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.

Electron Solvation and the Unique Liquid Structure of a Mixed‐Amine Expanded Metal: The Saturated Li–NH3–MeNH2 System

“…The inter‐atom potentials in EPSR are comprised of a Lennard‐Jones potential and Coulombic term, and is iterated throughout the simulation until a suitable convergence to experimental F ( Q ) spectra is reached. The seed potentials used in this work were the same as used previously for metal–amine liquid structure studies 12, 13, 14, 15…”

“…The differing metallic properties of Li–NH 3 and Li–MeNH 2 are also reflected in their distinct liquid structures 13, 14. All metallic metal amines share the trait of being highly structured liquids,13, 14, 15, 16 with distinct M (solv) –M (solv) correlations.…”

“…All metallic metal amines share the trait of being highly structured liquids,13, 14, 15, 16 with distinct M (solv) –M (solv) correlations. In both Li–NH 3 and Li–MeNH 2 solutions, Li is found to be four‐coordinate, which has subsequently been found to be the case in the gas phase,17, 18, 19 and through computational investigation 1, 19, 20, 21.…”

“…The volumetric expansion of these liquid metals across their insulator–metal transition is also accompanied by the appearance of void spaces, which is presumably linked to the conduction electron density. In Li–NH 3 these voids take the form of channels between Li(NH 3 ) 4 units,13 whereas in Li(MeNH 2 ) 4 the voids are spatially isolated 14. This is concordant with magnetic measurements that suggest electronic conduction in Li(MeNH 2 ) 4 is via rapid migration of electrons between these polaronic voids, which begin to localize in the solid state 22, 23…”

“…The single principal peak at 1.75 Å −1 , and importantly the existence of a sharp pre‐peak at 0.97 Å −1 , attest to the homogeneity of the 20 MPM solution. The pre‐peak is a structural feature witnessed in solvated‐electron systems as they transition to the metallic state, and arises owing to strong intermediate range ordering in the liquid 13, 14, 15. Integration of the principal peak in the Δ G Li ( r ) distribution corresponds to the coordination number of Li, and is consistent with approximately 4 solvent molecules, as reported previously for other Li–RNH 2 systems 13, 14…”