ELECTRONIC-STRUCTURE OF LI(9-CROWN-3)2 - A MOLECULE WITH A RYDBERG-TYPE GROUND-STATE

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 compl ex is rather accurately described by the formula Li+ (9C3)2e-; (2) the spin density at the Li nucleus drops by about three-orders of magnitu de, as compared to the free lithium atom; (3) the character of the gro und state, and the first few excited states, is predominantly Rydberg- type, centered 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 aw are, this is the first example of a neutral molecule with a Rydberg-ty pe ground-state. We also discuss the relation of our results to the pr operties of the crystalline electrides.