Bulky alkane dimer breaks record for distance between hydrogens on different molecules

Two atoms are said to “touch” each other when they come closer together than the sum of their respective van der Waals radii. The van der Waals radius for hydrogen, for example, is 120 pm, meaning that the closest two nonbonded hydrogen atoms normally get is 240 pm. But intermolecular hydrocarbon H•••H contacts can be pushed to less than 160 pm, reports a team led by Peter R. Schreiner of Justus Liebig University (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01879). The researchers studied crystals of tri(3,5-tert-butylphenyl)methane dimers at 20 K using neutron and X-ray diffraction, measuring the distance between the central methane hydrogens as 156.7 pm. They also probed the crystal and gas-phase structures computationally. Their results suggest that, besides crystal packing, other forces must also contribute to the tight contact. Schreiner and coworkers point to the tert-butyl groups as the stabilizing influence: Because the tert-butyl groups are highly polarizable, their electron clouds can orient to induce temporary dipoles that allow for attractive London dispersion forces between the dimer’s molecules to hold them closely together.