Computer simulation of water and concentrated ionic solutions. Potential fluctuations and electron localization

Abstract

The possibilities of trapping an excess electron by potential traps in liquid water and aqueous ionic solutions have been investigated by means of a computer simulation method. The equilibrium configurations of water molecules are generated by the molecular dynamics (MD) method and the molecular configurations are searched for local minima of the potential energy. The analysis of a large set of the minima allows us to obtain an extensive statistical description of the microscopic trapping sites. The estimated concentration of the electron traps in liquid water is ca. 0.5 mol dm–3. The possibility of electron trapping depends very strongly on the lifetime of the potential traps. The simulations yielded the distribution of the trap lifetime with an average of 84 fs. A substantial fraction (20%) of the traps live longer than 100 fs, a small fraction (0.2%) live as long as 1 ps. These values can be compared with experimental measurements of the electron hydration time of the order of 100 fs. The calculations of the localized excess electron in concentrated solutions of LiCl and NaCl have been performed. The MD simulations of the solutions provided an ensemble of configurations of ions and water molecules. The quantum calculations (based on density functional theory, non–local version) of an excess electron in the clusters of ions and water molecules extracted from the MD configurations have been performed for a few hundred cases. It appears that most of the hydrated Li+ cations can trap an excess electron. The histograms of the distribution of energy of the HOMO orbitals and a histogram of the electron excitation spectrum have been constructed.