Crystal Structure Survey and Theoretical Analysis of Bifurcated Halogen Bonds

The possibility that two Lewis bases can share a single halogen atom within the context of a bifurcated halogen bond (XB) is explored first by a detailed examination of the CSD. Of the more than 22,000 geometries that fit the definition of an XB (with X = Cl, Br, I), less than 2% are bifurcated. There is a heavy weighting of I in such bifurcated arrangements as opposed to Br, which prefers monofurcated bonds. The conversion from mono to bifurcated is associated with a smaller number of short contact distances, as well as a trend toward lesser linearity. The two XBs within a bifurcated system are somewhat symmetrical: the two lengths generally differ by less than 0.05 angstrom, and the two XB angles are within several degrees of one another. Quantum calculations of model systems reflect the patterns observed in crystals and reinforce the idea that the negative cooperativity within a bifurcated XB weakens and lengthens each individual bond.

Automated summary

The possibility of two Lewis bases sharing a single halogen atom within the context of a bifurcated halogen bond (XB) is explored using a detailed examination of the Cambridge Structural Database (CSD). The study reveals that less than 2% of the geometries fitting the definition of an XB are bifurcated, with a preference for I over Br, which tends to form monofurcated bonds. The conversion from monofurcated to bifurcated bonds is associated with fewer short contact distances and a trend towards lesser linearity, with the two XB lengths and angles being somewhat symmetrical. Quantum calculations of model systems confirm the observed patterns in crystals and suggest that the negative cooperativity within a bifurcated XB weakens and lengthens each individual bond.