Study of the structure of molecular complexes. Coordination numbers for Li+, Na+, K+, F− and Cl− in water

A cluster of 200 water molecules containing a single ion (either Li⁺ or Na⁺ or K⁺ or F^? or Cl^?) has been studied at T = 298 K using Monte Carlo techniques. The waterwater interaction is obtained from a quantum-mechanical study of CI type; the ionwater potentials have been obtained from HartreeFock type computations. The computed coordination numbers in the first shell for Li⁺, Na⁺, K⁺, F^? and Cl^? are 4.0, 4.3, 5.1, 3.85 and 4.3, respectively; the corresponding first hydration shell radii are 2.28 Å, 2.59 Å, 3.27 Å, 1.99 Å and 2.85 Å, respectively. A discussion of the second and third hydration shell radii and coordination numbers is given.     

Coordination polyhedra of hydration shells of Na+ and K+ cations in aqueous solutions

Simulation of the hydration of Na⁺ and K⁺ cations in dilute solution was performed by the Monte Carlo method. A novel approach to structural analysis of hydration shells of ions was developed. Specific sets of coordination polyhedra formed by water molecules of the first coordination sphere were found. Structural and energy characteristics of hydration were calculated. The effect of Na⁺ and K⁺ cations on the structure of the network of H-bonds and mobility of water molecules in hydration shells was studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 852–857, May, 1999.     

Nucleation of water vapor on Na<Superscript>+</Superscript>Cl<Superscript>−</Superscript> ion pairs: Computer simulation

The Monte Carlo method is used to calculate the free energy, entropy, and work of water cluster formation in the field of Na⁺Cl⁻ ion pairs. A detailed model is used that allows for polarization and covalent many-particle interactions, as well as the effects of ion charge reversal. The model is matched to the experimental data on the free energy of ion hydration and the results of the quantum-chemical calculations of stable configurations. The hydration leads to the cleavage of an ion pair in a molecular cluster after approximately ten water molecules are captured. As vapor molecules are added, the stable interion distance monotonically elongates. The low free energy barrier separating the dissociated and nondissociated states of the ion pair in an equilibrium cluster does not hinders the reversible spontaneous transitions between the states, which are responsible for strong fluctuations and the instability of the system. Unlike hydroxonium-containing ion pairs, the formation of long-lived metastable states of hydrated Na⁺Cl⁻ pairs is impossible.     

Solvent structures around Na+ and Cl− ions in water

The structural orientation of water in the hydration shells of Na⁺ and Cl^– has been obtained from a Monte Carlo simulation of a 0.55 molal NaCl solution, using the MCY model for water. The probability of first shell coordination numbers has been calculated and is compared with data of previous studies using various model systems.

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Zur Lösungsmittel-Struktur rund um Na⁺ und Cl^– Ionen in Wasser

Zusammenfassung Es wurde mittels einer Monte Carlo-Simulation einer 0.55 molalen NaCl-Lösung unter Verwendung des MCY-Modells für Wasser die Lösungsmittel-orientierung von Wasser in der Hydrat-Hülle von Na⁺ und Cl^–-Ionen erhalten. Die wahrscheinlichsten Koordinationszahlen der ersten Hydrat-Schale werden diskutiert und mit den Daten aus früheren Untersuchungen verschiedener Modellsysteme verglichen.

     

Structure and dynamics of high-spin Ru in aqueous solution: Ab initio QM/MM molecular dynamics simulation

The structural and dynamical properties of high-spin Ru²⁺ in aqueous solution have been theoretically studied using molecular dynamics (MD) simulations. The conventional MD simulation based on pair potentials gives the overestimated average first shell coordination number of 9, whereas the value of 5.9 was observed when the three-body corrected function was included. A combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to take into account the many-body effects on the hydration shell structure of Ru²⁺. The most important region, the first hydration shell, was treated by ab initio quantum mechanics at UHF level using the SBKJC VDZ ECP basis set for Ru²⁺ and the 6-31G^∗ basis sets for water. An exact coordination number of 6 for the first hydration shell was obtained from the QM/MM simulation. The QM/MM simulation predicts the average Ru²⁺–O distance of 2.42 Å for the first hydration shell, whereas the values of 2.34 and 2.46 Å are resulted from the pair potentials without and with the three-body corrected simulations, respectively. Several other structural properties representing position and orientation of the solvate molecules were evaluated for describing the hydration shell structure of the Ru²⁺ ion in dilute aqueous solution. A mean residence time of 7.1 ps was obtained for water ligands residing in the second hydration shell.     

Structural and dynamic investigations of aqueous clusters of Na+ and K+

Using computer modeling, we have studied Na⁺–24H₂O and K⁺–24H₂O clusters. We propose ion-water interaction potentials. We obtain structural, energy, and dynamic characteristics of the studied clusters. We show different mechanisms for exchange of water molecules surrounding the Na⁺ and K⁺ ions: single-particle in the case of Na⁺, and close to K⁺, along with single-particle exchange, a large percentage of multiparticle cooperative exchange of water molecules. This difference is explained by the different degrees of orientation of the water molecules surrounding these ions, by the presence of a unified deformed network of H bonds in the K⁺ cluster and its absence in the Na⁺ cluster. We propose a negative hydration mechanism for the K⁺ ion.Institute of General and Inorganic Chemistry, Russian Academy of Sciences. Institute of Physical Chemistry, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 96–104, March–April, 1993.     

Two exponential decay of 3371 å laser excited CS2 fluorescence

We have observed time resolved CS₂ fluorescence excited by an N₂⁺ pulsed laser at 3371 Å. The optical absorption in this region is unassigned; we find two fluorescing states with collision free lifetimes of 2.9 ± 0.3 and 17 ± 2 μsec. Deactivation rates for both states are reported for the collision partners CS₂, Ar, O₂, and N₂. All rates are near gas kinetic; the 2.9 ± 0.3 μsec state exhibits exceptionally fast deactivation, with the rate constant for CS₂ being (7.9 ± 1.2) × 10⁻¹⁰ cm³/molecule sec.     

Hydration of Fe+: A Monte Carlo simulation of water clusters and of a dilute aqueous solution

Monte Carlo statistical thermodynamic computer simulations are reported for several clusters Fe⁺ (H₂O)_n at different temperatures and for a dilute aqueous solution of Fe⁺ at 298 K. The energy of each configuration has been calculated in the pairwise additivity approximation using the MCY potential for the water–water interaction and an ab initio analytical potential built by us for the Fe⁺–H₂O interaction. Energy and structural analysis of the generated configurations lead to the prediction of a coordination number of six for the first hydration shell of the Fe⁺ ion, both in clusters and in dilute solution. Finally, the variation in the distance to the Fe⁺ ion of the energy and orientation of water molecules in the solution are discussed.     

Vibrational spectroscopy of Cs+ solvated by methylamine

We report the vibrational spectra of the cluster ions Cs⁺(CH₃NH₂)_N, N=3–22. Bands in the 1015–1050 cm^?1 region of the infrared are due to the v8 mode (CN stretch) of methylamine molecules displaying different degrees of interaction with the central ion. Monte Carlo simulations of the solvated Cs⁺ ion indicate that nine methylamine molecules fill the first solvation shell of Cs⁺ and that possible rearrangements in cluster structure occur at N=14?15. No absorptions due to bulklike methylamine molecules are observed through N=22.     

Theoretical study of PO and PO−

The diatomic systems, PO and PO^– are studied, using numerical Hartree-Fock (NHF) and coupled-cluster calculations. The latter employs a hybrid NHF and Slater orbital basis set. Highly accurate CCSD methods predict bond lengths accurate to <0.004 Å and frequencies to 60 cm^–1. In addition the electron affinity of PO is computed to be 0.89 eV compared to an experimental value of 1.09±0.01. Comparisons are made with SCF and MBPT(2) results for PO⁺ using conventional basis sets.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayGuggenheim Fellow     

Interaction of DNA fragments with counterions in aqueous solution

Monte Carlo simulation of the hydration of metal ion—DMP⁻ and metal ion—9-methylguanine complexes was performed. A comparative analysis of the results for Na⁺ and K⁺ ions was carried out. The main stages of dissociation were revealed. The energy effects of dissociation were evaluated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2174–2177, November, 1998.     

The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte

The structure of the electrochemical double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte has been probed using in-situ surface X-ray scattering (SXS). Detailed modeling of the SXS data at negative potential (E = − 1.0 V versus SCE) is consistent with the presence of an hydrated K⁺ cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at positive potential (E = − 0.2 V), indicates that the presence of OH_ad stabilizes the hydrated K⁺ cations through a non-covalent interaction forming a compact double layer structure in which the Ag-K⁺ distance is reduced to 3.6 ± 0.2 Å.     

Spatial correlations of ions in nanoclusters of an aqueous solution of NaCl

The Monte Carlo method is used to calculate the radial distribution functions (RDFs) of Na⁺Cl^?, Na⁺Na⁺, and Cl^?Cl^? ion pairs in water clusters that differ both in composition and size. An analysis of the RDFs shows that like-charged ions can approach one another at extremely small distances (direct ion contact) without the participation of a third ion of the opposite sign. The Na⁺Na⁺ (interionic distance 3.5 Å) and Cl^?Cl^? (interionic distance 5.25 Å) ion pairs are most likely to form in very dilute solutions. The composition of ionic associates changes with increasing concentration of ions in the solution (it is indicated by the emergence of the RDF peak at 4.25 Å and its further growth with concentration and by a selective visual analysis of instantaneous cluster configurations) so that groups of three, five, etc. ions of the opposite sign bearing a net charge of 1+ or 1? appear.     

Effect of hydrophilic walls on the hydration of sodium cations in planar nanopores

A computer simulation of the structure of Na⁺ ion hydration shells with sizes in the range of 1 to 100 molecules in a planar model nanopore 0.7 nm wide with structureless hydrophilic walls is performed using the Monte Carlo method at a temperature of 298 K. A detailed model of many-body intermolecular interactions, calibrated with reference to experimental data on the free energy and enthalpy of reactions after gaseous water molecules are added to a hydration shell, is used. It is found that perturbations produced by hydrophilic walls cause the hydration shell to decay into two components that differ in their spatial arrangement and molecular orientational order.     

A theoretical study of the hydration of Rb+ by Monte Carlo simulations with refined ab initio-based model potentials

An infinitely diluted aqueous solution of Rb⁺ was studied using ab initio-based model potentials in classical Monte Carlo simulations to describe its structural and thermodynamic features. An existing flexible and polarizable model [Saint-Martin et al. in J Chem Phys 113(24) 10899, 2000] was used for water–water interactions, and the parameters of the Rb⁺–water potential were fitted to reproduce the polarizability of the cation and a sample of ab initio pair interaction energies. It was necessary to calibrate the basis set to be employed as a reference, which resulted in a new determination of the complete basis set (CBS) limit energy of the optimal Rb⁺–OH₂ configuration. Good agreement was found for the values produced by the model with ab initio calculations of three- and four-body nonadditive contributions to the energy, as well as with ab initio and experimental data for the energies, the enthalpies and the geometric parameters of Rb⁺(H₂O) _n clusters, with n = 1,  2,…, 8. Thus validated, the potential was used for simulations of the aqueous solution with three versions of the MCDHO water model; this allowed to assess the relative importance of including flexibility and polarizability in the molecular model. In agreement with experimental data, the Rb⁺–O radial distribution function (RDF) showed three maxima, and hence three hydration shells. The average coordination number was found to be 6.9, with a broad distribution from 4 to 12. The dipole moment of the water molecules in the first hydration shell was tilted to 55° with respect to the ion’s electric field and had a lower value than the average in bulk water; this latter value was recovered at the second shell. The use of the nonpolarizable version of the MCDHO water model resulted in an enhanced alignment to the ion’s electric field, not only in the first, but also in the second hydration shell. The hydration enthalpy was determined from the numerical simulation, taking into account corrections to the interfacial potential and to the spurious effects due to the periodicity imposed by the Ewald sums; the resulting value lied within the range of the various different experimental data. An analysis of the interaction energies between the ion and the water molecules in the different hydration shells and the bulk showed the same partition of the hydration enthalpy as for K⁺. The reason for this similarity is that at distances longer than 3 Å, the ion–water interaction is dominated by the charge-(enhanced) dipole term. Thus, it was concluded that starting at K⁺, the hydration properties of the heavier alkali metal cations should be very similar.     

Hydration of guanidinium depends on its local environment

Hydration of gaseous guanidinium (Gdm⁺) with up to 100 water molecules attached was investigated using infrared photodissociation spectroscopy in the hydrogen stretch region between 2900 and 3800 cm^–1. Comparisons to IR spectra of low-energy computed structures indicate that at small cluster size, water interacts strongly with Gdm⁺ with three inner shell water molecules each accepting two hydrogen bonds from adjacent NH₂ groups in Gdm⁺. Comparisons to results for tetramethylammonium (TMA⁺) and Na⁺ enable structural information for larger clusters to be obtained. The similarity in the bonded OH region for Gdm(H₂O)₂₀ ⁺ vs. Gdm(H₂O)₁₀₀ ⁺ and the similarity in the bonded OH regions between Gdm⁺ and TMA⁺ but not Na⁺ for clusters with <50 water molecules indicate that Gdm⁺ does not significantly affect the hydrogen-bonding network of water molecules at large size. These results indicate that the hydration around Gdm⁺ changes for clusters with more than about eight water molecules to one in which inner shell water molecules only accept a single H-bond from Gdm⁺. More effective H-bonding drives this change in inner-shell water molecule binding to other water molecules. These results show that hydration of Gdm⁺ depends on its local environment, and that Gdm⁺ will interact with water even more strongly in an environment where water is partially excluded, such as the surface of a protein. This enhanced hydration in a limited solvation environment may provide new insights into the effectiveness of Gdm⁺ as a protein denaturant.     

Modeling of the hydration sphere of gadolinium(III) ion using density functional theory

The hydrated gadolinium(III) ion cluster Gd³⁺ (H₂O)_x, with x = 8,9, was studied using density functional theory. The different conformations of the first hydration shell were calculated. For x = 8, the results for the cubic conformation correspond to previously published Hartree-Fock and MP2 results, whereas much lower energies were found for the square antiprismatic and dodecahedral conformations. For x = 9, the energy of the tricapped trigonal prism is nearly identical to the one of the antiprism plus an extra free water molecule. © 1997 John Wiley & Sons, Inc.     

Structure and hydration of polycytidylic acid from the data of infrared spectroscopy, EHF dielectrometry and computer modeling

The aim of this paper is to summarize the original results concerned with the elucidation of the role of water environment in the formation of different structures of polyribocytidylic acid (poly(rC)), depending on the pH, temperature and ion content. To solve this problem, we studied the hydration of poly(rC)-K⁺ in films differing in water content, by methods of infrared spectroscopy and piezogravimetry, and in solution, using the dielectric measurements at a wavelength of 7.6 mm (extremely high frequencies — EHF). The experimental results were confirmed by a Monte Carlo simulation of the interaction between water molecules and single-strand and double-strand poly(rC) fragments in clusters of 800 water molecules. A model of hydration of double-strand complex of poly(rC)⁺-poly(rC) has been proposed based on our results and the known X-ray parameters of the complex. The obtained results and proposed structure of poly(rC)⁺-poly(rC) suggest that, the stabilization of this complex occurs due to intra- and inter-chain water bridges, together with the hydrogen bonds between neutral and protonated cytosines in pairs.     

Intermolecular nuclear spin—spin coupling and scalar relaxation. A quantum-mechanical and statistical-mechanical study for the aqueous fluoride ion

Coupling between¹⁹F and magnetic nuclei (¹H,¹⁷O) in water molecules in the first hydration sphere of F^? are calculated using Monte Carlo simulations. The simulations are based on intermolecular potentials from the literature and on variations of coupling constants with geometry obtained by coupled Hartree-Fock calculations. Average coupling constants are ≈20–40 Hz. Intermolecular scalar relaxation in aqueous solutions is discussed.     

Structures of the nearest surroundings of the K<Superscript>+</Superscript>, Rb<Superscript>+</Superscript>, and Cs<Superscript>+</Superscript> ions in aqueous solutions of their salts

Published data on structural characteristics of hydration of K⁺, Rb⁺, and Cs⁺ ions in aqueous solutions of their salts under standard conditions, including authors’ X-ray diffraction data, are summarized and correlated. The structural parameters of the nearest surrounding of the K⁺, Rb⁺, and Cs⁺ ions, such as the coordination numbers, interparticle distances, and types of ionic association, are discussed. It is noted that, because of weak tendency of these cations to hydration, the parameters of their coordination spheres strongly depend on the concentration and chemical nature of counterions.