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Water clusters are hydrogen bonded molecular assembly of water molecules. They have been extensively studied using experiments, ab initio calculations and molecular simulations. The molecular arrangement of water in water cluster provide significant insights into behavior of water in bulk, confinement and close to surfaces. Here molecular dynamics simulations of TIP4P/2005 water clusters are performed in weak electrid fields. These fields introduce additional forces of varying magnitude 0.1–25 % of forces existing in without-field water clusters. Autocorrelation functions of distance, energy and velocities are analysed. Molecular arrangements in water clusters does not disintegrate under additional forces.

The present letter reports unusual high fluorescence anisotropy of excited state intramolecular proton transfer (ESIPT) prone 2-(2′-hydroxyphenyl)benzoxazole (HBO) in water followed by its imidazole (HBI) and thiazole (HBT) analogues. The low fluorescence anisotropy of the acetate compound of HBO in water reveals the key role of the hydrogen bonded cage-like probe-water cluster for the findings in water. Formation of the aquated cluster is controlled possibly by the combined effect of the electronegativity and mesomeric effect of the heteroatom away from the ESIPT site. The work invites further explorations, preferably from computational perspective, for a proper explanation.

In this letter, a possible reason behind selective host-guest organization in the initial stage of sI methane hydrate nucleation is provided, through density functional theory based calculations. In doing so, we have connected earlier experimental and theoretical observations on the structure and energetics of sI methane hydrate to our findings. Geometry and relative stability of small (H 2 O) 5 and (H 2 O) 6 clusters, presence of CH 4 guest, integrity and cavity radius of (H 2 O) 20 and (H 2 O) 24 , as well as the weak van der Waals type of forces, particularly dispersion interaction, are major factors responsible for initial formation of methane encapsulated dodecahedron cavity over tetrakaidecahedron.

We introduce the orbital-resolved electron density projected integral (EDPI) along the H-bond in the real space to quantitatively investigate the specific contribution from the molecular orbitals (MOs) aspect in (H 2 O) 2 . Calculation results show that, the electronic occupied orbital (HOMO-4) of (H 2 O) 2 accounts for about surprisingly 40% of the electron density at the bond critical point. Moreover, the electronic density difference analysis visualizes the electron accumulating effect of the orbital interaction within the H-bond between water molecules, supporting its covalent-like character. Our work expands the understanding of H-bond with specific contributions from certain MOs.

Chemical denaturation of protein represents a secondary/tertiary structural change, which has been measured by absorption and CD spectra in the liquid phase. In this study, we used gas-phase photoelectron detachment yield (PDY) spectra for elucidating isolated structures of bovine serum albumin (BSA) ions and their conjugated ions with cetyltrimethylammonium bromide (CTAB), whose conjugation causes a significant chemical denaturation in the protein structure. The obtained PDY spectrum of BSA ions shows a broad peak near 208 nm, which significantly disappears in that of CTAB-denatured BSA. This result indicates the CTAB-denatured BSA has a stable unfolding structure in the isolated water cluster.

The isomerization processes of 3-hydroxy-2(1H)-pyridinethione with water or water cluster in the aqueous solution were explored at B3LYP-D3/6-311++G(3df,3pd) level of theory. The results suggest that the formation of P2 dominates over that of P1 in the isolated reaction, whereas the barrier difference of transition states between them will decrease gradually with increasing of water number from one to three and the production of P1 will be competitive to that of P2 with water dimer or water trimer. Furthermore, the location of water or water cluster between N and S is more suitable for the formation of active component P1.

A nitrate-water cluster [(NO 3 ) 2 (H 2 O) 8 ] 2− confined in the hydrophobic cavity of a cucurbit[8]uril complex with Sr 2+ has been observed. It represents the first example of inorganic anion-water cluster formation inside the cucurbit[ n ]uril hydrophobic cavity.

Water clusters (H2O)N from the size range N ¯ ≈ 10 –150 have been investigated by electron ionization (EI) and photoionization after Na-doping (NaPI) under identical experimental conditions. The NaPI yields cluster sizes in good agreement with a semiempirical formula derived previously. The EI leads to large cluster fragmentation even at electron energies near the ionization threshold at 13 eV. The fragmentation does not change significantly between 15 eV and 90 eV. For N ¯ = 138 the mean cluster ion fragment size corresponds to only about 10% of the original neutral cluster size.

Recent matrix isolation (p-H-2, Ne, Ar, Kr) studies of water clusters made in Lund are reviewed. Gas phase positions of water cluster bands are estimated from matrix data. Intermolecular spectra of several isotopomers of the water dimer have been obtained, and approximate band strengths have been measured. Acceptor switching of the water dimer has been observed in matrices. A few intermolecular vibrations of (H2O)(3), (D2O)(3), (H2O)(4), and (D2O)(4) have been measured. The pseudorotation of the water trimer is not quenched in matrices, and pentamer pseudorotation appears to be fast as well. Spectra of partially deuterated water tetramers support a rigid cyclic tetramer structure. The hydrogen rotation in p-H-2 may interfere with guest vibrations at frequencies near the J = 2

Time-dependent density functional theory (TDDFT) was used to investigate the excited-state proton transfer (ESPT) dynamics of 6-hydroxyquinolinium (6HQc) in aqueous solvent, resulting in the excited zwitterionic form (6HQz). The optimized excited-state energy profiles of 6HQc:(H(2)O)(n) complexes have been calculated along the phenolic O-H bond to simulate the minimum energy pathway (MEP) in the excited state. The results suggested that the threshold of the size of the water cluster is 3 for the excited-state proton transfer of 6HQc in aqueous solvent, since the conformation of the stable hydrated proton requires proton transferring to the second or deeper shell of water solvent. Moreover, the stability of the hydrated proton can be improved significantly by adding one more H(2)O molecule to form an Eigen cation in the excited-state 6HQz:H(9)O(4)(+). The effect of the size of water cluster on the proton transfer is investigated theoretically in the excited state for the first time. (C) 2011 Elsevier B.V. All rights reserved.

Photo physical and spectral property of indomethacin has been harnessed to sense the cluster structures that exist at the microscopic level in binary mixtures of 1-propanol–water. The ability of indomethacin to switch from a fluorescent complex with alcohol self-association cluster at low water content, to a non-fluorescent complex with hydrogen-bonded water cluster at high water content, imparts the drug with its unique microheterogeneity sensing property. Similar results are obtained for methanol–water and 1-butanol–water binary mixtures.

The efficiency of Basin Paving method [Phys. Rev. E 73 (2006) 015701(R)], a Monte Carlo based optimization algorithm, is investigated by applying it to (H 2 O) 20 and (H 2 O) 50 . It is found that Basin Paving does a better sampling of low energy minima compared to the well known Basin Hopping method. The large number of minima obtained from this work is used to form a network of water cluster minima. From the properties of the network, it is found that (H 2 O) 50 minima are more uniformly distributed in the conformational space than that of (H 2 O) 20 .

Here we present a structural study of different gas hydrates using the B3LYP hybrid DFT exchange-correlation functional. A new concept for viewing the icosahedral cluster as an expansion of dodecahedral subclusters is introduced. The investigated structures consist of 280 water molecules. Structural and orientational features of various guest molecules occupying the central cavity of the clusters are established. It was found that water as the guest molecule has the highest stabilization energy in studied clusters. The conformational changes in dimer and trimer water molecules upon incorporation into hydrate cavity are discussed. The influence of second- and third-order solvent shells is illustrated on example of icosahedral water cluster derivatives.

The rate constants for concerted exchange reaction of the small cyclic water clusters ( H 2 O ) n ( 3 , 4 ) were calculated with the YL method. The anharmonic effect on the concerted hydrogen exchange reaction was examined. For the trimer water, at temperature 300 K, the anharmonic rate constant is k = 4.70 × 10 - 4 s - 1 , and for the tetramer water, the value is 0.55 s - 1 . The tunneling effect is very small for this two concerted hydrogen exchange reactions and can be neglected in the two reactions. The results are in reasonable agreement with the value given by Loerting et al. Besides that, the anharmonic effect is not significant for the concerted hydrogen exchange reaction of ( H 2 O ) n ( 3 , 4 ) .

The electrostatic potential in the Fragment Molecular Orbital (FMO) method describes the effect of the environment upon fragments, but approximations are necessary to achieve linear scaling. We have developed the derivative of the point charge approximation in this study to enable accurate and fast gradient calculations for geometry optimizations and molecular dynamics of large systems. The accuracy is tested in comparison with the numeric gradient for solvated sodium cation, water cluster, α-helix of polyalanine, and hydrated chignolin. The errors are found to be reduced by approximately one order of magnitude.

The dissociation rate constant of the water dimer was calculated by using the method proposed by Yao and Lin (YL method). The dividing surface method and RRKM theory are used to calculate the pseudo transition state and the rate constant, respectively. For the canonical case at temperature range of 243–1000 K and for the microcanonical system at total energy 1411–4000 cm −1 , the anharmonic rate constant, around 10 9 –10 11 s −1 , is close to the experimental prediction. The present results indicate that the anharmonic effect should be included and YL method is suitable for calculating dissociation rate constants of such small flexible water cluster.

Born–Oppenheimer molecular dynamics for a phenol molecule in a cluster with 32 water molecules is reported. The results for the dynamics provide new theoretical informations on the role played by phenol as an hydrophobic/hydrophylic probe. Significant differences between the short-time hydrogen bond dynamics related to water–water and phenol–water interactions are observed. The vibrational spectra calculated from the Fourier transform of time correlation functions were compared with experimental data on phenol–water clusters and it was found that the average red-shift of the phenol ν OH frequency (78 ± 36 cm −1 ) is less important than what is observed in small phenol–water clusters at low temperatures (133–312 cm −1 ).

Semiempirical, ab initio and density functional theory calculations are used to locate many low-energy minima on the potential energy surface of the CH 2 OHCOOH–(H 2 O) 16 complex. The lowest-energy structures consist of a fused, pentagonal-prism (H 2 O) 16 cluster hydrogen-bonded to the carboxylic group of the glycolic acid; the intramolecular hydrogen bond remains intact in the glycolic acid. Low-energy structures with the acid inside a cage-like water cluster were not found.

We made a theoretical study of the proton transfer path from C 6 H 6 + ( H 2 O ) 4 to C 6 H 5 ( H 2 O ) 4 H + with a planar cyclic geometry of (H 2 O) 4 H + by the hybrid density functional B3LYP method. The proton transfer proceeds in a three-step manner. In the first step, the proton in C 6 H 6 + ( H 2 O ) 4 , moves from the terminal H 2 O molecule in the (H 2 O) 4 cluster to yield a (H 2 O) 3 H + cluster. In the last third step, one H 2 O molecule leaves from C 6 H 6 (OH) in C 6 H 6 (OH) (H 2 O) 3 H + to yield a C 6 H 5 ( H 2 O ) 4 H + complex through the H-abstraction by the OH radical. It is found that the proton transfer and redistribution from π to σ of the radical electron proceed separately.

The adsorption of water molecule on a Cu(1 1 1) surface at 25 K was studied by infrared reflection absorption spectroscopy (IRAS). The four characteristic absorption bands in a ν OD stretching region were observed on Cu(1 1 1) surface and assigned to be OD stretchings from dimer and hexamer water cluster molecules. The appearances of the key OD stretching bands observed on M(1 1 1) (M = Cu, Ni, Pt) and Ru(0 0 1) surfaces indicate that water dimer and hexamer clusters are coadsorbed dominantly on those surfaces at 25 K.

Electronic spectra of benzene–(water) n cluster cations ([Bz–(H 2 O) n ] + , n = 1–4) in the visible and near ultraviolet regions were measured. The spectral feature of [Bz–(H 2 O) 1–3 ] + is similar to that of bare Bz + , showing that the electronic structure of the benzene cation is preserved in the cluster cations up to n = 3. At n = 4, however, a remarkable decrease of the absorption intensity takes place, indicating an essential change of the electronic structure of the benzene moiety. This result is consistent with the size dependent intracluster proton transfer reaction which has been suggested by the previous infrared spectroscopic study.

Collisional reactions of size- and energy-selected unprotonated water clusters (H 2 O) n + ( n =2 and 3) in a single-collision with D 2 O were investigated at very low collision energy ranging from 0.05 to 2.0 eV using guided-ion beam tandem mass spectrometry. First, in the unimolecular and collision-induced dissociations of (H 2 O) 2 + and (H 2 O) 3 + , only the H 2 O + fragment, but not the H 3 O + fragment, was observed. These results suggest that the structure of (H 2 O) 2 + is primarily hemibonded ‘(H 2 O⋯OH 2 ) + ’ rather than proton-transferred ‘(H 3 O + ⋯OH)’. Second, the measured incorporation cross-section of (H 2 O) 2 + was found to be smaller than that of H + (H 2 O) 2 . From the dipole moment obtained by ab initio calculation, we found that the small values of the incorporation cross-section for (H 2 O) 2 + were caused by the small value, 0.0 debye, of the dipole moment of the cluster.

The electron capture dynamics in water dimer (H 2 O) 2 has been investigated by means of direct ab initio trajectory method. It was found that the solvent reorientation of the water cluster anion occurs spontaneously following the vertical electron capture of the neutral water dimer: namely, the linear form water dimer which is the most stable structure of neutral water dimer, was drastically changed by accepting an excess electron to a dipole orientation form in which the two dipoles of the water molecules coordinate equivalently to the excess electron. The electron was located around the center of two water molecules at the final structure. The water dimer’s solvation time was calculated to be about 150–400 fs. The mechanism of the solvent reorientation was discussed on the basis of theoretical results.

A quantitative investigation of surface vs. interior solvation in iodide–water clusters was performed by evaluating the potentials of mean force and structural properties of I−(H2O)n clusters (n=32, 64) from Monte Carlo simulations with both non-polarizable and polarizable model potentials. Simulation results clearly indicate that the iodide ion tends to reside at the surface of a water cluster of size 32, whereas entropy and polarization effects make the interior solvation state more likely for a cluster size of 64. This is consistent with previous analyses of cluster experimental and model data, which suggest a transition from surface to bulk behavior around a cluster size of 60.

Interface water cluster anions (H 2 O) n − ( n ⩽ 12 ) composed of two to four fragments are simulated in the unrestricted Hartree–Fock approximation with the second order Moeller–Plesset perturbation theory corrections taken into account with the 6-31++G ** basis set either augmented or not with the floating center of eight s functions. A linear dependence of the circumsphere radius involving central molecules of the anions on 1/ n provides an estimate of the excess electron radius in condensed water (about 2.5 A). Vertical detachment energies, approximated with linear dependences on n −1/3 , are extrapolated to the values around 3.4 eV for bulk water.

Photoexcitation of the (H 2 O) − n ( n =20–100) clusters with 100 fs pulses at 800 nm results in an increasing propensity for two-photon electron photoejection with increasing cluster size. This increase correlates with the size range ( n ≈30) where the first excited electronic state drops below the electron continuum, and the electronic absorption band approaches the energy of the 800 nm pump photon. No above-threshold, two-photon detachment is observed for n =20. Differences in the shape of the resonant two-photon photoelectron spectrum compared to that arising from direct (high energy) photodetachment are interpreted in terms of the vibrational state selection created in the resonant step.

Hydrogen(H)-bonded structures of 7-hydroxyquinoline-(H 2 O) n clusters were investigated based on the analysis of the infrared spectra of the OH stretching vibrations in combination with ab initio calculations. For 7-hydroxyquinoline-(H 2 O) 1 , two isomers due to different H-bonding sites were found to coexist; in one of them 7-hydroxyquinoline acts as a proton donor, and in the other the OH group of water is H-bonded to the nitrogen site of 7-hydroxyquinoline. For 7-hydroxyquinoline-(H 2 O) 2–3 , the structures were determined to be bridge forms, in which a linear-form water cluster is H-bonded to the OH hydrogen and to the nitrogen of 7-hydroxyquinoline.

High level electronic structure calculations show that chlorine nitrate is readily hydrolysed in aqueous sulfate aerosols. For the reaction catalysed by H2SO4 and three water molecules, the reaction products involve H3O+NO3−/H2SO4/HOCl. Catalysis by HSO4− leads to an essentially spontaneous reaction, where the reaction products involve NO3−/H2SO4/HOCl or HONO2/HSO4−/HOCl depending on the size of the water cluster.

In addition to isomers having a dipole-bound electron, the internally bound electron isomers of trimer, tetramer and hexamer water anions are found using an ab initio molecular orbital method. The latter isomers have a characteristic OH{e}HO structure. The interaction between the excess electron {e} and the surrounding OH bonds holds the structure stable. The calculated vibrational infrared spectrum for a hexamer anion with two double proton-acceptor water molecules shows a qualitatively similar vibrational spectrum with the one observed. A strong correlation between the vertical detachment energy and the distribution of the excess electron is also found.

The adsorption of D 2 O on Pt(111) at 20 K has been investigated by infrared reflection absorption spectroscopy (IRAS). Water was found to form as a dimer at low coverages and it grew a tetramer molecule at high coverages. The structures of the cluster water molecules on the surface were derived from the intensities and the frequencies of symmetric and asymmetric ν OD stretching absorptions. The annealing at higher temperatures than 130 K was needed to form an ice bilayer island structure.

Rhodamine 3B perchlorate coexists solubilized in mixtures of water:toluene as a distribution of ion pair (74%) and cation (26%). The quantum yield variation shows that the fluorescence quenching process of the ion pair is diffusion controlled with a rate constant oscillating within the limits of slip and stick boundary conditions. The driving force for the excited-state conversion is the unbalance of electrostatic forces and changes in inter-ionic distances in the water cluster solvation sphere created around the ion pair by dielectric enrichment.

An intramolecular potential term (expressed as a series expansion in terms of internal coordinates) has been introduced in the water many-body model TCPE to estimate the water cluster OH stretch vibrational spectrum. Model parameters are derived from experimental results on (H 2 O) n clusters ( n =1–5). OH stretch vibrational spectra of three water hexamers computed with TCPE are in good agreement with available ab initio computations, and, as compared to experimental data, TCPE results also agree with the assignment of the experimental benzene–(H 2 O) 6 OH spectrum to a structure where water adopts a three-dimensional cage like structure π-hydrogen bonded to benzene.

The emission of small fragments during collisions of ( H 2 O ) n ( n ≤ 4000) with graphite surfaces is studied to characterize the cluster decomposition dynamics. Angular distributions for the emitted flux are measured for non-normal cluster incidence with 1375 ± 20 m/s. In the surface temperature range 800–1400 K, the distributions are well described by a model where fragments thermally evaporate from parent clusters gliding along the surface plane. When the surface temperature is decreased, clusters are slowed down by friction forces before complete evaporation takes place, and the decomposition is instead dominated by evaporation of monomers following a cosine law dependence.

Water cluster anions were efficiently generated by attachment of slow photoelectrons ejected from laser-excited zirconium metal surface, and photodetachment cross sections for (H2O)n− (n=2, 6, 7, and 11) were measured for a photon energy range of 0.74–2.54 eV. A gradual decrease of the cross section with the photon energy was observed for all the species. It was also found that the larger cluster exhibits the larger cross section. Size-dependent features of the spectra were discussed in relation to the excess electron localization with evolution of the cluster size.

The excited state proton transfer reaction of 4-hydroxy-1-naphthalenesulphonate in 1-propanol—water mixtures has been studied at six different temperatures ranging from 5 to 60°C. Because of the hydrophobic interaction in these mixtures, the dissociation of the probe cannot be explained by a simple water dimer model which we found to be applicable to methanol—water and ethanol—water mixtures. In 1-propanol—water mixtures the proton acceptor concentration, however, shows a linear dependence on the water concentration above a certain critical concentration of water. This behavior is explained by a new model assuming a water dimer within a water cluster to be the effective proton acceptor.

Twenty nine structures of the (H 2 O) 8 are examined. Calculations were carried out with a Hartree-Fock wavefunction using the 6-311G ∗ , and 6-311G ∗∗ basis sets. SCF force constants were calculated with both basis sets and the information used to generate thermodynamic data. The enthalpies, entropies, and free energies of the twenty nine structures are presented and compared. It is shown that entropy scales linearly with spatial extent of the cluster size. This allows calculation of entropy per volume of water cluster. Infrared and Raman spectra of three structures, cube, di-ring and ring, are also presented.

Analyses of the excited-state proton transfer reaction of 1-naphthol in sapphire/water interface layers have been performed using fluorescence decay data obtained by a picosecond time-resolved total internal reflection fluorescence spectroscopy. The rate constant of the excited-state proton transfer reaction, k1, of 1-naphthol in an interface layer was reduced as compared to that in a bulk aqueous solution. Addition of NaCl to aqueous solution of 1-naphthol resulted in a decrease in k1 for both in the interface layer and in the bulk solution, suggesting that a proton acceptor from excited 1-naphthol is not a single water molecule but a water cluster and that water clusters in an interface layer are also destroyed by NaCl.

A 3×3×3 cubic grid of oxygen atoms has 54 internal bonds and this can be an idealised model of a completely hydrogen-bonded water cluster (H2O)27. A combinatorial study gave 456 distinct isomers of such ideal structures, each of which should have similar energies. Computer simulation of (H2O)27 with a realistic intermolecular potential has produced such structures with energies of −237 kcal/mol. Similar cubic structures, but with a few external OH bonds have also been found and these have the lower energy of −241kcal/mol.

The binding energy of a hydrated electron in the vicinity of a water cluster is considered. Our consistent quantum-mechanical treatment results in an effective potential different form the classical image potential. Various approximate forms of the dielectric function are tested and compared.

The structure in a water cluster restricted by a hydrophobic wall was studied as a function of the distance from the wall. Significant orientational preference was observed near the surface. This preference is very similar to that present in liquid water near planar hydrophobic surfaces of infinite extend.

The calculated geometries of water clusters, (H 2 O) n with n = 2–6, obtained from the semi-empirical AM1 procedure differ from geometries based on ab initio calculations. The AM1 hydrogen-bonded linear dimer with C s symmetry reported by Dewar et al. [J. Am. Chem. Soc. 107 (1985) 3902], which corresponds to the most stable ab initio or experimental geometry, is only a local minimum. The AM1 C s dimer is found to be 2.0 kcal less stable than an unusual cyclic bifurcated structure (approximately C 2h symmetry) in which one of the lone pairs of electrons from each water molecule bisects the HOH bond angle of the other molecule. An additional local minimum, 1.7 kcal more stable than the reported C s structure, consists of a C 2v framework in which the two hydrogen atoms of one water molecule coordinate to both lone pairs of the second molecule. These results lead to questions regarding the use of the present AM1 parameterization in studies of hydrogen bonding.

The appearance potentials for producing the water cluster ions (H2O)2+, )H2O)3+, (H2O)2H+ and (H2O)3H+ have been determined by synchrotron radiation to be 10.87±0.06, 10.92±0.04, 11.18±0.02, and 11.10±0.02 eV, respectively, using water-argon binary mixture in supersonic jet. The result are discussed within the framework of intracluster excess energy dissipation processes.

We present experiments in which a beam of neutral (H 2 O) N clusters are scattered at various solid surfaces. It is shown that at cluster sizes corresponding to N ≈ 300 cluster cations, anions and electrons appear in the scattered flow. As N increases the current of cluster cations I + and anions I − becomes greater. The currents I + and I − are not identical and depend on the target material and the angle of incidence of the water cluster beam on the target. The phenomenon of charge formation and separation can be explained by the autoprotolysis of water molecules in clusters: H 2 O(aq)→H + (aq)+OH-(aq). This reaction is enhanced by increasing the cluster collision energy. The difference between I + and I − seems to be connected with the different rates of neutralization of positive and negative ions in clusters by the surface. The presence of electrons in the scattered particle flow is explained by Auger neutralization of some of the cations.

Classical trajectories have been numerically constructed for a proton interacting with an octameric water cluster. The “polarization model” was used to represent the 75-dimensional potential energy hypersurface. The most probable outcome is production of H7O+3 and five diverging water monomers, with one of the latter carrying away the incoming proton.