Dihydrogen bond: Difference between revisions

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| year = 1968
| doi = 10.1039/C19680001551
}}</ref> They observed intense absorptions in the [[IR spectroscopy|IR band]]s at 3300 and 3210&nbsp;cm<sup>−1</sup> for a solution of (CH<sub>3</sub>)<sub>2</sub>NHBH<sub>3</sub>. The higher energy band is assigned to a normal N-H vibration whereas the lower energy band is assigned to the same bond, which is interacting with the B-&minus;H. Upon dilution of the solution, the 3300&nbsp;cm<sup>−1</sup> band increased in intensity and the 3210&nbsp;cm<sup>−1</sup> band decreased, indicative of intermolecular association.
 
Interest in dihydrogen bonding was reignited upon the crystallographic characterization of the molecule [[ammonia borane|H<sub>3</sub>NBH<sub>3</sub>]]. In this molecule, like the one studied by Brown and Hazeltine, the hydrogen atoms on nitrogen have a partial positive charge, denoted H<sup>δ+</sup>, and the hydrogen atoms on boron have a partial negative charge, often denoted H<sup>δ-</sup>.<ref>{{Cite journal
| date = 1996-01-01
| pmid = 19904922
}}</ref> In other words, the amine is a protic acid and the borane end is hydridic. The resulting B-&minus;H<sup>...</sup>H-&minus;N attractions stabilize the molecule as a solid. In contrast, the related substance ethane, H<sub>3</sub>CCH<sub>3</sub>, is a gas with a boiling point 285 °C lower. Because two hydrogen atoms are involved, this is termed a dihydrogen bond.
 
Formation of a dihydrogen bond is assumed to precede formation of H<sub>2</sub> from the reaction of a hydride and a protic acid. A very short dihydrogen bond is observed in NaBH<sub>4</sub><sup>'''.'''</sup>2H<sub>2</sub>O with H---&minus;H contacts of 1.79, 1.86, and 1.94 Å.<ref>{{Cite journal
| doi = 10.1021/cr000021b
| volume = 101
}}</ref>
 
This kind of H---&minus;H interaction is distinct from the H---&minus;H bonding interaction in [[transition metal]] complexes having dihydrogen bound to a metal.<ref>{{Cite book
| edition = 1
| publisher = Springer