Structure of Solutions of Lithium in Methylamine across the Metal−Nonmetal Transition

Abstract

We present the first neutron diffraction studies of the structure of lithium-methylamine solutions as they cross the metal-nonmetal transition. A shift in the principle scattering peak immediately reflects the overall decrease in density as the solvent expands to accommodate the excess electrons. 6 Li/ 7 Li isotopic labeling then allows us to answer key questions concerning cation solvation. We find that each lithium cation is coordinated to four methylamine molecules. However, the cation solvation shell expands as the system becomes metallic: a direct structural signature of electron delocalization. As a result of correlations between strongly solvated lithium ions, the solutions are highly structured over intermediate length scales. The valence electrons then reside primarily in polaronic cavities, formed by the solvated cations and remaining solvent molecules.

Coordination structures of lithium-methylamine clusters from infrared spectroscopy and ab initio calculations

“…This quantity is quite strongly dependent on the concentration of the solution, but the Li-N separation was found to be in the region of 4.0 Å. 17 The distance between the Li atom and the second shell N atom in the Li͑NH 2 CH 3 ͒ 5 cluster was calculated to be 4.14 Å using DFT and 3.93 Å with MP2, values which match the neutron diffraction estimate. Consequently, the calculations carried out on small clusters in the present work seem to provide a reasonable description of the coordination of lithium by methylamine in concentrated bulk solutions.…”

“…16,17 Their findings suggest a maximum of four methylamine molecules in the inner solvation shell, with an approximately tetrahedral coordination of the lithium atom by the nitrogen atoms in each methylamine molecule. This description fits nicely with the calculated global minimum energy structure for the Li͑NH 2 CH 3 ͒ 4 cluster calculated in the present work.…”

“…More recent studies using neutron diffraction have also been applied to lithium/ methylamine solutions and indicate a solvation structure around the lithium ion consisting of approximately four methylamine molecules in the inner solvation shell. 8,9 As far as we are aware, spectroscopic studies of clusters of alkali atoms with ammonia derivatives have so far been restricted to a single report by Seegue et al 10 This investigation focused on the cationic clusters Cs͑NH 2 CH 3 ͒ n + , where n = 3 -22. Mass-selective photodissociation was achieved using IR absorption in the 1000-1100 cm −1 region ͑corre-sponding to excitation of the CN stretch͒ and, through a combination of the measured band shifts relative to free methylamine and Monte Carlo simulations, the tentative conclusion reached was that nine methylamine molecules are required to fill the first solvation shell.…”

“…Both the calculations and the spectra are consistent with a full inner solvation shell at n = 4, a conclusion that tallies with the neutron diffraction findings of bulk lithium-methylamine solutions. 8,9 II. EXPERIMENT Full details of the experimental procedure have been provided elsewhere.…”

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

“…Thed iffering 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.I nb oth Li-NH 3 and Li-MeNH 2 solutions,L ii s found to be four-coordinate,w hich has subsequently been found to be the case in the gas phase, [17][18][19] and through computational investigation. [1,[19][20][21] Thevolumetric 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.I nLi-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.…”