The structure of an aqueous ammonium chloride solution

Abstract

X-ray measurements have been performed on a 2 molal NH4 Cl solution at 25 °C. The scattering data have been evaluated with the first neighbor model (FNM). These results are compared with molecular dynamics (MD) simulations where the ST2 model for water is employed. The NH4+ is described by a rigid tetrahedron with point charges at the positions of the hydrogen atoms and the Cl− by a Lennard-Jones sphere with an elementary charge at the center. Good agrement has been found between the measurements and the simulation. Contrary to previous results the nearest neighbor distance for NH4+–water is found to be significantly different from the one for water–water. The structure of the solvent described by the nearest neighbor distance and its root mean square deviation is in the limits of error the same as in pure water and significantly different from the solvent structure in alkali halide solutions.

Structure and dynamics of hydrated NH: An ab initio QM/MM molecular dynamics simulation

“…Compared to the experimentally observed coordination number of 8, 10 good agreement can then be achieved. In fact, comparison between experiments and theories is not always straightforward because most of the experimental methods for structural analysis have to be performed with solutions of relatively high concentrations, while the theoretical approaches mostly refer to very dilute solutions.…”

“…8 -20 Of particular interest, recent NMR measurements 6,8 have shown that NH 4 ϩ rotates quite fast in aqueous solution, despite the expected strong hydrogen bonding between the NH 4 ϩ and its surrounding water molecules. To explain such surprising phenomenon, several rotational mechanisms have been proposed based on either ab initio geometry optimizations [11][12][13] as well as Monte Carlo (MC) 14 and molecular dynamics (MD) 10,[15][16][17][18][19][20] simulations. Most of the previous studies have provided useful information on the dynamical properties of the hydrated NH 4 ϩ , such as its fast diffusive rotational motion.…”

“…This structural aspect is supported by experimental X-ray and neutron diffraction studies. 9,10 The second group of water molecules is much less strongly oriented than the first, and its structure is a source of major disagreement between the theoretical observations, consisting of 1-8 water molecules. 10,14 -20 Thus, the comprehensive knowledge of how NH 4 ϩ interacts with water molecules remains incomplete.…”

“…25,26 In addition, although both exemplary approaches can provide quite reasonable detailed information on the rotational dynamics of NH 4 ϩ in water, the observed coordination numbers of 5.8 17 and 5.3 18 are considerably low when compared to the results from the X-ray measurement. 10 Because both theoretical and experimental observations have provided a rather inhomogeneous picture of NH 4 ϩ in water, the motivation of the present work was to obtain more reliable data for the structure and dynamics of this hydrate complex. An alternative approach to elucidate such detailed information is to apply the so-called combined ab initio quantum mechanical/molecular mechanical (QM/ MM) MD technique.…”

Structural Properties of an Aqueous LiI Solution Derived from a Molecular Dynamics Simulation

“…The combination of x-ray measurements and MD simulations has been shown to be a good way to elucidate structures of aqueous electrolyte solutions [6,7]. Therefore, in a subsequent paper [8] new x-ray measurements on a Lil solution are reported and compared with the structural properties discussed here.…”