Computer Simulation of Water Clusterization on Chlorine Ions: 1. Thermodynamic Properties

Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor were simulated using the Monte Carlo method. Free energy and the work of clusters formation at room temperature and temperature corresponding to polar stratosphere were calculated. Clusters retain their stability over the entire investigated size range even at multiple vapor supersaturation; however, when supersaturation increases further, the cluster grows in an avalanche-like manner. In clusters with n > 20, the effect of ion field on the free energy of added molecules diminishes dramatically retaining, however, its stabilizing function.     

Computer simulation of molecular complexes H3O+(H2O) n under conditions of thermal fluctuations: II. Work of formation and structure

The free energy, entropy, and work of formation of H₃O⁺(H₂O)_n clusters (n=1–27) in water vapor (300 K) were calculated by the Monte Carlo method. Binary correlation functions were calculated. The calculations are based on the nonpair interaction model presented in the previous publication. The hydration shell of the ion is thermally stable in the size range under study. Nonpair interactions exert an essential effect on the structure of the cluster. Fitting the cluster behavior to its experimental thermodynamic characteristics shows that the excess charge of the ion is spatially delocalized at room temperature, and the role of hydrogen bonds is strengthened on this background. Clusters formed on electric charges have such a fundamental characteristic as transition size. The transition size is independent of vapor pressure and demarcates two qualitatively different mechanisms of holding molecules in a cluster. A change in the holding mode is reflected on the mechanism of vapor nucleation.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1585–1592.Original Russian Text Copyright © 2004 by Shevkunov.For communication I, see [1].This revised version was published online in April 2005 with a corrected cover date.     

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.     

Effect of Ligands on Formation of Photosensitive Oxide Layers on Copper Electrode*

Photosensitive oxide layers are found to develop on copper electrode exposed to solutions containing Cu(II), different ligands, and K₂SO₄ as a supporting electrolyte. Two mechanisms of Cu₂O formation are discussed: corrosion of copper in naturally aerated Cu(II)-free solutions, and interaction between Cu and Cu²⁺ yielding intermediate Cu⁺ ions. Oxide layers formed in the supporting electrolyte at pH 5 and 7 exhibit n-type conduction; the n–p transition is observed at pH 10. An addition of ligands suppresses the oxide formation. The correlation between the photoelectrochemical effects and the stability of Cu(II) complexes is revealed: the higher the complexation degree, the lower the level of photoresponse. A model of nonuniform Cu₂O-containing layer with predominant n- and p-type properties at copper/oxide and oxide/solution interfaces, respectively, is discussed.     

Electronic structure and reactivity of Mg+(H2O)n cluster ions

Electronically excited states of magnesium-water cluster ions, Mg⁺(H₂O) _n ,n=1–5, are studied by photodissociation after mass selection. The observed photodissociation spectra are assigned to the²P–²S type transitions localized on the Mg⁺ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced intracluster reaction to produce MgOH⁺(H₂O) _n is found to occur efficiently, with pronounced size dependence. The intriguing features observed in the mass spectrum of nascent cluster ions are discussed in relation to the stepwise solvation of this reaction.     

Ionization of water in interfacial electric fields: An electrochemical view

High electric fields promote ionization of water, yet relatively little is known about this topic due to the difficulty of generating such fields. The high field capability of field emitter tips enables study of ionization in water layers. Results from this work include ionization fields, water layer morphology, dielectric properties, coadsorbate interactions, cluster distributions of hydrated hydronium ions H⁺(H₂O)_m, and field ionization images. These experimental results, combined with theoretical findings, are interpreted in the context of four examples from electrochemistry; double layer structure, hydrogen oxidation, CO oxidation, and oxygen reduction; to reveal the research frontier in interfacial ionization of water.     

Encasing of Na+ ion in dimer‐formed acetic acid clusters

Peaks with anomalous abundance found in the mass spectra are associated with ions with enhanced stability. Among the scientific community focused on mass spectrometry, these peaks are called ‘magic peaks’ and their stability is often because of suggestive symmetric structures. Here, we report findings on ionised Na‐acetic acid clusters [Na⁺‐(AcA)_n] produced by Na‐doping of (AcA)_n and UV laser ionisation. Peaks labelled n = 2, 4, 8 are clearly distinguishable in the mass spectra from their anomalous intensity. Ab initio calculations helped elucidate cluster structures and energetic. A plausible interpretation of the magic peaks is given in terms of (AcA)_n formed by dimer aggregation. The encasing of Na⁺ by twisted dimers is proposed to be the origin of the enhanced cluster stability. A conceivable dimer‐formed tube‐like closed structure is found for the Na⁺‐(AcA)₈. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydration of gas-phase gramicidin S (M + 2H)2+ ions formed by electrospray: The transition from solution to gas-phase structure

The hydration of doubly protonated gas-phase ions of gramicidin S formed by electrospray ionization was investigated. Under “gentle” electrospray conditions, a near Gaussian distribution of (M + 2H + nH₂O)²⁺ ions with n up to 50 can be readily formed. These extensively hydrated gas-phase ions should have structures similar to those in solution. For intermediate extents of hydration, the “naked” or unsolvated ion is present in unusually high abundance. This is attributed to a competition between solvation of the charges by water vs intramolecular self-solvation via hydrogen bonding. In addition, “magic” numbers of attached water molecules are observed for n = 8, 11, and 14. These magic numbers are attributed to favorable arrangements of water molecules surrounding the charge and surface of the peptide in the gas phase. These results are indicative of a gentle stepwise transformation from the solution-phase structure of the ion to the preferred gas-phase structure as solvent evaporates from the hydrated ions.     

Crystal Structure of di-(L-alanine)Monophosphite Monohydrate [{\rm C}_3{\rm O}_2{\rm NH}_7 ]_2 \cdot {\rm H}_3 {\rm PO}_3 \cdot {\rm H}_2{\rm O}

An X-ray analysis of the crystal structure of di-(L-alanine)monophosphite monohydrate was carried out. The symmetrically nonequivalent L-alanine molecules were found to be present in the structure in two different forms coupled by a strong hydrogen bond: monoprotonated positively charged [CH₃CH(NH₃)COOH] molecule and CH₃CH(NH₃)COO zwitterion. Two layers are distinguished in the structure: one is a positively charged layer formed by L-alanine molecules and the other is a negatively charged layer composed of phosphite ions and water molecules. These layers, alternating along the a axis, are connected to each other by a network of hydrogen bonds.     

Organic cluster ions from continuous-flow fast atom bombardment

Cluster ions from fast atom bombardment of liquid alcohols and nitriles were examined using a continuous-flow technique. Protonated molecular M_nH⁺ species are the dominant cluster ions observed in molecules of formula M. The abundances of the M_nH⁺ cluster ions decrease monotonically with increasing n, and within a homologous series the M_nH⁺ abundance diminishes more rapidly for higher molecular mass compounds. Reaction products (ROH)_n(H₂O)H⁺ and (ROH)_n(ROR)H⁺ are observed also in the case of alcohols, and the ion abundances decrease with increasing n. Radiation damage yields fragment ions and ionic alkyl reaction products which are captured in solvent clusters. Semi-empirical molecular orbital methods were used to examine the energetics of cluster ion formation and decomposition pathways. Metastable decomposition processes exhibit only evaporative loss of monomers, with the probability of loss increasing sharply with n. The evaporative ensemble model of Klots was used to predict the cluster size-dependent trends of metastable dissociation processes observed for alcohol and nitrile cluster ions.     

A molecular dynamics study on Rh3+ hydration: development and application of a first principles hydrated ion–water interaction potential

The Rh³⁺ aquaion exhibits one of the largest residence times of water molecules in the first hydration shell. The extreme stability of this hexahydrated ion in water solutions makes Rh³⁺ an extremely suitable candidate to be studied using the hydrated ion model. According to this approach, the representative cationic entity in aqueous solution is the ion plus its first hydration shell (i.e. the hydrated ion) and not the bare ion. Our group has successfully applied that concept in the framework of classical statistical simulations based on first principles ion–water interaction potentials. The methodology is now applied to the [Rh(H₂O)₆]³⁺ case based on a previous generalization in which some of the contributions were found to be transferable among the cases already studied (Cr³⁺, Al³⁺, Mg²⁺, Be²⁺). In this contribution a flexible hydrate model is presented, in which rigid first-shell water molecules have rotational and translational degrees of freedom, allowing for internal dynamics of the hydrated ion entity. The potential presented is thoroughly tested by means of a set of molecular dynamics simulations. Structural, dynamical, energetic and spectroscopic information is retrieved from the simulations, allowing the estimation of properties such as ion hydration energy, vibrational spectra of the intermolecular modes, cation mobility, rotational dynamics of the hydrated ion and first-shell water molecules and residence times of the second-shell water molecules. Extension of the Ewald sum to terms r^–4, r^–6 and r^–7 is presented and applied to systems of different size ([Rh(H₂O)₆]³⁺+(n–6)H₂O, n=50, 100, 200, 500, 1000 and 2500) and cutoff radii.Contribution to the Jacopo Tomasi Honorary Issue     

Theoretical analysis of the gas-phase hydration of common atmospheric pre-nucleation and (H3O)(H2SO4)(H2O)n cluster ions

Small atmospheric ions are always hydrated. The hydration affects their mobility, stability, and lifetime, which are critically important for the determination of nucleation rates. The gas-phase hydration of anions and cations of the atmospheric nucleation precursor H₂SO₄ has been studied using the Density Functional Theory (DFT). We found that the interaction between the common atmospheric ions and polar water molecules lead to the formation of the strongly hydrogen bonded hydrate complexes, whose thermodynamic stability is much higher than that of neutral atmospheric hydrates (H₂SO₄)(H₂O)_n. Both negatively and positively charged hydrates are much more stable than the aforementioned neutral form; however, the hydration of cations is much stronger than that of anions. The difference in hydration free energies is very large (10 kcal mol⁻¹); however, it decreases quickly when the cluster is growing. The observed positive hydration sign preference is consistent with previous studies for pure water.     

Effect of intercalation of metal ions on the colloidal and solid properties of vanadium pentaoxide hydrate,V2O5 nH2O

The colloidal stability of V₂O₅ nH₂O was studied on the basis of the measurements of critical flocculation concentration (CFC) by metal ions, amount of ions exchanged (or intercalated), and -potential. In total, the CFC values obeyed the Schulze Hardy law and strong Hofmeister's series was found in the systems including alkaline ions. The sequence of colloidal stability of V₂O₅ nH₂O in the electrolyte solutions was related to the intercalation of metal ions in the interlayer spaces of the solid. The largest CFC value for Li⁺ (87 mmol dm^–3) was explained by smaller affinity of Li⁺ to be intercalated in V₂O₅ nH₂O as well as smaller Hamaker constant of the intercalated solid compared to the other systems.Effect of intercalation of metal ions on the crystalline properties of the materials was measured by use of XRD and electron microscope. Under highly dehydrated condition the ions whose radii are smaller than 0.1 nm are captured in the structure of V₂O₅ nH₂O without changing interlayer distances, while those larger than 0.1 nm increase the interlayer distance. In a saturated H₂O vapor interlayer distances increased with increasing charge of intercalated ions. However, when intercalated with ions carrying the same valency the interlayer distances of the sample decreased with decrease in the hydration property of ions. Hydrolyzable Cr³⁺ gave exceptionally larger interlayer distances, both in a vacuum and in H₂O vapor.     

Does in-source decay occur independent of the ionization process in matrix-assisted laser desorption?

The influence of the acidic and basic characters of constituent amino acid residues on the peptide fragment ions produced by in-source decay under matrix assisted laser desorption/ionization (MALDI) conditions has been studied using positive- and negative-ion experiments. Whereas the in-source decay spectra of peptides containing basic Arg and/or Lys residues near the N-terminus showed so-called c_n- and a_n-series ions in positive-ion mode, a peptide that has an acidic amino acid cluster near the N-terminus and a basic residue near the C-terminus characteristically formed y_n- and z_n-series ions in the positive-ion in-source decay spectrum. These results indicated that fragment ion series produced by in-source decay depend strongly upon the acidic and basic characters of the constituent amino acid residues and the near N- and C-termini. It was suggested that in-source decay processes occur intrinsically at NH–C^α and CO–NH bonds independent of the formation of molecular-related ions, and that the cleavages at the NH–C^α and CO–NH bonds occurred independently and were dependent on the matrix used.     

Electrical field effects on dipole moment, structure and energetic of (H2O)n (2 n 15) cluster

The effect of an external electric field on water clusters of the (H₂O)_n type, with [1 n 15], in the ground state was analyzed at the B3LYP/cc-pVTZ level of theory. The calculations showed that an external electric field changes the number of hydrogen bonds, reduces the cluster sizes and increases the strength of the inter-cluster hydrogen bonds. The particular symmetry of the cluster and the null dipole moment in these specific configurations suggest that their stability can be associated with a perfect alignment of the water molecules, maximizing attractive electrostatic interactions caused by changes in the charge distribution of the clusters.     

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^.?.     

catena‐Poly[tetra­kis(3‐phenyl­propyl­ammonium) [iodo­plumbate(II)‐tri‐μ‐iodo‐plumbate(II)‐tri‐μ‐iodo‐iodo­plumbate(II)‐di‐μ‐iodo]]

The title compound, {(C₉H₁₄N)₄[Pb₃I₁₀]}_n, crystallizes as an organic–inorganic hybrid. As such, the structure consists of a two‐dimensional inorganic layer of [Pb₃I₁₀]_n⁴ⁿ⁻ ions extending along [100]. The asymmetric unit contains two independent Pb atoms, viz. one in a general position and the other on an inversion centre. Each Pb atom is octa­hedrally coordinated by six iodide ions and exhibits both face‐ and corner‐sharing with adjacent atoms in the inorganic layer. These anionic layers alternate with 3‐phenyl­propyl­ammonium cations, which hydrogen bond to the iodides. Simple face‐to‐edge σ–π stacking inter­actions are observed between the aromatic rings that stabilize the overall three‐dimensional structure. This net structure has only been observed five times previously.     

Size-dependent reactivity of cobalt cluster ions with nitrogen monoxide: Competition between chemisorption and decomposition of NO

Reactions of NO molecules on cobalt cluster ions were studied in a beam-gas geometry by using a tandem mass spectrometer. Single-particle collision reactions of Co_mNO⁺ (m = 3–10) with NO were found to proceed in such a manner that NO decomposition dominates at m = 4–6 with the maximum reaction cross section at m = 5 and chemisorption dominates in m ≥ 7. On the other hand, in two-particle collision reactions of Co_n⁺ (n = 2–10) with NO, NO decomposition at n ≥ 5 and chemisorption of two NO molecules with Co atoms loss at n ≥ 8 were found to proceed. These results indicate that the size-dependency of the multiple collision reactions originates from secondary attacking of an NO molecule to primary products of the initial single collision reactions. The DFT calculation supports the scheme that both the decomposition and chemisorption of two-particle collision reactions proceed via a common intermediate, Co_mN₂O₂⁺, in which the two NO molecules are dissociatively chemisorbed on the cobalt cluster ion, and the size-dependency of the two-particle collision reactions is explained in terms of the structure of this reaction intermediate.     

Stability of molecular form of HCl in water vapor: A computer simulation

The free energy and entropy of the dissociation of HCl molecule into ions in water vapor, HCl(H₂O) _n + mH₂O → H₃O + (H₂O) _n+m -1Cl^?, were calculated. The dependences of various parameters on the interionic distance at 273 K and various vapor pressures were obtained. A detailed model taking into account unpaired covalent-type interactions, polarization interactions, charge transfer effect, and hydrogen bonds was applied. The numerical values of the parameters were reconstructed from the experimental data on the free energy and enthalpy of the first reactions of addition of vapor molecules to ions, and also from the results of quantum-chemical calculations of the energy and geometry of locally stable configurations of clusters HCl(H₂O) _n . Despite lower internal energy of the dissociated state, the molecular form is absolutely stable in clusters of water molecules. The dissociated state is relatively stable. Accumulation of unrecombined ion pairs in clusters is possible with a decrease in the temperature to 200 K.     

Interlayer reactions of the silver molybdate Ag6Mo10O33

Silver molybdate Ag₆Mo₁₀O₃₃ exchanges silver ions for organic cations, particularly surface-active agents such as long-chain n-alkylammonium ions C_nH_2n+1NH⁺₃. The alkylammonium ions penetrate between the layers and aggregate as bimolecular structures. The alkyl chains in the interlayer are not in all-trans conformation but are isomerized into conformers with gauche-bonds. These chains aggregate as gauche-blocks because the polar chain ends (NH⁺₃ and NH₂ groups) interacting with the molybdate layer cannot be close-packed. The specially favored formation and pronounced stability of gauche-blocks impede the quantitative exchange of the silver ions. No more than 20% of the silver ions are exchanged by alkylammonium nitrate. The gauche-blocks are stabilized by additional uptake of alkylamine molecules. Silver molybdate also reacts with alkylamine and forms long-spacing complexes with long segments of the alkyl chains perpendicular to the layers.