Molecular Dynamics Study of a Lithium Ion in Ammonia

Abstract

A Molecular Dynamics simulation of a solution of one Li+ in 215 NH3 molecules has been performed at an average temperature of 235 K. A newly developed flexible model for NH3 is employed and the Li + -NH3 interactions are derived from ab initio calculations. The structure of the solution is described by radial distribution functions and the orientation of the molecules. A solvation number of six is found for Li+ and a strong preference of the solvation shell molecules exists for an orientation where the Li + -N vector and the dipole moment direction of NH3 are parallel. The self-diffusion coefficient, the hindered translational motions and librations are calcula ted separately for the ammonia molecules in the solvation shell and in the bulk. The effect of Li + on intramolecular geometry and vibrations is reported. The physics of metal solutions in liquid ammonia is an interesting topic in modern physical chemistry. For example, striking differences between solvation of metal atoms in water and liquid ammonia exist. Diss olving alkali metal atoms in water leads to oxidation to the monovalent cations, the formation of hydroxide ions, and H2 evolution. On the contrary, in liquid ammonia free electrons exist if the alkali metal con centration exceeds a few mole percent [1], The electri cal conductivity of a 20 mole percent lithium solution is higher than that of liquid mercury at room tempera ture [2], Structural information about bulk ammonia as well as metal ion and electron solvation is necessary in order to understand the properties of these solutions. The only X-ray scattering experiment on liquid am monia reported so far has been performed by Narten [3]. Only the nitrogen-nitrogen radial distribution function (RDF) can be extracted reliably from these data. The available neutron scattering data [4] are not analysed in terms of atom-atom RDFs. Neutron scat tering experiments with isotopic substitution, which yield information also about the nitrogen-hydrogen and hydrogen-hydrogen RDFs, have not been per formed. Considerable efforts have been made in attempts to overcome this lack of experimental information on liquid ammonia by means of computer simulations [5][6][7][8][9], which yield information about all three atomatom RDFs. Recently, the electron solvation in liquid ammonia has been treated by the quantum path inte gral Monte Carlo method on the basis of a pseudopotential ansatz for the electron ammonia interactions [10].In this work, the structure of the solvation shell of a lithium ion dissolved in NH3 will be discussed in detail and compared with some preliminary results from a neutron diffraction experiment [4] and a recent Molecular Dynamics (MD) simulation [11]. In addi tion, the dynamical properties of ammonia in the bulk and in the solvation shell of the ion have been calcu lated separately from the simulation and are com pared with experimental data. The flexibility of the ammonia model employed permits the investigation of the effect of Li+ on the intramolecular vibrations of NH3 and the comparison...

THE STRUCTURE OF CONCENTRATED Li-AMMONIA SOLUTIONS AS DERIVED FROM MD SIMULATIONS

“…The results for this dilute case reproduce the findings presented already in reference [15] and proved to be unchanged when 3-body interactions were included [10]. In accordance with these findings the solvation number remained six while for the higher concentrations the addition of the 3-body term reduced the solvation number to four.…”

“…All of the direct contributions to the potentials have been derived from ab initio calculations and are given for the ammonia-ammonia interactions by [15]: …”

“…In both investigations a pronounced shoulder at about 2.5Å is obvious which reduces with the increasing concentration. The formation of the shoulder has been discussed in reference [15] for the dilute solution. It is a result of the existence of two kinds of hydrogen atoms.…”

Three-body Effects in Calcium(II)-ammonia Solutions: Molecular Dynamics Simulations

“…Both inter-and intramolecular potentials of ammonia molecules, which are the same as those used in [9][10][11][14][15][16], were taken from [17,18]. Two molecular dynamics simulations were performed for an average temperature of 240 K and atmospheric pressure.…”

Simulations of Liquid Ammonia Based on the Combined Quantum Mechanical/Molecular Mechanical (QM/MM) Approach

“…The rest of the system is described by the classical NH 3 -NH 3 pair potentials. 11,36 In general, the post-HF methods with the extended basis sets are most suitable for the treatment of hydrogen-bonded systems. It turns out, however, to be computationally prohibitive.…”