Solvation Shell Of Molecules Research Articles

The structure of the solvation shell of Na + in aqueous solutions

The structure of the solvation shell of the Na + ion in a aqueous solution was obtained from radial distribution functions, the ion—water dipoles and ion-HH vectors angular—radial distribution functions, the angular—angular frequencies obtained from the angles formed by the vectors dipole and by the vectors HH and, finally, by the frequencies of the azimuthal angles formed by three water molecules in solvation shell. These functions were obtained by Monte Carlo simulation of a system composed of TIP4P water and one Na + ion. The most important structural features of the solvation shell are: the dipole moment and the ion are co-linear, the two vectors defined by the ion and the oxygen atom and by the two hydrogen atoms of the same molecule (HH) are perpendicular, the angles formed by the dipoles of two water molecules are close to 105, 120 and 180°, the angles formed by their HH vectors are 90°. This information allows us to identify three main structures for the solvation shell: the first one is non—planar and derived from the tetrahedron geometry, the second structure is an equilateral triangle and the last one is planar also and is formed by the three water molecules occupying three of four vertices of a square. In all cases, the Na + ion occupies the centre of the geometric figure.

A model calculation on structures and excitation energies of the hydrated electron

Model calculations were made on the hydrated electron by using the ab initio MO method combined with the MR-SD-Cl method and the coupled cluster theory. The models used in the calculations were water clusters denoted by [e−(H2O)n(H2O)m], where n = 2,3,4, and 6 for the first solvation shell and m = 0–28 for the second and third solvation shells. In these model calculations, the interactions between the excess electron and the water molecules in the first solvation shell are explicitly calculated by ab initio MO methods and the water molecules in the second and third solvation shells were represented by the fractional charges obtained at the MP2/D95V** level. The stabilization energies and the solvation radius r(e−–O), in terms of the distance between the center of the cavity and an oxygen atom of the surrounding water molecules, increased monotonically with the number of water molecules in the first solvation shell. On the other hand, the first excitation energy was not dependent on the number of water molecules in solvation shells, but constant, with the value of ca. 2.0 eV. On the basis of the present calculations, we suggest that (1) the energetic stability of excess electrons depends on both short-range interaction and long-range interaction, (2) the first excitation energy is critically affected by only the short-range interactions, and the excitation is theoretically attributed to the1s→2p transition of the excess electron.

Free energy profiles for sodium cation adsorption on a metal electrode

Monte Carlo statistical mechanics simulations have been used to determine free energy profiles for the approach of a sodium ion to a model meal electrode in aqueous solution and in tetrahydrofuran (THF). The ion and 200-300 solvent molecules are explicitly represented in a periodic cell bounded in the ±z directions by soft walls that are polarizable via image charges. Statistical perturbation theory provided the free energy profiles from the wall out 8-10 A into the solvent. In both cases, a single minimum is found corresponding to the sodium ion with its first shell of solvent molecules in contact with the wall

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 calculated separately for the ammonia molecules in the solvation shell and in the bulk. The effect of Li + on intramolecular geometry and vibrations is reported.

Solvent molecular dynamics in regions of hydrophobic hydration

We present a dynamical analysis of the solvent in an aqueous solution of two nonpolar atomic solutes which are constrained to an interatomic distance corresponding to a solvent-separated free energy minimum. The results are obtained from a molecular dynamics simulation using ST2 model water. Molecular mobility for solvent near the solutes is seen to be retarded, as evidenced in translational diffusion and rotational reorientation. These slower net motions are analogous to pure solvent dynamics at a temperature reduced by 10–15 °C. An analysis of intermolecular hydrogen bonding reveals that solvation shell molecules have correspondingly longer bond half-lives compared to bulk molecules, by a factor of 1.5–2.0. The spectral densities for intermolecular vibrations are computed from translational and rotational velocity autocorrelation functions for shell and bulk motions. These densities are seen to correlate well with the local binding energy distributions.

Dynamics of solute-solvent interactions

Abstract

Electron spin echo studies of solvation structure

The development and application of electron spin echo modulation analysis for deducing detailed geometrical information about the solvation shell structure of paramagnetic species is described. This technique makes it possible to measure weaker hyperfine interactions in disordered systems such as frozen solutions and surfaces than is normally possible. The number, distance, and orientation of first solvation shell molecules can then be determined. Applications of this technique to silver atom solvation, electron solvation, anion solvation, cation solvation, and cation solvation on surfaces are described.

A semicontinuum model with a structured first solvation shell for excess electrons in liquid and glassy alkanes

The microdipole model for excess electrons in alkanes seems inconsistent with recent electron spin echo data. We develop a new model for electron binding in alkanes based on a semi-continuum potential framework in which the electron-polarizability energy is dependent on molecular structure by considering the electron interaction with the polarizability of each C—C bond of the first solvation shell molecules rather than with point molecules. In addition we incorporate the effect of hydrogen–hydrogen repulsion between first solvation shell molecules. This model accounts well for the variety of experimental data known for localized electrons in 3-methylpentane glass at 77 K and for electron localization or nonlocalization in various liquid alkanes consistent with electron mobility data.

Geometrical structure of solvated electrons in γ-irradiated 3-methylpentane glass at 77 K from an analysis of the modulation of the electron spin echo decay envelope

Deuterium modulation of the electron spin echo decay envelope from trapped electrons in γ-irradiated 3-methylpentane-d14 glass at 77 K has been analyzed to give a solvation structure characterized by 18–21 equivalent nearest neighbor deuterons to the electron at a distance of 3.0–3.2 Å and with an isotropic coupling constant of essentially zero. From the structure of 3-methylpentane it is suggested that there are six to seven first solvation shell molecules around the electron.

Pulse radiolysis studies of time dependent spectral shifts of the solvated electron in ethanol and deuterated ethanol glasses at 76 K

The initial (?200 nsec) spectra of the trapped electron are similar in C2H5OH, C2H5OD, C2D5OH, and C2D5OD glasses at 76 K with λmax ∼1300 nm. The kinetics of the spectral shifts to the stable values of λmax near 540 nm are similar for C2H5OH and C2H5OD and resemble the kinetics of a series of first order processes or of tunneling reactions. Structure seems to be present in the partially relaxed spectra and may result from specific geoemetrical orientations of first solvation shell molecules.

Matrix proton ENDOR studies of the overlap of trapped electron wavefunctions with first solvation shell molecules in methanol glass at 77 K

First Page

ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

ESR line-shape measurements of Cu(ClO4)2 solutions in methanol and water at X- and Q-band frequencies are reported. In both frequencies, above room temperature, the over-all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g-tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/mole for the aqueous complex and τi(25°C)=1.2× 10−11 sec−1, ΔEi=1.1 kcal/mole for the methanol complex. NMR relaxation measurements of Cu(ClO4)2 solutions in methanol between −90°C and 100°C are also reported and are interpreted in terms of kinetic and magnetic parameters of the Cu2+ solvation complex. Since there is fast intramolecular inversion of the complex, only weighted averaged parameters over the axial and equatorial ligand molecules are obtained. The pseudo-first-order rate constant for the exchange of solvation shell molecules is 1/ τM(25°C)=7.4× 107 sec−1 with an activation energy ΔE=6.6 kcal/mole; the molecular tumbling time is τR(25°C)=4.6× 10−11 sec with ΔER=3.0 kcal/mole; the isotropic ligand hyperfine interaction constants are AOH/h=8.8× 105 Hz, ACH3/h=10.6× 105 Hz for the protons and AO/h=2.9× 107 Hz for 17O. Previous NMR results in aqueous solutions are discussed and reinterpreted.