History
In 1863, Frederick Guthrie gave the first report on the ability of halogen atoms to form well-defined adducts with electron donor species. In his experiment, he added I2 to a saturated solution of ammonium nitrate to form NH3I2. When the compound was exposed to air, it spontaneously decomposed into ammonia and iodine which allowed Guthrie to conclude that he had formed NH3I2.
In the 1950s, Robert S. Mulliken developed a detailed theory of electron donor-acceptor complexes, classifying them as being outer or inner complexes. Outer complexes were those in which the intermolecular interaction between the electron donor and acceptor were weak and had very little charge transfer. Inner complexes have extensive charge redistribution. Mulliken’s theory has been used to describe the mechanism by which XB formation occurs.
Around the same time period that Mulliken developed his theory, crystallographic studies performed by Hassel began to emerge and became a turning point in the comprehension of XB formation and its characteristics.
The first X-ray crystallography study from Hassel’s group came in 1954. In the experiment, his group was able to show the structure of bromine 1,4-dioxanate using x-ray diffraction techniques. The experiment revealed that a short intermolecular interaction was present between the oxygen atoms of dioxane and bromine atoms. The O−Br distance in the crystal was measured at 2.71 Å, which indicates a strong interaction between the bromine and oxygen atoms. In addition, the distance is smaller than the sum of the van der Waals radii of oxygen and bromine (3.35 Å). The angle between the O−Br and Br−Br bond is about 180°. This was the first evidence of the typical characteristics found in halogen bond formation and led Hassel to conclude that halogen atoms are directly linked to electron pair donor with a bond direction that coincides with the axes of the orbitals of the lone pairs in the electron pair donor molecule.
In the 1980s continued work was carried out using analytical methods such as infrared spectroscopy and Fourier transform spectroscopy. These methods allowed the isolation of complexes formed between Lewis bases and halogen molecules for further studies.
Read more about this topic: Halogen Bond
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