Charge‐transfer‐to‐solvent absorption spectra of I−(H2O)3–5 at a finite temperature via simulation

Ground‐state equilibrium Born–Oppenheimer molecular dynamics on I^?(H₂O)_3–5 clusters at ～200 K are performed to sample configurations for calculating the charge‐transfer‐to‐solvent (CTTS) absorption spectra for these clusters. When there are more water molecules in clusters, the calculated CTTS spectra are found to become more intense with the absorption maxima shifting to higher energies, which is in agreement with experimental results. In addition, compared with the findings for optimized structures, the absorption energies of the iodide 5p orbitals are red‐shifted at ～200 K because, on average, the distances between the iodide and the dangling hydrogen atoms are increased at finite temperatures which weakens the interactions between the iodide and water molecules in the clusters. Moreover, the number of ionic hydrogen bonds in the clusters are also reduced. However, it is found that all dangling hydrogen atoms must be considered to obtain a good correlation between the CTTS excitation energy and the average distance between the iodide and the dangling hydrogen atoms, which indicates the existence of the strong interactions of the CTTS electron with all of the dangling hydrogen atoms.     

Infrared and NMR Spectroscopic Fingerprints of the Asymmetric H7+O3 Complex in Solution

Infrared (IR) absorption in the 1000–3700 cm⁻¹ range and ¹H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H₇⁺O_3, in acetonitrile. The core H₇⁺O₃ motif persists in larger protonated water clusters in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible proton transport promoted by propagating the asymmetric H₇⁺O₃ structure in solution. The QM/MM calculations allow for the successful simulation of the measured IR absorption spectra of H₇⁺O₃ in the OH stretch region, which reaffirms the assignment of the H₇⁺O₃ spectra to a hybrid-complex structure: a protonated water dimer strongly hydrogen-bonded to a third water molecule with the proton exchanging between the two possible shared-proton Zundel-like centers. The H₇⁺O₃ structure lends itself to promoting irreversible proton transport in presence of even one additional water molecule. We demonstrate how continuously evolving H₇⁺O₃ structures may support proton transport within larger water solvates.     

Heterogeneous Electrofreezing of Super-Cooled Water on Surfaces of Pyroelectric Crystals is Triggered by Trigonal Planar Ions

Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO₃ and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO₃⁻) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H₃O⁺) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO₃⁻ ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO₃⁻ and guanidinium (Gdm⁺), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl⁻ and SO₄²⁻ ions of different configurations reduce the icing temperature.     

Heterogeneous Electrofreezing of Super‐Cooled Water on Surfaces of Pyroelectric Crystals is Triggered by Trigonal Planar Ions

Electrofreezing experiments of super‐cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO₃ and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO₃^?) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H₃O⁺) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO₃^? ions self‐assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice‐like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO₃^? and guanidinium (Gdm⁺), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl^? and SO₄^2? ions of different configurations reduce the icing temperature.     

Ab initio investigation of possible lower-energy candidate structure for cationic water cluster (H2O) 12+ via particle swarm optimization method

Detecting the underlying performance of hydrated electrons and hydroxyl radicals in the cationic water cluster can greatly help to understand the inter reaction mechanism in the liquid water and aqueous solutions. Based on our previous (H₂O)₁₀⁺ research, we have paid attention to more problems of larger cationic clusters in this work, including the existence of hemibonded type, long-range correction functions, and hydrogen-bonded site analyses. The lower-energy structures of the cationic water cluster (H₂O)₁₂⁺ have been comprehensively explored, and more experienced functions are introduced to check the ground state and vibration spectrum. Unlike the configuration regularity of neutral (H₂O)₁₂ clusters and small cationic water clusters, those new-found structures for (H₂O)₁₂⁺ are inclined to adopt three dimension (3D) cage-like structures and the H₂O-OH₂ structure appears in the higher energy isomer. The calculation reveals that the lowest stable isomer is the 3D cage structure W14 predicted at MP2 level, which has not been reported yet. In the thermal simulation, structure transition from the cage-like to the ring-like occurs at T?>?≈256 K, and the two dimension (2D) ring-like structure occupies a dominant position at high temperature range. The infrared spectra explain that the difference of the spectra between the 2D net structures and 3D cage-structures is mainly caused by the weight fluctuation of single acceptor-single donor (AD), double acceptor-single donor (AAD), and single acceptor-double donor (ADD) sites in these isomers. This further gives a similarity relation between (H₂O)₁₂⁺ and H⁺(H₂O)₁₂ clusters in the shape of the network and spectral characteristics. By molecular orbitals and topological analysis, we find that the lone pair orbital on hydroxyl radical dominates the reactivity and stability of cationic system. The present research may be helpful for exploring the evolution law of the larger cationic water clusters in the future.

     

Collision-induced dissociation studies of protonated alcohol and alcohol—water clusters by atmospheric pressure ionization tandem mass spectrometry. 1?Methanol

Cluster size distribution and collision-induced dissociation (CID) studies of protonated methanol and protonated methanol—water clusters yield information on the structure and energetics of such ions. Ions were formed at atmospheric pressure in a corona discharge source, and were subjected to CID in the center quadrupole of a triple quadrupole mass spectrometer. Cluster ions containing up to 13 molecules of methanol and/or water were observed and examined using CID experiments. The CID of all (CH₃OH)_n · H₂O · H⁺ clusters, where n ? 8, showed that water loss was statistically favored over methanol loss and that the preferred dissociation channel involved loss of water with methanol molecules. These results support a model employing a chain of hydrogen-bonded solvent molecules rather than one in which fused rings of ligands surround a central hydronium ion. However, CID of larger clusters, where n ? 9, showed that loss of one methanol was equal to or less than loss of water, reflecting a change in structure.     

Chemi-Inspired Silicon Allotropes—Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets,Single-Walled Nanotubes,and Nanoparticles

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si₉]⁴⁻ clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si–Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si₉ clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si₉]^4– based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.     

Cover Feature: Formation of Eigen or Zundel Features at Protonated Water Cluster–Aromatic Interfaces (ChemPhysChem 16/2023)

Interfacial interactions of protonated water clusters adsorbed at aromatic surfaces play an important role in biology, and in atmospheric, chemical and materials sciences. Here, we investigate the interaction of protonated water clusters ((H⁺H₂O)_n (where n=1–3)) with benzene (Bz), coronene (Cor) and dodecabenzocoronene (Dbc)). To study the structure, stability and spectral features of these complexes, computations are done using DFT-PBE0(+D3) and SAPT0 methods. These interactions are probed by AIM electron density topography and non-covalent interactions index (NCI) analyses. We suggest that the excess proton plays a crucial role in the stability of these model interfaces through strong inductive effects and the formation of Eigen or Zundel features. Also, computations reveal that the extension of the π-aromatic system and the increase of the number of water molecules in the H-bounded water network led to a strengthening of the interactions between the corresponding aromatic compound and protonated water molecules, except when a Zundel ion is formed. The present findings may serve to understand in-depth the role of proton localized at aqueous medium interacting with large aromatic surfaces such as graphene interacting with acidic liquid water. Besides, we give the IR and UV-Vis spectra of these complexes, which may help for their identification in laboratory.     

FTIR study of water clusters in water–triethylamine solutions

The FTIR study presented in this work, on water dissolved in triethylamine (TEA), reveals the formation of water clusters in the TEA liquid phase at tenths of water molar concentrations. In the OH stretching region, the FTIR spectra of water in TEA show, at high frequencies, a narrow band at 3682 cm⁻¹ and, at low frequencies, a wide band that can be resolved into four peaks with maxima at 3249 cm⁻¹, 3348 cm⁻¹, 3440 cm⁻¹ and 3545 cm⁻¹. The results have been rationalised assuming the formation of clusters containing tens of three- and four-coordinated water molecules. TEA molecules surrounding the clusters are hydrogen bonded to one OH of the water molecules at the surface, leaving dangling protons. Further, the analyses of the spectra suggest that, in the used range, the water cluster mean size does not depend strongly on the water concentration.     

Observable Structures of Small Neutral and Anionic Gold Clusters

Since gold clusters have mostly been studied theoretically by using DFT calculations, more accurate studies are of importance. Thus, small neutral and anionic gold clusters (Au_n and Au_n^?, n=4–7) were investigated by means of coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] calculations with large basis sets, and some differences between DFT and CCSD(T) results are discussed. Interesting isomeric structures that have dangling atoms were obtained. Structures having dangling atoms appear to be stable up to n=4 for neutral gold clusters and up to n=7 for anionic clusters. The relative stabilities and electronic properties of some isomers and major structures are discussed on the basis of the CCSD(T) calculations. This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom‐scale materials science including nanocatalysts.     

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 mechanism of the ambient pressure pyroelectric ion source (APPIS) and its applications to chemical nerve agent detection

We present studies of the ionization mechanism operative in the ambient pressure pyroelectric ionization source (APPIS), along with applications that include detection of simulants for chemical nerve agents. It is found that ionization by APPIS occurs in the gas-phase. As the crystal is thermally cycled over a narrow temperature range, electrical discharges near the surface of the crystal produce energetic species which, through reactions with atmospheric molecules, result in reactant ions such as protonated water clusters or clusters of hydroxide and water. Reactant ions can be observed directly in the mass spectrometer. These go on to react with trace neutrals via proton transfer reactions to produce the ions observed in mass spectra, which are usually singly protonated or deprotonated species. Further implicating gas-phase ionization, observed product distributions are highly dependent on the composition of ambient gases, especially the concentration of water vapor and oxygen surrounding the source. For example, basic species such as triethylamine are observed as singly protonated cations at a water partial pressure of 10 torr. At a water pressure of 4 torr, reactive oxygen species are formed and lead to observation of protonated amine oxides. The ability of the APPIS source to detect basic molecules with high proton affinities makes it highly suited for the detection of chemical nerve agents. We demonstrate this application using simulants corresponding to VX and GA (Tabun). With the present source configuration pyridine is detected readily at a concentration of 4 ppm, indicating ultimate sensitivity in the high ppb range.     

Express analysis of the structural properties of water clusters containing more than 12 water molecules and impurities

An express analysis technique for the determination of the structural properties of water clusters with impurities and the number of contained water molecules more than 12 is developed. The technique is based on the derivation of three matrices for a water cluster, which makes it possible to write down the unique code characterizing its structure. The technique significantly decreases the computation time (approximately 10⁴ times) in comparison with the procedure developed previously [1], and consequently, allows the analysis of the structural properties of aqueous systems with a large number of molecules on usual PCs. In order to illustrate the technique the code for the typical configuration of the (H₂O)₁₃ cluster is determined within the water model at a temperature of 268 K and a density of 0.99930 g/cm³.     

Long range influence of an excess proton on the architecture of the hydrogen bond network in large-sized water clusters

Infrared spectra of completely size-selected protonated water clusters H+(H2O)n are reported for clusters ranging from n=15 to 100. The behavior of the dangling OH stretch bands shows that the hydrogen bond structure in H+(H2O)n is uniquely different to that of (H2O)n up to the size of n=100, at least. This finding indicates that the presence of an excess proton creates a characteristic morphology in the hydrogen bond network architecture of more than 100 surrounding water molecules.     

Ordering and Clathrate Hydrate Formation in Co‐deposits of Xenon and Water at Low Temperatures

Local ordering in co‐deposits of water and xenon atoms produced at low temperatures can be followed uniquely by ¹²⁹Xe NMR spectroscopy. In water‐rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ～140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder Xray diffraction. There is evidence that Xe ? 20 H₂O clusters are prominent in the early stages of crystallization. In xenon‐rich deposits at 77 K there is evidence of xenon atoms trapped in Xe ? 20 H₂O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.     

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.     

The investigation on the relative stability of different clusters of thiourea dioxide in water using gas phase quantum chemical calculations

The relative stability of different clusters of thiourea dioxide (TDO) in water is examined using gas phase quantum chemical calculations at the MP2 and B3LYP level with 6‐311++G(d,p) basis set. The possible equilibrium structures and other energetic and geometrical data of the thiourea dioxide clusters, TDO‐(H₂O)_n (n is the number of water molecules), are obtained. The calculation results show that a strong interaction exists between thiourea dioxide and water molecules, as indicated by the binding energies of the TDO clusters progressively increased by adding water molecules. PCM model is used to investigate solvent effect of TDO. We obtained a negative hydration energy of ?20.6 kcal mol^?1 and free‐energy change of ?21.0 kcal mol^?1 in hydration process. On the basis of increasing binding energies with adding water molecules and a negative hydration energy by PCM calculation, we conclude thiourea dioxide can dissolve in water molecules. Furthermore, the increases of the C? S bond distance by the addition of water molecules show that the strength of the C? S bonds is attenuated. We find that when the number of water molecules was up to 5, the C? S bonds of the clusters, TDO‐(H₂O)₅ and TDO‐(H₂O)₆ were ruptured. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Effect of nitrogen adsorption on the mid-infrared spectrum of water clusters

Experimental Fourier-transform infrared spectra and DFT calculated infrared spectra are compared to investigate the effect of adsorbed nitrogen on the OH-stretch band complex of water clusters. Using a collisional cooling experiment, pure as well as partially and completely N(2)-covered water clusters consisting of 20-200 water molecules have been generated in thermal equilibrium in the aerosol phase within the temperature range of 5-80 K. Computational IR-spectra simulations have been performed for discrete pure and N(2)-covered water clusters including 10, 15, 20, and 30 water molecules. The adsorbed N(2) molecules especially affect the three-coordinated water molecules at the cluster surface which could be observed as a blue shift of the companion O-H band at 2900 cm(-1) and a red shift of the dangling O-H band at 3700 cm(-1) by about 20 cm(-1) in both cases. The most striking effect of the N(2) adsorbate is an intensity increase of the dangling O-H band by a factor of 3-5. Furthermore, the onset temperature of nitrogen adsorption at the water cluster surface was experimentally found to be roughly 30 K for cluster sizes of about 100 water molecules. Experimental and computational results are in good agreement. The presented results are based on and support the work of V. Buch, J. P. Devlin, and co-workers (e.g., J. Phys. Chem. B, 1997; J. Phys. Chem. A, 2003; Int. Rev. Phys. Chem., 2004).     

Structure of protonated water clusters: low-energy structures and finite temperature behavior

The structure of protonated water clusters H+(H2O)n (n=5-22) are examined by two Monte Carlo methods in conjunction with the OSS2 potential [L. Ojamae, I. Shavitt, and S. J. Singer J. Chem. Phys. 109, 5547 (1998)]. The basin-hopping method is employed to explore the OSS2 potential energy surface and to locate low-energy structures. The topology of the "global minimum," the most stable low-energy structure, changes from single ring to multiple ring to polyhedral cage as the cluster size grows. The temperature dependence of the cluster geometry is examined by carrying out parallel tempering Monte Carlo simulations. Over the temperature range we studied (25-330 K), all water clusters undergo significant structural changes. The trends are treelike structures dominating at high temperature and single-ring structures appearing in slightly lower temperatures. For n> or =7, an additional transition from single ring to multiple rings appears as the temperature decreases. Only for n> or =16 do polyhedral structures dominate the lowest temperature range. Our results indicate very dynamic structural changes at temperature range relevant to atmospheric chemistry and current experiments. The structures and properties of medium-sized protonated clusters in this temperature range are far from their global minimum cousins. The relevance of these findings to recent experiments and theoretical simulations is also discussed.     

Models for the description of the H3O+ and OH− ions in water

Structures of the H₃O⁺-OH ion pair surrounded by up to three water molecules have been studied. Since the ion-pair structure is always above the corresponding neutral water structure, a constrained geometry optimization is needed. The energy difference between the ion-pair structure and the neutral water structure is studied as a function of the number of surrounding water molecules. The effect of the surrounding water solvent is also studied by placing the model system in a spherical cavity in a dielectric medium. The main results are that the energy difference stabilizes at 10–20 kcal/mol for the larger clusters and that an effect indicating a mechanism for charge separation can be noticed on the geometries of these clusters. Results obtained using gradient-corrected density-functional theory are compared to a configuration interaction treatment using a scaling procedure of the correlation energy. © 1996 by John Wiley & Sons, Inc.