinal chemistry 14, 15. Together, they comprise some of the strongest known intermolecular interactions 6, 9, 10. Both interactions are nearly linear attractions between a nucleophile (an HB or XB-acceptor) and an electrophilic site on the terminus of either a hydrogen or halogen atom bonded to an electron-withdrawing group (an HB or XB-donor, respectively) 5, 6, 7, 8. Halogen bonding (XB) is a strong intermolecular interaction that is commonly compared to hydrogen bonding (HB) 1, 2, 3, 4, 5, 6, 7. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. The possibility of π-covalency in a halogen bond is not contemplated in any known models. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σ-symmetric component of halogen bonding. Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. ![]()
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