Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium as Electron Sources

Abstract

A free electron in solution, known as a solvated electron, is the smallest possible anion. Alkali and alkaline earth atoms serve as electron donors in solvents that mediate outer-sphere electron transfer. We report herein ab initio molecular dynamics simulations of lithium, sodium, magnesium, and calcium in liquid ammonia at 250 K. By analyzing the electronic properties and the ionic and solvation structures and dynamics, we systematically characterize these metals as electron donors and ammonia molecules as electron acceptors. We show that the solvated metal strongly modifies the properties of its solvation shells and that the observed effect is metal-specific. Specifically, the radius and charge exhibit major impacts. The single solvated electron present in the alkali metal systems is distributed more uniformly among the solvent molecules of each metal's two solvation shells. In contrast, alkaline earth metals favor a less uniform distribution of the electron density. Alkali and alkaline earth atoms are coordinated by four and six NH3 molecules, respectively. The smaller atoms, Li and Mg, are stronger electron donors than Na and Ca. This result is surprising, as smaller atoms in a column of the periodic table have higher ionization potentials. However, it can be explained by stronger electron donor-acceptor interactions between the smaller atoms and the solvent molecules. The structure of the first solvation shell is sharpest for Mg, which has a large charge and a small radius. Solvation is weakest for Na, which has a small charge and a large radius. Weak solvation leads to rapid dynamics, as reflected in the diffusion coefficients of NH3 molecules of the first two solvation shells and the Na atom. The properties of the solvated electrons established in the present study are important for radiation chemistry, synthetic chemistry, condensed-matter charge transfer, and energy sources.

Excess electron solvation in ammonia clusters

“…Experimental 5,6,7,8,9 and theoretical studies 10,11,12 have explored a wide spectrum of physical properties of the ammoniated electron both in bulk liquid and clusters. Still, conceptually important structural questions need more elaborate clarification.…”

“…The only many-electron based study that attempted to simulate the temporal behavior of the ammoniated electron is the DFT based ab initio molecular dynamics study by Chaban and Prezhdo. 12 They simulated the dynamics of alkali and alkaline earth metals solvated in a bath of 32 ammonia molecules in a periodic simulation cell. To the best of our knowledge, the dynamics of the ammoniated electron clusters (without the interference of counter ions) have not been investigated using many electron models.…”

Synthesis of K2Se solar cell dopant in liquid NH3 by solvated electron transfer to elemental selenium

“…Computational work by Chandra and Marx [26] and Hoffmann et al [19] confirms that sufficiently diluted solutions contain unbound electron species near empty cavities of the solvent. Therefore, such systems contain the smallest possible anion [27] and constitute the ultimate environment for electron transfer processes in homogeneous media. These are in contrast to electron transfers to or from the surface of electrodes immersed in solution, which form a heterogeneous interface [28,29].…”

Structure and some physicochemical and functional properties of water treated under ammonia with low-temperature low-pressure glow plasma of low frequency

“…Alkaline earth atoms readily coordinate O-ligands and that ability can be controlled by their counterions [18]. These central atoms can also coordinate N-ligands.…”