J. Am. Chem. Soc. 137, 3631–3637 (2015)

Credit: AMERICAN CHEMICAL SOCIETY

Electrides are something of a chemical curiosity. They are ionic materials within which electrons act as anions and they find roles in catalysis and as reducing agents in organic synthesis. They have been shown, both theoretically and experimentally, to exist in several materials at high pressures. One theoretical framework for high-pressure electrides, put forward by Mao-sheng Miao at California State University and Roald Hoffmann at Cornell University, argues that the interstitial spaces in an elemental or ionic lattice feature quantized energy levels analogous to those of atomic orbitals. With increasing pressure, the respective energy levels of atoms on lattice sites and of interstitial spaces change, and electrides form when the energy of the interstitial space is less than that of the valence orbital of the lattice site atoms. The electrons that then occupy those spaces have been dubbed interstitial quasiatoms (ISQs).

Now, Miao and Hoffmann have carried out theoretical studies on a number of materials at high pressures to examine the behaviour of ISQs and compare them with 'real' atoms and molecules. By calculating electron density in double hexagonal close-packed sodium and hexagonal magnesium, at 300 and 800 GPa respectively, they show that ISQs can behave analogously to anions — for instance, the sodium high-pressure electride carries a charge in the interstitial spaces very close to that on the sulfur atoms in Na2S at high pressure. They can also interact with each other by forming metal-like bonding: the model for the magnesium high-pressure electride shows planes of delocalized electron density.

Having noted these similarities to conventional bonding, they also carried out calculations on a 'compression chamber' made up of 108 helium atoms in a face-centred cubic geometry. When two electrons are added to the system, features that bear striking resemblance to a conventional HOMO and LUMO are seen in the calculated electron-density maps (pictured). Similar features were also seen when ISQs were located adjacent to Li or Mg dopants in the same lattice, suggesting that interstitial quasiatoms could effectively form so-called quasimolecules.