Hydrogen air space violated

28 May 2017 - Chemistry book of world records

shortHH distance Grimme 2017.PNGWith thanks to Mevans86@twitter for the alert, a group of European researchers have discovered something unusual in tri(3,5-tert-butylphenyl)-methane, ordinarily a truly unremarkable compound. They write about it in JACS ( Rösel et al. DOI) The unusual property was uncovered when crystals of the compounds were subjected to XRD at cryogenic temperatures. It turns out that in the crystal structure dimers are formed with the two hydrogen atoms at the central carbon atom pointing towards each other and this with an extremely short hydrogen to hydrogen (-H~H-) distance, at 20K just 1.566 angstrom. A record in the chemistry book of world records and the hydrogen Van der Waals radius is just 1.2 A.

The hydrogen atoms are clearly violating each others air space. Other compounds exist with such small contact distances but these tend to have the hydrogen atoms sterically confined in a cavity. The solution to this riddle according to the authors is in so-called dispersion energy donors (DED). London dispersion forces are small but attractive forces between atoms and in the compound in question due to the many tert-butyl groups there are 33 attractive interactions offsetting the one repulsive HH interaction.