Electron Tunneling in Lithium–Ammonia Solutions Probed by Frequency-Dependent Electron Spin Relaxation Studies

Abstract

Electron transfer or quantum tunneling dynamics for excess or solvated electrons in dilute lithium-ammonia solutions have been studied by pulse electron paramagnetic resonance (EPR) spectroscopy at both X- (9.7 GHz) and W-band (94 GHz) frequencies. The electron spin-lattice (T1) and spin-spin (T2) relaxation data indicate an extremely fast transfer or quantum tunneling rate of the solvated electron in these solutions which serves to modulate the hyperfine (Fermi-contact) interaction with nitrogen nuclei in the solvation shells of ammonia molecules surrounding the localized, solvated electron. The donor and acceptor states of the solvated electron in these solutions are the initial and final electron solvation sites found before, and after, the transfer or tunneling process. To interpret and model our electron spin relaxation data from the two observation EPR frequencies requires a consideration of a multi-exponential correlation function. The electron transfer or tunneling process that we monitor through the correlation time of the nitrogen Fermi-contact interaction has a time scale of (1–10)×10−12 s over a temperature range 230–290K in our most dilute solution of lithium in ammonia. Two types of electron-solvent interaction mechanisms are proposed to account for our experimental findings. The dominant electron spin relaxation mechanism results from an electron tunneling process characterized by a variable donor-acceptor distance or range (consistent with such a rapidly fluctuating liquid structure) in which the solvent shell that ultimately accepts the transferring electron is formed from random, thermal fluctuations of the liquid structure in, and around, a natural hole or Bjerrum-like defect vacancy in the liquid. Following transfer and capture of the tunneling electron, further solvent-cage relaxation with a timescale of ca. 10−13 s results in a minor contribution to the electron spin relaxation times. This investigation illustrates the great potential of multi-frequency EPR measurements to interrogate the microscopic nature and dynamics of ultra fast electron transfer or quantum-tunneling processes in liquids. Our results also impact on the universal issue of the role of a host solvent (or host matrix, e.g. a semiconductor) in mediating long-range electron transfer processes and we discuss the implications of our results with a range of other materials and systems exhibiting the phenomenon of electron transfer.

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

“…[28] Figure 3s hows the EPSR fits to the experimental data, with the EPSR model fitting well the experimentally obtained partial structure factors.T he corresponding real-space distribution of solvent molecules around the lithium ions is given in Figure 4. TheL i À Nd istance for both NH 3 and MeNH 2 is about 2.0 ,i nc lose agreement with that observed for both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 .I ntegration of this first solvation shell gives an average co-ordination number of 2.01 and 1.78 for Li-MeNH 2 and Li-NH 3 ,r espectively.T he orientational distribution of the coordinated solvent about Li can be extracted from the EPSR model (Figure 4a-d). Only asmall distortion from tetrahedral geometry for agiven Li (solv) unit is found, with the MeNH 2 ligands acting to compress the tetrahedra slightly along the tetrahedral C 2 axis.T he dipole moment of NH 3 is orientated directly towards the cation, whereas the Li-MeNH 2 angle is about 1098 8.T he relative orientation of neighboring Li (solv) units is not isotropic,w ith the vertex (that is,t he NH 3 /MeNH 2 molecules) of one tetrahedral unit approaching the face and edges of the adjacent tetrahedral unit.…”

“…[8] Along with metal concentration, chemical tunability of the electronic properties of these systems can be achieved by varying the amine.L ithium will also dissolve in MeNH 2 to yield as ystem whereby solvated electrons transition to am etallic state. [3,[9][10][11][12] TheM IT in Li-MeNH 2 occurs at 15 MPM, an otably higher concentration than in Li-NH 3 . No liquid-liquid phase separation has been detected across the full concentration range of Li-MeNH 2 ,a nd the solution remains ad eep blue,a lbeit with am etallic luster.…”

“…Lithium will also dissolve in MeNH 2 to yield a system whereby solvated electrons transition to a metallic state 3,9, 10, 11, 12. The MIT in Li–MeNH 2 occurs at 15 MPM, a notably higher concentration than in Li–NH 3 .…”

“…[1,2] Va rying the concentration of metal in these liquids dramatically alters the electronic,magnetic,and structural properties of the solutions,a nd enables us to experimentally determine the manner in which liquid systems accommodate excess electron density.A talow concentration of metal/electrons, the solutions are electrolytic, whereby the metal valence electrons have been ionized into solution and exist as solvated electrons propagating between solvent cavities. [3] Increasing the concentration results in metallization in the liquid phase, which for the Li À NH 3 system occurs at amere 4mol %metal (MPM). [2] Interestingly at lower temperatures,b elow T C = 210 K, the point of the Mott-type metal-insulator transition (MIT) is obscured by ap ronounced liquid-liquid phase separation.…”

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

“…Varying the concentration of metal in these liquids dramatically alters the electronic, magnetic, and structural properties of the solutions, and enables us to experimentally determine the manner in which liquid systems accommodate excess electron density. At a low concentration of metal/electrons, the solutions are electrolytic, whereby the metal valence electrons have been ionized into solution and exist as solvated electrons propagating between solvent cavities . Increasing the concentration results in metallization in the liquid phase, which for the Li−NH 3 system occurs at a mere 4 mol % metal (MPM) .…”