Monte Carlo bicanonical ensemble simulation for sodium cation hydration free energy in liquid water

A series of bicanonical ensemble Monte Carlo (BC MC) simulations has been performed to calculate Na⁺ hydration Gibbs energy in aqueous solution. The hydration Gibbs energy of Na⁺ ion in aqueous solution is the difference between formation free energies of Na⁺ (H₂O)_n and (H₂O)_n clusters at n → α. The convergence of the hydration free energy to bulk water value is fast, and the results at n = 60 turned out to be in good agreement with experimental ones and those calculated using free energy perturbation method [1]. The ion–water interaction has been described by Aqvist's pair potential [1] and SPC model [2] has been used for water–water interactions. The behaviour of the absolute Gibbs energy, the entropy, the internal energy of the clusters and the development of hydration shells’ structure with the increase of the number of water molecules are discussed.     

Ab initio copper–water interaction potential for the simulation of aqueous solutions

A new ab initio effective two-body potential that aims at mimicking the average copper–water interaction energy of the first solvation shell was developed. This new potential, together with the MCY water–water potential and a three-body ion–water–water induction potential, is tested in simulations of gas-phase clusters [Cu²⁺? (H₂O)₂₀] and diluted solutions [Cu²⁺? (H₂O)₂₀₀] at T = 298 K. The results of simulations with conventional ab initio pair potentials, with and without three-body induction corrections, are also presented. The different types of copper–water interaction potentials are evaluated comparatively and the efficiency of the newly proposed effective pair potential is discussed. © 1993 John Wiley & Sons, Inc.     

On the Search for H5O2+centered Water Clusters in the Gas Phase

In searching for H₅O₂⁺-centered water clusters, we employed vibrational predissociation spectroscopy and ab initio calculations. Structures of the clusters were characterized by the free- and hydrogen-bonded-OH stretches of ion cores and solvent molecules. Systematic examination of H⁺(H₂O)_5–7 in a supersonic expansion reveals the presence of both cyclic and noncyclic forms of H₅O₂⁺-centered water clusters. The proton transfer intermediate H₅O₂⁺(H₂O)₄ was identified, for the first time, by its characteristic hydrogen-bonded-OH stretches of the ion core at 3178 cm^?1. Also discovered at n = 7 is the H₅O₂⁺-containing five-membered ring isomer, whose existence is evidenced by the observation of a bonded-OH stretching doublet at 3544 and 3555 cm^?1 of the solvent molecules. The observations are in accord with ab initio calculations which forecast that H₅O₂⁺(H₂O)₄ and H₅O₂⁺(H₂O)₅ are, respectively, the lowest-energy isomers of protonated water hexamers and heptamers.     

Ionization-recombination equilibrium in cold cluster plasma under conditions of retardation by high energy barrier

The kinetics of charge separation in a cold plasma was studied with the degradation reaction in molecular clusters HCl(H₂O) _n + m(H₂O) ? H₃O⁺(H₂O) _{n + m ?1}Cl^?, taken as an example, which precedes chlorine adsorption on the ice surface in the stratosphere. The formation of a vast population of H⁺, Cl^? ion pairs stabilized in water clusters ensures the intense binding of chlorine in ice microcrystals that occur in stratospheric clouds. The accumulation of chlorine in the stratosphere is recognized as the main cause of the destruction of the protective ozone layer. The ion buildup effect is a result of the balance between opposite ionization and recombination processes in the presence of a high energy barrier that retards ion recombination in water clusters. A kinetic equation for the process was obtained and its solution was analyzed. The parameters of the barrier were calculated by computer simulation.     

Global Minimum‐Energy Structure and Spectroscopic Properties of I2.−⋅n H2O Clusters: A Monte Carlo Simulated Annealing Study

The vibrational (IR and Raman) and photoelectron spectral properties of hydrated iodine‐dimer radical‐anion clusters, I₂^.? ? n H₂O (n=1–10), are presented. Several initial guess structures are considered for each size of cluster to locate the global minimum‐energy structure by applying a Monte Carlo simulated annealing procedure including spin–orbit interaction. In the Raman spectrum, hydration reduces the intensity of the I? I stretching band but enhances the intensity of the O? H stretching band of water. Raman spectra of more highly hydrated clusters appear to be simpler than the corresponding IR spectra. Vibrational bands due to simultaneous stretching vibrations of O? H bonds in a cyclic water network are observed for I₂^.? ? n H₂O clusters with n≥3. The vertical detachment energy (VDE) profile shows stepwise saturation that indicates closing of the geometrical shell in the hydrated clusters on addition of every four water molecules. The calculated VDE of finite‐size small hydrated clusters is extrapolated to evaluate the bulk VDE value of I₂^.? in aqueous solution as 7.6 eV at the CCSD(T) level of theory. Structure and spectroscopic properties of these hydrated clusters are compared with those of hydrated clusters of Cl₂^.? and Br₂^.?.     

Interaction of O+ ion with water molecules: A quantum-chemical study

The interaction of O⁺ ion with several (from one to four) water molecules was studied by theab initio (UMP4/4-31G^*) and semiempirical (AM1) quantum-chemical methods. It was found that the energy of binding the O⁺ ion to the first water molecule is appreciably higher than those of binding to the subsequent water molecules. In the complex with a water molecule, whose structure corresponds to that of water oxide, the O⁺ ion retains high reactivity. The barrier to the transfer of O⁺ ion to another water molecule is much lower than the barrier to analogous transfer of O atom from the molecule of water oxide, despite the lower dissociation energy of the H₂O−O bond. Consideration of subsequent interactions with water molecules leads to an increase in the barrier to the transfer of O⁺ ion. The doublet and quadruplet excited states of the O⁺+2 H₂O system were also studied. In these cases, the formation energies are well described by the ion-dipole model. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 981–988, June, 2000.     

Non-additivity in water-ion-water interactions

The three-body system Li⁺(H₂O)₂ was analyzed to study that non-additive part of the interaction potential which can be obtained by the Hartree-Fock approximation.For long and intermediate distances the three-body correction was found to be well represented by the induction energy, where bond dipoles are induced on each water molecule by point charges located on the (unpolarizable) lithium ion and on the other molecule respectively: for shorter distances this approximation was corrected by means of an exponential repulsive term. Such a potential model for non-additive interactions was extended to the more general situation Li⁺(H₂O)_n, and Monte-Carlo calculations were carried out on clusters containing up to six water molecules; comparison with other simulation results and with available data showed a significantly improved agreement with experiment. Tentative values for H are presented for n =7, 8,..., 20, where experimental data are not available.     

The Arrangement of First- and Second-shell Water Molecules Around Metal Ions: Effects of Charge and Size

Structural features of clusters involving a metal ion (Li⁺, Na⁺, Be²⁺, Mg²⁺, Zn²⁺, Al³⁺, or Ti⁴⁺) surrounded by a total of 18 water molecules arranged in two or more shells have been studied using density functional theory. Effects of the size and charge of each metal ion on the organization of the surrounding water molecules are compared to those found for a Mg[H₂O]₆²⁺• [H₂O]₁₂ cluster that has the lowest known energy on the Mg²⁺• [H₂O]₁₈ potential energy surface (Markham et al. in J Phys Chem B 106:5118–5134, 2002). The corresponding clusters with Zn²⁺ or Al³⁺ have similar structures. In contrast to this, clusters with a monovalent Li⁺ or Na⁺ ion, or with a very small Be²⁺ ion, differ in their hydrogen-bonding patterns and the coordination number can decrease to four. The tetravalent Ti⁴⁺ ionizes one inner-shell water molecule to a hydroxyl group leaving a Ti⁴⁺(H₂O)₅ (OH⁻) core, and an H₃O⁺• • • H₂O moiety dissociates from the second shell of water molecules. These observations highlight the influence of cation size and charge on the local structure of hydrated ions, the high-charge cations causing chemical changes and the low-charge cations being less efficient in maintaining the local order of water molecules. Electronic Supplementary Material: Supplementary material is available for this article at http://dx.doi.org/10.1007/S00214-005-0056-2.     

<Emphasis Type="Italic">In-situ</Emphasis> discrimination of the water cluster size distribution in aqueous solution by ToF-SIMS

The size distribution and molecular structure of water clusters play a critical role in the chemical, biological and atmospheric process. The common experimental study of water clusters in aqueous solution is challenged due to the influence of local Hbonding environments on vibration spectroscopies or vacuum requirements for most mass spectrometry technologies. Here, the time-of-flight secondary ion mass spectrometry (ToF-SIMS) combining with a microfluidic chip has been applied to achieve the in-situ discrimination of the size distribution for water clusters in liquid water at room temperature. The results demonstrated that the presented method is highly system stable, reproducible and accurate. The comparison of heavy water with pure water was made to further demonstrate the accuracy of this technique. These results showed that (H₂O)₃H⁺ and (D₂O)₄D⁺ are the most dominant clusters in pure and heavy water, respectively. This one water molecule difference in the dominant cluster size may due to the nuclear quantum effects on water’s hydrogen bonded network. It is the first time to experimentally show the size distribution of water clusters over a wide range (n=1–30) for pure (H₂O) and heavy (D₂O) water from molecular level. This technique provides an achievable method for liquid water, which offers a bridge to close the gap between theoretical and experimental study of water cluster in aqueous solution.     

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.     

Structural and Infrared Spectroscopic Study on Solvation of Acetylene by Protonated Water Molecules

The effect of solvation on the conformation of acetylene has been studied by adding one water molecule at a time. Quantum chemical calculations of the H⁺(C₂H₂)(H₂O)_n (n=1-5) clusters indicate that the H₂O molecules prefer to form the OH…π interaction rather than the CH…O interaction. This solvation motif is different from that of neutral (C₂H₂)(H₂O)_n (n=1-4) clusters, in which the H₂O molecules prefer to form the CH…O and OH…C H-bonds. For the H⁺(C₂H₂)(H₂O)_n cationic clusters, the first solvation shell consists of one ring structure with two OH…π H-bonds and three water molecules, which is completed at n=4. Simulated infrared spectra reveal that vibrational frequencies of OH…π H-bonded O-H stretching afford a sensitive probe for exploring the solvation of acetylene by protonated water molecules. Infrared spectra of the H⁺(C₂H₂)(H₂O)_n(n=1-5) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.     

A dual functional 1D Cd-based coordination polymer for the highly luminescent sensitive detection of Fe3+ and picric acid

Fluorescent coordination polymers have drawn extensive attention in sensing applications. Herein, we report a carbazole-based one-dimensional coordination polymer [CdL(H₂O)(DMF)₂]·DMF ( CdL , H₂L = 9H-carbazole-2,7-dicarboxylic acid, DMF = N,N-dimethylformamide). In CdL , each Cd²⁺ ion is four-bridged by carboxylates, which is further linked by the carbazole units to form a one-dimensional Cd–O–C chains along the c-axis. CdL displays high water stability in the pH range of 3–10. Luminescence experiments indicate that CdL could selectively detect Fe³⁺ during the concentration range of 0–0.1 mm in water with a K_sv of 8022 m ⁻¹ and picric acid (PA) within the concentration range of 0–0.05 mm in methanol solution with a K_sv of 17948 m ⁻¹ respectively. The above results reveal that CdL can be applied as a multiresponse luminescence sensor for selectively sensing for Fe³⁺ in water and PA in methanol solution.     

Structure and energy of the positively ionized water clusters

Geometry optimization of small (H₂O)_n⁺ clusters (n ≤ 4) at the UHF/4–31 + + G^** level indicates that the cations consist of two fragments: the OH radical and the H_2n−1 O⁺_n−1 ion. The latter can be considered as a thermodynamically stable combination of a distorted H₃O⁺ ion and (n−2) H₂O molecules. The H bond between the fragments becomes weaker with increasing cluster size. Extrapolation of the adiabatic ionization potentials calculated for the (H₂O)_n oligomers (n ≤ 4) at the MP2 level to an infinite cluster size provides the value of approximately 8.7 eV, which can be presumably necessary for the ionization of liquid water in a vacuum. © 1997 John Wiley & Sons, Inc.     

Dynamics and structural changes of small water clusters on ionization

Despite utmost importance in understanding water ionization process, reliable theoretical results of structural changes and molecular dynamics (MD) of water clusters on ionization have hardly been reported yet. Here, we investigate the water cations [(H₂O)_{n = 2–6}⁺] with density functional theory (DFT), Möller–Plesset second‐order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The complete basis set limits of interaction energies at the CCSD(T) level are reported, and the geometrical structures, electronic properties, and infrared spectra are investigated. The characteristics of structures and spectra of the water cluster cations reflect the formation of the hydronium cation moiety (H₃O⁺) and the hydroxyl radical. Although most density functionals fail to predict reasonable energetics of the water cations, some functionals are found to be reliable, in reasonable agreement with high‐level ab initio results. To understand the ionization process of water clusters, DFT‐ and MP2‐based Born‐Oppenheimer MD (BOMD) simulations are performed on ionization. On ionization, the water clusters tend to have an Eigen‐like form with the hydronium cation instead of a Zundel‐like form, based on reliable BOMD simulations. For the vertically ionized water hexamer, the relatively stable (H₂O)₅⁺ (5sL4A) cluster tends to form with a detached water molecule (H₂O). © 2013 Wiley Periodicals, Inc.     

Inception of Acetic Acid/Water Cluster Growth in Molecular Beams

The influence of carboxylic acids on water nucleation in the gas phase has been explored in the supersonic expansion of water vapour mixed with acetic acid (AcA) at various concentrations. The sodium‐doping method has been used to detect clusters produced in supersonic expansions by using UV photoionisation. The mass spectra obtained at lower acid concentrations show well‐detected Na⁺?AcA(H₂O)_n and Na⁺?AcA₂(H₂O)_n clusters up to 200 Da and, in the best cooling expansions, emerging Na⁺?AcA_m(H₂O)_n signals at higher masses and unresolved signals that extend beyond m/e values >1000 Da. These signals, which increase with increasing acid content in water vapour, are an indication that the cluster growth taking place arises from mixed water–acid clusters. Theoretical calculations show that small acid–water clusters are stable and their formation is even thermodynamically favoured with respect to pure water clusters, especially at lower temperatures. These findings suggest that acetic acid may play a significant role as a pre‐nucleation embryo in the formation of aerosols in wet environments.     

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.     

Gas phase reactions of the ion C5H5Fe+: A kinetic study

The kinetics of gas phase reactions of the ion C₅H₅Fe⁺ with oxygen (Me₂CO, Me₂O, MeOH, iso-propanol, H₂O) and nitrogen (NH₃, NH₂Me, NHMe₂, NMe₃) donor ligands have been studied by ion trap mass spectrometry. While in the literature reactions of the ion Fe⁺, with the same ligands, the principal reaction path involves fragmentation in almost all the reactions of the ion C₅H₅Fe⁺, formation of adduct ions is the major reaction path. The reactivity of these two ions is briefly compared in the ion trap conditions. Kinetic data for the ion C₅H₅Fe⁺ indicate that the reactions show a large range of efficiency and a linear correlation is found between the log of the reaction rate constants and the ionization energy of ligands with the same donor atom.     

Theoretical study of the gas-phase Fe<Superscript>+</Superscript>-mediated oxidation of ethane by N<Subscript>2</Subscript>O

We report herein a comprehensive study of the gas-phase Fe⁺-mediated oxidation of ethane by N₂O on both the sextet and quartet potential energy surfaces (PESs) using density functional theory. The geometries and energies of all the relevant stationary points are located. Initial oxygen-atom transfer from N₂O to iron yields FeO⁺. Then, ethane oxidation by the nascent oxide involves C–H activation forming the key intermediate of (C₂H₅)Fe⁺(OH), which can either undergo C–O coupling to Fe⁺ + ethanol or experience β-H shift giving the energetically favorable product of FeC₂H₄ ⁺ + H₂O. Reaction of FeC₂H₄ ⁺ with another N₂O constitutes the third step of the oxidation. N₂O coordinates to FeC₂H₄ ⁺ and gets activated by the metal ion to yield (C₂H₄)Fe⁺O(N₂). After releasing N₂ through the direct H abstraction and/or cyclization pathways, the system would be oxidized to ethenol, acetaldehyde, and oxirane, regenerating Fe⁺. Oxidation to acetaldehyde along the cyclization –C–to–C hydrogen shift pathway is the most energetically favored channel.     

A Monte Carlo simulation of Fe2+ aqueous solvation

A Monte Carlo simulation of Fe²⁺ aqueous solvation, at 298 K, including 100 water molecules, has been done using periodic boundary conditions under the minimum image conversion. The energy has been calculated in the pair-potential approach, employing the MCY potential for the H₂OH₂O interaction and an ab initio analytical potential generated by us for the Fe²⁺H₂O interaction. The examination of interaction energies and of the radial distribution functions clearly show that the first hydration shell is formed by eight water molecules. By classifying the generated configurations into different significant structures of the solvent, it has been found that the eight water molecules of the first hydration shell are situated in a lightly distorted D_4d structure which maximizes the water—solute stabilization and minimizes the water—water repulsion. Finally, the validity of our theoretical predictions is discussed.     

Can Water Clusters Ionize HCl under the Conditions of Arctic Stratosphere? 1. Intermolecular Interactions

Pseudopotential model was constructed to simulate the H₃O⁺(H₂O) _n Cl^– clusters at room and stratosphere temperatures using the Monte Carlo method. Numerical values of interaction parameters were restored from the experimental data on the free energy and entropy of vapor nucleation on ions in the combination with the data of quantum chemical calculations of the optimal configurations of HCl(H₂O) _n clusters. The stability of various cluster structures and the probability of the rupture of intramolecular HCl bond in these clusters were analyzed.