We report unrestricted Hartree–Fock (UHF) results for the molecule Li(9C3)2, obtained with several basis sets, including diffuse functions, up to 6-31G++**. These results show that (1) the spatial distribution of the highest occupied molecular orbital (HOMO), or valence, electron is predominantly ‘‘outside’’ the Li(9C3)2 complex. That is, the complex is rather accurately described by the formula Li+(9C3)2e−; (2) the spin density at the Li nucleus drops by about three-orders of magnitude, as compared to the free lithium atom; (3) the character of the ground state, and the first few excited states, is predominantly Rydberg-type, centered on the lithium, with the valence electron ‘‘loosely’’ bound to the complex. We argue that this complex is thermodynamically stable, and show graphically what happens to the spin density as the 9-crown-3 (9C3) molecules complex the neutral lithium atom. As far as we are aware, this is the first example of a neutral molecule with a Rydberg-type ground-state. We also discuss the relation of our results to the properties of the crystalline electrides.

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.

The interesting Letter by Kohanoff et al. [1] presented ab initio calculations which show the valence electron in the lithium-tetra-amine molecule Li(NH3)4 appreciably We thank S.D. Mahanti for useful discussions. We acknowledge partial support by

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