![]() 10 To precisely define the interaction, IUPAC launched a project 11 which concluded that “A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity”. 7 With the development of modern quantum mechanics, it was eventually revealed that a covalently bonded halogen atom, owing to the anisotropy of its electron density distribution, 8 could generate a locally electron-depleted region, namely the σ-hole, 9 upon elongation of the covalent bond, which is able to form attractive interactions with Lewis bases possibly through an electrostatic way. 6 These findings partially demonstrated an orbital-based origin of the interaction between halogenated molecules and Lewis bases, which was also discussed in various pioneering research studies. In the middle of the 20th century, reports on the X-ray structure of the Br 2:dioxane adduct 4 revealed a substantially shorter distance of Br⋯O than the sum of their van der Waals radii, which also demonstrated the inconsistent role of halogens that are traditionally accepted as nucleophiles.įollowing these early studies, intermolecular charge transfer was indicated by Mulliken to rationalize the structure of halogen–aromatic–molecule complexes, 5 while the structure of the Br 2:dioxane adduct was described as electron-pair donors bridged by halogen molecules according to Bent. 1 Subsequently, various unconventional iodinated complexes were reported throughout the whole 19th century, 2 and extended to other halogens, 3 indicating the potential ability of halogen atoms to interact with Lewis bases. Introduction Precursory research on halogen bonding can probably be traced back to 1814, conducted by Colin, revealing that dry iodine forms a metallic-colored liquid with dry gaseous ammonia. ![]() The results should further promote the application of halogens in all related areas. Accordingly, this study demonstrates that the orbital-based origin of halogen bonds could successfully interpret the complicated behaviour of differently charged XB complexes, while electrostatic interaction may dramatically change the overall bonding strength. Intramolecular charge redistribution inside both the donor and the acceptor is found to be system-dependent but always leads to a more stable XB. These observations could be attributed to the intrinsic σ-hole of the XB donor and the intrinsic electronic properties of the XB acceptor regardless of their charge states. Orbital and dispersion interactions are found to be always attractive while unidirectional intermolecular electron transfer from a XB acceptor to a XB donor occurs in all XB complexes. ![]() The results revealed that all XBs could be stable, with binding energies after removing background interaction as strong as −1.2, −3.4, and −8.3 kcal mol −1 for Cl, Br, and I involved XBs respectively. In this paper, 9 XB systems mimicking all possibly charged halogen bonding interactions were designed and explored computationally. ![]() Studies on halogen bonds (XB) between organohalogens and their acceptors in crystal structures revealed that the XB donor and acceptor could be differently charged, making it difficult to understand the nature of the interaction, especially the negatively charged donor's electrophilicity and positively charged acceptor's nucleophilicity.
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