On the electronic structure of electrides

Abstract

We explore a mechanism for the remarkable charge isolation of the localized, or trapped, electrons found in the crystalline electrides Cs(18C6)2 and Cs(15C5)2. 133Cs NMR measurements show only ≊ 0.05% atomic character of the spin density at the Cs nucleus, consistent with many features of the structure and measured properties which indicate that the localized electron distribution is centered at the anion vacancies. The optical absorption data suggest that the localized electrons, which give rise to the Curie-law spin susceptibility, must penetrate appreciably into the crown ethers, (18C6) and (15C5), which encapsulate the Cs. We suggest that the large reduction of the spin density at the Cs nucleus is due to a Coulomb barrier resulting from negative charge on the oxygens. A crude model, one electron moving in two spherical charged shells surrounding the Cs core, illustrates the mechanism and accounts accurately for the ratio of spin densities at the Cs nucleus found in the 18C6 and the 15C5 electrides. Hartree–Fock calculations for an idealized model of an isolated Cs(18C6)2 molecule, namely Li(9C3)2, tend to support the mechanism.

Theoretical Descriptors of Electrides

“…Specifically, the observation was made that certain trimer configurations would form a strong dipole and the excess electron would occupy an extremely diffuse molecular orbital, localized at one end of the cluster. Note that these diffuse orbitals are very similar to the Rydberg-like states observed by Rencsok et al in theoretical studies of the Li + (9C3) 2 e – complex . Further, those configurations of the anionic water trimer possess exceptionally large hyperpolarisabilities.…”

Relation of Li(NH3)4to Electrides

“…Finally, we note the neutron diffraction study of Li(ND3) 4 [3], which found that one of the Li-N distances 0.3 -Lĩ Li(9C3)ĩ s much larger than the others (by 0.5 A). In LDA [1] this difference did not occur in the molecule, and only a small distortion of the Li-N distances occurred in the solid.…”

“…It was argued [4 -6] that essentially similar behavior should occur for the molecules Cs(18C6)z, Cs(15C5)2, and the various alkali cryptand complexes, which are the basic molecular units of experimentally studied crystals [2]. Figure 1 shows the HOMO for Li(NH3) 4 and Li(9C3) 2, both in the Hartree-Pock approximation (HFA) [7]. The similarity of the two cases with regard to the greatly expanded HOMO is clear.…”

“…The picture arrived at, via empirical arguments, is that the valence electrons are confined mainly to interstitial regions, far from the positive alkali ions [2]. This was supported by the finding of the Rydberg-like ground state in a single molecule, an encapsulated alkali atom [4,5], and by an LDA band calculation for Cs(15C5)~ [9]. Although this picture is somewhat controversial, there is little doubt that it is essentially correct [6].…”

“…Two examples for which ab initio calculations have demonstrated this property are Li(9C3) 2 [4] and H(C111) cryptate [5]. Here, 9C3 (read 9-crown-3) is a crown ether molecule and C111 is a cryptand molecule.…”