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

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 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.

Metal ammonia solutions – of which lithium-ammonia solutions are a prototypical example – have long been studied because of their fascinating physical and chemical characteristics; these include their spectacular colors, their composition-induced transition from a liquid electrolyte to liquid metal, their unique and potent reducing power, and their remarkable liquid-liquid phase separation. Upon dissolution in anhydrous liquid ammonia, elemental lithium is spontaneously ionized such that its outer-valence shell 2s electron is introduced into this liquid host solvent with the formation of solvated Li+ ions and solvated electrons, esol−1. It has previously been noted that this dissolution process is formally akin to the ionization process in highly excited gas-phase atomic states of the alkali metals, but with the ionized or ejected electron now entering the host, liquid ammonia. At low concentrations of lithium in liquid ammonia, approximately 1 to 4 mole percent metal (MPM), the solution is intensely blue and electrolytic in nature. In such dilute lithium-ammonia solutions, a broad optical absorption, peaked at around 0.85 eV, has a tail extending into the visible range which gives the solutions their characteristic blue color. As the concentration of metal is gradually increased, the solution continuously transforms to a highly conducting liquid until at metal concentrations between approximately 6 MPM to saturation (ca. 20 MPM), the solution takes on a spectacular copper bronze metallic luster and, to many intents and purposes, behaves as a liquid metal. One of the earliest – perhaps the earliest – comments on the nature of the solvated electron, was made over a century ago by Kraus. He had determined the primary carrier of electric current in these solutions to be of negative charge and massless by chemical standards. In 1908 Kraus noted perceptively: “The negative ion constitutes a new species of anion. It consists of a negative charge, an electron surrounded by an envelope of solvent molecules” Kraus first proposed that an alkali metal dissociates in liquid ammonia according to the process and in 1916 the first use of the description “solvated electrons” appears.  A model in which the electron resides, and also moves in a cavity of radius ca. 3 Å and the surrounding ammonia liquid is polarized or solvated as it is around a cation, was first put forward by Ogg and significantly developed by Jortner, who showed that that it was able to account for the optical absorption spectrum as being due to the 1s – 2p transitions of the electron located within the cavity (Figure 1). Jortner’s model also accounted for the very large volumetric expansion of the liquid which occurs upon dissolution of the metal.

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