Water Cluster Anions Research Articles

Reaction dynamics of temperature-variable anion water clusters studied with crossed beams and by direct dynamics

We present a study of the different product channels in the reactions of OH and OH-(H2O) with methyl iodide over a range of collision energies. Direct dynamics classical trajectory simulations are employed to obtain an atomistic comparison with the experimental results. For the experiments we have combined a crossed beam ion imaging setup with a multipole rf ion trap. The trap allows us to prepare the molecular and cluster ions with a controlled internal temperature and thus provides well-defined initial conditions for reaction experiments at low collision energy. Changing the internal temperature of the cluster ions was found to have a profound effect on their reactivity.

“Liquid-like” type (COO−)2(H2O)10 anion water clusters in three-dimensional supramolecular structure of cucurbit[6]uril

The three-dimensional (3D) supramolecular network structure of the guest 1,1′-(hexane-1,6-diyl)dipyridinium-4-carboxylate inner salt (HBPC) and the host cucurbit[6]uril (CB[6]) is self-assembled through an intricate array of hydrogen-bonding interactions involving the contributions of two-dimensional (2D) infinite network (COO−)2(H2O)10 anion water clusters and (H2O)2 water clusters. These water clusters are notably have the liquid-like water characteristics in the solid and are reversibly dehydrated and rehydrated, while the pseudorotaxane retains its integrity in the process, and is stable up to 390 °C.

Self-assembly of ordered water tetramers in an encapsulated [Br(H2O)12]− complex

An unanticipated anion-water cluster is assembled by one bromide and three highly-ordered "water tetramers" within the cavity of a receptor, providing a perfect C(3) symmetric propeller-shaped bromide-water cluster of [Br(H(2)O)(12)](-).

Theoretical Characterization of Four Distinct Isomer Types in Hydrated-Electron Clusters, and Proposed Assignments for Photoelectron Spectra of Water Cluster Anions

Water cluster anions, (H(2)O)(N)(-), are examined using mixed quantum/classical molecular dynamics based on a one-electron pseudopotential model that incorporates many-body polarization and predicts vertical electron detachment energies (VDEs) with an accuracy of ~0.1 eV. By varying the initial conditions under which the clusters are formed, we are able to identify four distinct isomer types that exhibit different size-dependent VDEs. On the basis of a strong correlation between the electron's radius of gyration and its optical absorption maximum, and extrapolating to the bulk limit (N → ∞), our analysis supports the assignment of the "isomer Ib" data series, observed in photoelectron spectra of very cold clusters, as arising from cavity-bound (H(2)O)(N)(-) cluster isomers. The "isomer I" data reported in warmer experiments are assigned to surface-bound isomers in smaller clusters, transitioning to partially embedded isomers in larger clusters. The partially embedded isomers are characterized by a partially formed solvent cavity at the cluster surface, and they are spectroscopically quite similar to internalized cavity isomers. These assignments are consistent with various experimental data, and our theoretical characterization of these isomers sheds new light on a long-standing assignment problem.

Can a Dipole-Bound Electron Form a Pseudo-Atom? An Atoms-In-Molecules Study of the Hydrated Electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, Møller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.

Quantum-classical simulation of electron localization in negatively charged methanol clusters

A series of quantum molecular dynamics simulations have been performed to investigate the energetic, structural, dynamic, and spectroscopic properties of methanol cluster anions, [(CH(3)OH)(n)](-), (n = 50-500). Consistent with the inference from photo-electron imaging experiments, we find two main localization modes of the excess electron in equilibrated methanol clusters at ∼200 K. The two different localization patterns have strikingly different physical properties, consistent with experimental observations, and are manifest in comparable cluster sizes to those observed. Smaller clusters (n ≤ 128) tend to localize the electron in very weakly bound, diffuse electronic states on the surface of the cluster, while in larger ones the electron is stabilized in solvent cavities, in compact interior-bound states. The interior states exhibit properties that largely resemble and smoothly extrapolate to those simulated for a solvated electron in bulk methanol. The surface electronic states of methanol cluster anions are significantly more weakly bound than the surface states of the anionic water clusters. The key source of the difference is the lack of stabilizing free hydroxyl groups on a relaxed methanol cluster surface. We also provide a mechanistic picture that illustrates the essential role of the interactions of the excess electron with the hydroxyl groups in the dynamic process of the transition of the electron from surface-bound states to interior-bound states.

The identification of a solvated electron pair in the gaseous clusters of Na−(H2O)n and Li−(H2O)n

By first principles calculations, we explore the possibility that Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n) clusters, which have been measured previously by photoelectron experiments, could serve as gas-phase molecular models for the solvation of two electrons. Such models would capture the electron-electron interaction in a solution environment, which is missed in the well-known anionic water clusters (H(2)O)(n) (-). Our results show that by n = 10, the two loosely bound s electrons in Li(-)(H(2)O)(n) are indeed detached from lithium, and they could exist in either the singlet (spin-paring) or the triplet (spin-coupling) state. In contrast, the two electrons would prefer to stay on the sodium atom in Na(-)(H(2)O)(n) and on the surface of the cluster. The formation of a solvated electron pair and the variation in solvation structures make these two cluster series interesting subjects for further experimental investigation.

Nature's most squishy ion: The important role of solvent polarization in the description of the hydrated electron

The aqueous electron, e −(aq), and its finite analogues, the anionic water clusters , have attracted significant attention from both theory and experiment over the past few decades. Nevertheless, some of the most basic structural aspects of these systems, as well as the interpretation of certain spectroscopic features, remain controversial or else have defied theoretical explanation altogether. Due to the solvent-supported nature of the ion, a large number of water molecules is required in order to obtain a realistic model of e −(aq), and a wide variety of structural morphologies are available in clusters. These aspects severely limit the role of ab initio quantum chemistry in elucidating the properties of solvated-electron systems, but at the same time, the fundamentally quantum-mechanical nature of the ion must be taken into account. Most theoretical studies have therefore relied upon one-electron pseudopotential models and mixed quantum/classical molecular dynamics. In view of the highly diffuse, polarizable nature of the ion, however, it is surprising how little attention has been paid to the development of polarizable one-electron models. This article presents an overview of our efforts to develop such a model, as well as computational evidence to suggest that self-consistent, many-body electron–water polarization is qualitatively important in the description of both clusters and e −(aq) in bulk water.

Inclusion of unique four-clawed crown-like nitrate–water cluster [(NO3)6(H2O)6]6− anions into the inter-spaces of a 3D H–bonded cationic net formed by a cationic calix[4]arene

Reaction of 5,11,17,23-tetramino-25,26,27,28-tetrahydroxy-calix[4]arene (1) with MeI and NaOAc·3H2O in DMF produced a tetracationic calix[4]arene [H4L]I4 (2) {H4L = [5,11,17,23-tetrakis(trimethylammonium)-25,26,27,28-tetrahydroxycalix[4]arene]} in 90% yield. Anion exchange of 2 with [Ag(MeCN)4](PF6) in MeCN afforded [H4L](PF6)4 (3). Treatment of 3 with Na2CO3 or NaNO3 in MeCN afforded two mono-deprotonated complexes [H3L](PF6)3·MeCN (4·MeCN) and [(H3L)2{(NO3)6(H2O)6}]·4MeCN (5·4MeCN). Compounds 4·MeCN and 5·4MeCN have been characterized by elemental analysis, IR spectra, 1H NMR, and single-crystal X-ray crystallography. 4·MeCN contains a 3D cationic H–bonded [H3L]n3n+ net in which PF6− anions are embedded into its 1D channels. 5·4MeCN consists of another 3D cationic H–bonded [H3L]n3n+ net in which the unique four-clawed crown-like nitrate–water cluster [(NO3)6(H2O)6]6− anions are included into the inter-spaces between the layers of [H3L]3+ trications. The formation of this nitrate–water cluster anion may demonstrate interesting information about the hydration of nitrate in water-related systems.

H 2 production from reactions between water and small molybdenum suboxide cluster anions

Reactions between molybdenum suboxide cluster anions, Mo(x)O(y)(-) (x=1-4; y < or = 3x), and water (H(2)O and D(2)O) have been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. Product distributions vary with the number of metal atoms in the cluster. Within the MoO(y)(-) oxide series, product masses correspond to the addition of one water molecule, as well as a H/D exchange with MoO(4)H(-). Within the Mo(2)O(y)(-) oxide series, product evolution and distribution suggest sequential oxidation via Mo(2)O(y)(-)+H(2)O/D(2)O-->Mo(2)O(y+1)(-)+H(2)/D(2) reactions for y<5, while for Mo(2)O(5)(-), Mo(2)O(6)H(2)/D(2)(-) is produced. Mo(2)O(6)(-) does not appear to be reactive toward water. For the Mo(3)O(y)(-) oxide series, sequential oxidation similarly is suggested for y<5, while Mo(3)O(5)(-) reactions result in Mo(3)O(6)H(2)/D(2)(-) formation. Mo(3)O(6)(-) appears uniquely unreactive. Mo(3)O(7)(-) and Mo(3)O(8)(-) react to form Mo(3)O(8)H(2)/D(2)(-) and Mo(3)O(9)H(2)/D(2)(-), respectively. Lower mass resolution in the Mo(4)O(y)(-) mass range prevents unambiguous mass analysis, but intensity changes in the mass spectra do suggest that sequential oxidation with H(2)/D(2) evolution occurs for y<6, while Mo(4)O(y+1)H(2)/D(2)(-) addition products are formed in Mo(4)O(6)(-) and Mo(4)O(7)(-) reactions with water. The relative rate constants for sequential oxidation and H(2)O/D(2)O addition for the x=2 series were determined. There is no evidence of a kinetic isotope effect when comparing reaction rates of H(2)O with D(2)O, suggesting that the H(2) and D(2) losses from the lower-oxide/hydroxide intermediates are very fast relative to initial reaction complex formation with H(2)O or D(2)O. The rate constants determined here are two times higher than those determined in identical reactions between W(2)O(y)(-)+H(2)O/D(2)O.

Response of Observables for Cold Anionic Water Clusters to Cluster Thermal History

We have used mixed quantum classical molecular dynamics simulations to explore the role of structural relaxation when binding an excess electron to neutral water clusters. The structural and spectral properties of the water cluster anions were investigated as a function of the size (n = 45 and 104), nominal temperature (T(nom) = 50, 100, and 150 K), and preparation method of the parent neutral clusters. In particular, we consider two different protocols for preparing the initial neutral clusters, which differ markedly in their thermal history. In the first, warm equilibrium neutral clusters are gradually quenched to increasingly lower temperature. In the second, neutral clusters are formed spontaneously at approximately 0 K and then warmed to the same target temperatures, yielding inherently metastable, nonequilibrium structures. Electron attachment to these alternative sets of clusters shows that below a critical temperature (approximately 200 K), the metastable water clusters bind a surface state excess electron significantly more strongly than the quenched, equilibrium clusters. The structural analysis indicates that these cluster anions with larger vertical detachment energies (VDEs) more frequently stabilize the electron by double-acceptor-type water molecules and exhibit a weak temperature dependence of the VDE compared with the quenched clusters. These results suggest that the alternative classes of cluster anions seen experimentally may reflect differences in the thermal history of such clusters.

Electronic relaxation dynamics in large anionic water clusters: (H2O)n− and (D2O)n− (n=25–200)

Electronic relaxation dynamics subsequent to s --> p excitation of the excess electron in large anionic water clusters, (H(2)O)(n)(-) and (D(2)O)(n)(-) with 25 < or = n < or = 200, were investigated using time-resolved photoelectron imaging. Experimental improvements have enabled considerably larger clusters to be probed than in previous work, and the temporal resolution of the instrument has been improved. New trends are seen in the size-dependent p-state lifetimes for clusters with n > or = 70, suggesting a significant change in the electron-water interaction for clusters in this size range. Extrapolating the results for these larger clusters to the infinite-size limit yields internal conversion lifetimes tau(IC) of 60 and 160 fs for electrons dissolved in H(2)O and D(2)O, respectively. In addition, the time-evolving spectra show evidence for solvent relaxation in the excited electronic state prior to internal conversion and in the ground state subsequent to internal conversion. Relaxation in the excited state appears to occur on a time scale similar to that of internal conversion, while ground state solvent dynamics occur on a approximately 1 ps time scale, in reasonable agreement with previous measurements on water cluster anions and electrons solvated in liquid water.

Low temperature photoelectron spectra of water cluster anions

Photoelectron spectra of cold (10 K) size selected water cluster anions (H(2)O)(n) (-) and (D(2)O)(n) (-) have been measured in the size range n=20-120. A new isomer with a higher binding energy than the so-called isomer I has been identified, which appears in the size range n=25-30 and for (H(2)O)(n) (-) becomes dominant at n=46. Magic numbers observed in the mass spectra of the cluster anions provide evidence that this new isomer class consists of clusters with an internal electron.

Calorimetric Observation of the Melting of Free Water Nanoparticles at Cryogenic Temperatures

We present an experimental study of the thermodynamics of free, size-selected water cluster anions consisting of 48 and 118 molecules. The measured caloric curves of the clusters are bulklike at low temperatures but show a well-defined, particle-size specific transition at 93+/-3 K for (H2O)48- and 118+/-3 K for (H2O)118-. At the transition temperature the heat capacity strongly increases, which marks the onset of melting.

Interior- and surface-bound excess electron states in large water cluster anions

We present the results of mixed quantum/classical simulations on relaxed thermal nanoscale water cluster anions, (H(2)O)(n)(-), with n=200, 500, 1000, and 8000. By using initial equilibration with constraints, we investigate stable/metastable negatively charged water clusters with both surface-bound and interior-bound excess electron states. Characterization of these states is performed in terms of geometrical parameters, energetics, and optical absorption spectroscopy of the clusters. The calculations provide data characterizing these states in the gap between previously published calculations and experiments on smaller clusters and the limiting cases of either an excess electron in bulk water or an excess electron at an infinite water/air interface. The present results are in general agreement with previous simulations and provide a consistent picture of the evolution of the physical properties of water cluster anions with size over the entire size range, including results for vertical detachment energies and absorption spectra that would signify their presence. In particular, the difference in size dependence between surface-bound and interior-bound state absorption spectra is dramatic, while for detachment energies the dependence is qualitatively the same.

Excess Electrons Bound to Small Ammonia Clusters

Dipole-bound anions of small water clusters (H2O) N- (N >or= 2) are well-known from experiment and theory. In contrast, the smallest ammonia cluster anion detected so far is the 13-mer (NH3)13-. Here dipole-bound states of small ammonia clusters (NH3)N- (N = 2, 3, 4) are investigated using coupled-cluster ab initio methods. The trimer is found to be the smallest ammonia cluster able to form a dipole bound state, and its vertical detachment energy is predicted to be 27 meV, somewhat smaller than that of the water dimer. For the ammonia tetramer dipole-bound states with triple-acceptor monmers are identified akin to the well-studied double-acceptor binding motif of water cluster anions. Moreover, a (NH3)6-)hexamer that has been considered as a model for a cavity-bound state is examined. Ab initio results for this system challenge the notion that an electron localized in an ammonia cavity can be thought of as a delocalized radical anion.

Electron solvation by clustered H-bond complexes of water with tri- n-butylphosphate

We report the occurrence of electron trapping by HOH···O P(OR) 3 complexes in tri- n-butyl phosphate (TBP) containing <0.4 mole fraction of water. This trapping shifts the absorption band of the electron to the blue with respect to neat TBP solvent, as the prevalent mode of electron solvation changes from that by aliphatic chains of TBP to that by dangling hydroxyl groups of the water–TBP complexes. In parallel, the rates of dissociative electron attachment to TBP and added scavenger both decrease. The structure of the trapping site is similar to that of the small, internally trapping water cluster anions in the gas phase.

Water Cluster Anions Studied by the Long-Range Corrected Density Functional Theory

Long-range corrected density functional theory (LC-DFT) is applied to a series of small water cluster anions(n= 2-6) to compute their vertical detachment energies (VDEs). The LC scheme is shown to eliminate an unphysical overestimation of the electron-water attraction in the hybrid functional by properly accounting for the long-range exchange repulsions. It is shown that a correct correlation energy behavior for a rapidly varying density is also important for describing a spatially extent, excess electron. The one-parameter progressive (OP) correlation functional, which satisfies this condition, leads to a remarkable improvement in the calculated VDE over the conventional one. The LC-BOP method produces highly accurate VDEs with a mean absolute deviation of 13.8 meV from the reference CCSD(T) results, reducing the error of B3LYP by more than 15 times. LC-BOP is found to be more accurate than MP2 which yields an excess electron underbound by 43.6 meV. The effect of basis sets on the calculated VDE is also examined. The aug-cc-pVDZ basis set with an extra diffuse function is found to be more accurate and reliable than the extended Pople-type basis sets used in the previous works. The extrapolation of the calculated VDE of different electron binding motifs is compared with the VDEs of experimentally observed three isomers (Verlet, J. R. R.; Bragg,A. E.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Science 2005, 307, 93).

Spectroscopy and dynamics of excess electrons in clusters

Anionic clusters comprising solvent molecules and excess electrons can provide new insights into electron solvation in liquids, an intrinsically bulk phenomenon. This paper reviews experimental and theoretical studies of this class of clusters, focusing primarily on water cluster anions, , but also on iodide-water clusters, I−(H2O) n , and methanol cluster anions . Issues of particular interest include the relationship of time-resolved dynamics in these clusters to those of bulk solvated electrons, and the nature of the electron binding motifs supported by these clusters.

Evaluation of Incremental Correlation Energies for Open-Shell Systems: Application to the Intermediates of the 4-Exo Cyclization, Arduengo Carbenes and an Anionic Water Cluster

A fully automated procedure for incremental closed-shell CCSD calculations has been extended to open-shell cases tractable with the restricted open-shell CCSD method. It is demonstrated that for monoradical intermediates of the 4-exo cyclization, the triplet state of Arduengo carbenes as well as for water cluster anions chemical accuracy can be reached with respect to the error introduced by the local correlation treatment. Finally, it is shown that the computationally less demanding evaluation of higher-order increments in a smaller basis set does not lead to significant errors.