title: Structural studies of ammonia and metallic lithium-ammonia solutions.
abstract: The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions.
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
authors.forename: Helen
authors.initials: H
authors.lastname: Thompson
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Wasse
authors.forename: Neal T
authors.initials: NT
authors.lastname: Skipper
authors.forename: Shusaku
authors.initials: S
authors.lastname: Hayama
authors.forename: Daniel T
authors.initials: DT
authors.lastname: Bowron
authors.forename: Alan K
authors.initials: AK
authors.lastname: Soper