Thompson, Helen; (2003) The structure and dynamics of alkali metal-amine solutions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The structure of lithium-ammonia and lithium-ammonia-methylamine solutions has been studied using the technique of isotopic substitution in neutron diffraction. In pure ammonia, there is evidence for ∼2.1 hydrogen bonds per nitrogen atom, with an average N-H distance of 2.4 Å. In the mixed solvent system, the ammonia molecules and the amine groups on the methylamine molecules are found to be responsible for the hydrogen-bonding present. The hydrogen-bonding is disrupted with increasing metal concentration due to both electron and ion solvation, such that no trace of hydrogen bonding remains at saturation. Classical simulation techniques have been used to produce three-dimensional molecular configurations which are consistent with the measured data. The resulting models have identified the structure of a second cation solvation shell, in which the nitrogen atoms reside above the faces and edges of the primary shell tetrahedron. The models have also enabled the investigation of orientational correlations between molecules, as well as the propensity and distribution of polaronic electron cavities. At dilute metal concentrations, the electrons are fully solvated by approximately 9 ammonia molecules. As the metal content is increased, the molecules re-orientate such that at saturation, percolation channels are formed between the solvated cation complexes. The dynamics of ammonia and lithium-ammonia solutions have been measured using the techniques of quasi-elastic neutron scattering and molecular dynamics simulation. The proton diffusion rates of the liquids at 230 K are found to increase with metal concentration to 7.84 × 10⁻⁵ cm² s⁻¹ at 12 MPM and decrease thereafter. The proton diffusion rate of the saturated solution decreases upon a decrease in temperature, and can be modelled as a jump-diffusion process. At temperatures below the liquid-solid phase transition temperature, a localised rotation of the solvent molecules in saturated lithium-ammonia compounds at 75 K and 40 K and ammonia at 150 K is observed.

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