Structure and Thermodynamics of H+(H2O)n (n = 9, 21, 40) Clusters between 0 and 300 K. A Monte Carlo Study

Effects of Temperature on Cs+(H2O)20 Clathrate Structure

Clusters consisting of 20 water molecules and a single cesium ion are especially stable due to their clathrate structure that is composed exclusively of three-coordinate water molecules. Clathrate stability was investigated using infrared photodissociation (IRPD) spectroscopy in the free-OH stretching region (∼3600-3800 cm^-1) at ion cell temperatures between 135 and 355 K. At 275 K and colder, IRPD spectra of Cs⁺(H₂O)₂₀ have just one acceptor-acceptor-donor band. At higher temperatures, a higher-energy acceptor-donor band emerges and grows in intensity. Non-clathrate Na⁺(H₂O)₂₀ structures contain both of these bands, which do not change significantly in intensity over the temperature range. These results indicate a rapid onset in the conversion from clathrate to non-clathrate structures with temperature and suggest that some clathrate population remains even at the highest temperatures investigated. These results provide new insights into the role of entropy in clathrate stability.

Insights into the dynamics of evaporation and proton migration in protonated water clusters from large-scale Born-Oppenheimer direct dynamics

Large-scale on-the-fly Born-Oppenheimer molecular dynamics simulations using recent advances in linear scaling electronic structure theory and trajectory integration techniques have been performed for protonated water clusters around the magic number (H(2)O)(n)H(+) , for n = 20 and 21. Besides demonstrating the feasibility and efficiency of the computational approach, the calculations reveal interesting dynamical details. Elimination of water molecules is found to be fast for both cluster sizes but rather insensitive to the initial geometry. The water molecules released acquire velocities compatible with thermal energies. The proton solvation shell changes between the well-known Eigen and Zundel motifs and is characterized by specific low-frequency vibrational modes, which have been quantified. The proton transfer mechanism largely resembles that of bulk water but one interesting variation was observed.

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.

Eigen and Zundel Forms of Small Protonated Water Clusters: Structures and Infrared Spectra

The spectral properties of protonated water clusters, especially the difference between Eigen (H3O+) and Zundel (H5O2+) conformers and the difference between their unhydrated and dominant hydrated forms are investigated with the first principles molecular dynamics simulations as well as with the high level ab initio calculations. The vibrational modes of the excess proton in H3O+ are sensitive to the hydration, while those in H5O2+ are sensitive to the messenger atom such as Ar (which was assumed to be weakly bound to the water cluster during acquisitions of experimental spectra). The spectral feature around approximately 2700 cm-1 (experimental value: 2665 cm-1) for the Eigen moiety appears when H3O+ is hydrated. This feature corresponds to the hydrating water interacting with H3O+, so it cannot appear in the Eigen core. Thus, H3O+ alone would be somewhat different from the Eigen forms in water. For the Zundel form (in particular, H5O2+), there have been some differences in spectral features among different experiments as well as between experiments and theory. When an Ar messenger atom is introduced at a specific temperature corresponding to the experimental condition, the calculated vibrational spectra for H5O2+.Ar are in good agreement with the experimental infrared spectra showing the characteristic Zundel frequency at approximately 1770 cm-1. Thus, the effect of hydration, messenger atom Ar, and temperature are crucial to elucidating the nature of vibrational spectra of Eigen and Zundel forms and to assigning the vibrational modes of small protonated water clusters.

Hydrogen peroxide and ammonia on protonated ice clusters

A temperature controlled source for protonated water clusters has been combined with high-resolution mass spectroscopy to study the stability pattern of ice clusters and compounds with ammonia and hydrogen peroxide depending on temperature. The stability pattern of pure protonated ice shows the two well known peaks at 21 and 28 molecules and also less pronounced structure up to n=55. Ammonia and hydrogen peroxide do not destroy this pattern but shift it by a number of water molecules. The additives are therefore integrated in the persisting crystalline structure of the pure protonated ice. Based on this structural information, density functional theory calculations reveal that hydrogen peroxide and ammonia occupy surface positions on a dodecahedral 21-molecule cluster and are not caged in the center.

Dynamics and fragmentation of van der Waals clusters: (H2O)n, (CH3OH)n, and (NH3)n upon ionization by a 26.5eV soft x-ray laser

A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5 eV/photon laser (pulse time duration of approximately 1 ns) is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2< or =n< or =22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be (0.6-2.7)x10(4), (3.6-6.0)x10(3), and (0.8-2.0)x10(4) s(-1) for the protonated water (9< or =n< or =24), methanol (5< or =n< or =10), and ammonia (5< or =n< or =18) clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200 K for water clusters (10< or =n< or =21), and 50-100 K for methanol clusters (6< or =n< or =10). Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters (NH(3))(n) (n>3) do not lose more than three H atoms in the photoionization/photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5 eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges.

Protonated clathrate cages enclosing neutral water molecules: (H+)(H2O)21 and (H+)(H2O)28

This paper describes a systematic study on the clathrate structure of (H+)(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced (H+)(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of (H+)(H2O)21 exhibits only a single feature at 3699 cm(-1), corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations. The observations, together with Monte Carlo simulations and density functional theory calculations, corroborate the notion for the formation of a distorted pentagonal dodecahedral (5(12)) cage with a H2O molecule in the cage and a H3O+ ion on the surface for this "magic number" water cluster ion. The dodecahedral cage melts at higher temperatures, as evidenced by the emergence of a free-OH stretching feature at 3717 cm(-1) for the two-coordinated water in (H+)(H2O)21 produced in a warmer molecular beam. Extension of this study to larger clusters strongly suggests that the experimentally observed isomer of (H+)(H2O)28 is most likely to consist of a distorted protonated pentakaidecahedral (5(12)6(3)) cage enclosing two neutral water molecules.

Protonated water clusters described by an empirical valence bond potential

The properties of low-lying stationary points on the potential energy surfaces of singly protonated water clusters (H(2)O)(n)H(+), are investigated using an empirical valence bond potential. Candidate global minima are reported for n=2-4, 8, and 20-22. For n=8, the variation in the energies and structures of low-lying minima with the number of valence bond states included in the model is studied. For n=4 and 8, disconnectivity graphs are also reported and are compared to results for the equivalent neutral water clusters as described by the rigid TIP3P potential. For the larger clusters, n=20-22, the structural properties of the low energy minima are compared with recently published spectroscopic data on these systems. The observed differences between the n=20 and n=21 systems are qualitatively reproduced by the model potential, but the similarities between the n=21 and n=22 systems are not.

Protons colliding with crystalline ice: proton reflection and collision induced water desorption at low incidence energies

We present results of classical trajectory (CT) calculations on the sticking of protons to the basal plane (0001) face of crystalline ice, for normal incidence at a surface temperature (Ts) of 80 K. The calculations were performed for moderately low incidence energies (Ei) ranging from 0.05 to 4.0 eV. Surprisingly, significant reflection is predicted at low values of Ei (< or = 0.2 eV) due to repulsive electrostatic interactions between the incident proton and the surface water molecules with one of their H-atoms pointing upward toward the gas phase. The sticking probability increases with Ei and converges to unity for Ei > or = 0.8 eV. In the case of sticking, the proton is trapped in the ice forming a Zundel complex (H5O2+), with an average binding energy of 9.9 eV with a standard deviation of 0.5 eV, independent of the value of Ei. In nearly all sticking trajectories, the proton is implanted into the ice surface, with a penetration depth that increases with Ei. The strong interaction with the neighboring water molecules leads to a local rupture of the hydrogen bonding network, resulting in collision induced desorption of water (puffing), a process that occurs with significant probability even at the lowest Ei considered. The probability of water desorption increases with Ei. In nearly all trajectories in which water desorption occurs, a single three-coordinated water molecule is desorbed from the topmost monolayer.

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.

Infrared Signature of Structures Associated with the H+(H2O)n (n = 6 to 27) Clusters

We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 </= n </= 27. A multiplet of such bands appears at small cluster sizes. This pattern simplifies to a doublet at n = 11, with the doublet persisting up to n = 20, but then collapsing to a single line in the n = 21 and n = 22 clusters and reemerging at n = 23. This spectral simplification provides direct evidence that, for the magic number cluster, all the dangling OH groups arise from water molecules in similar binding sites.