Molecular Perspective on Lithium-Ammonia Solutions

• Published in: Angewandte Chemie (International ed.) 2009-10, Vol.48 (44), p.8198-8232
• Main Authors: Zurek, Eva, Edwards, Peter P, Hoffmann, Roald
• Format: Article
• Language: English
• Subjects: ammonia; Ammonia - chemistry; Electrons; Free Radicals - chemistry; Hydrogen Bonding; lithium; Lithium - chemistry; metal-nonmetal transitions; molecular orbitals; Quantum Theory; solvated electrons; Solvents - chemistry; Thermodynamics
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.200900373

A detailed molecular orbital (MO) analysis of the structure and electronic properties of the great variety of species in lithium-ammonia solutions is provided. In the odd-electron, doublet states we have considered: e⁻@(NH₃)n (the solvated electron, likely to be a dynamic ensemble of molecules), the Li(NH₃)₄ monomer, and the [Li(NH₃)₄ ⁺ · e⁻@(NH₃)n] ion-pairs, the Li 2s electron enters a diffuse orbital built up largely from the lowest unoccupied MOs of the ammonia molecules. The singly occupied MOs are bonding between the hydrogen atoms; we call this stabilizing interaction HH bonding. In e⁻@(NH₃)n the odd electron is not located in the center of the cavities formed by the ammonia molecules. Possible species with two or more weakly interacting electrons also exhibit HH bonding. For these, we find that the singlet (S=0) states are slightly lower in energy than those with unpaired (S=1, 2[em leader]) spins. TD-DFT calculations on various ion-pairs show that the three most intense electronic excitations arise from the transition between the SOMO (of s pseudosymmetry) into the lowest lying p-like levels. The optical absorption spectra are relatively metal-independent, and account for the absorption tail which extends into the visible. This is the source of Sir Humphry Davy's "fine blue colour" first observed just over 200 years ago.