Water autoionization reaction 2H2O 3 H3O ؊ ؉ OH ؊ is a textbook process of basic importance, resulting in pH ‫؍‬ 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O ؉ and OH ؊ ions, density functional studies of (H2O)48H ؉ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O ؉ does, but OH ؊ does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.density functional theory ͉ IR spectroscopy ͉ molecular dynamics ͉ water autoionization ͉ ice nanocrystals I n room-temperature liquid, one in 6 ϫ 10 8 water molecules is autoionized, yielding the standard value of pH ϭ 7. Autoionization in crystal ice should be less favorable, because, in contrast to water, ice is a very poor solvent of ionic and polar substances (1). As recently realized (2-5), the chemistry and composition of aqueous surfaces are quite distinct from that of the bulk; therefore, autoionization behavior should be reexamined at the surface.A number of recent computations (6-8) indicated the preference of hydronium cations for surface positions. Surface propensity of H 3 O ϩ was also deduced from vibrational spectroscopy of large protonated water clusters (6), as well as vibrational sum frequency generation (8, 9) and second harmonic generation (10) spectroscopic experiments probing extended aqueous interfaces. Interestingly, zeta potential measurements and titration experiments on oil droplets dispersed in water indicated the presence of negative charges at the interface, interpreted as adsorbed OH Ϫ ions (11). Similar conclusions have also been drawn from zeta potentials of air bubbles in water (12). More work is clearly needed to reconcile this apparent discord between predictions of surface-selective spectroscopies and molecular simulations on one side and electrochemical measurements on the other side.H 3 O ϩ forms three strong proton-donor bonds to H 2 O, but acts as a poor proton acceptor. A surface position with only H atoms hydrogen-bonded is preferred to interior positions, because the latter are associated with disruption of the approximately tetrahedral hydrogen-bond network in water (10). The present work focuses on the effect of surface stabilization of hydronium on water autoionization and surface pH. CalculationsOverview. Modeling of proton-tranfer systems is a nontrivial problem, because standard (empirical) potential energy surfaces do not include a possibility of proton hopping between different water molecules or transitions between the two li...

The vibrational OH stretch spectra have been measured for size-selected pure water clusters ͑H 2 O͒ n , in the size range n 8 10. Comparison between experiment and calculations suggests that the spectra originate from a small number of "microcrystalline" structures, based on the cubic octamer. The n 8 spectra are caused by two isomers of D 2d and S 4 symmetry. The proposed lowest energy nonamer and decamer structures are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the cube edges. [S0031-9007 (98)05604-X] PACS numbers: 36.40.MrWater clusters have been the focus of intense interest during the last several years. One of the objectives is to understand how the properties of ͑H 2 O͒ n evolve towards condensed phase behavior. One may ask, for example, at which size a cold cluster starts displaying attributes of a small crystal. The initial stages of the evolution as a function of n have been largely understood. Recently, an elegant series of far-infrared vibration-rotation-tunneling [1,2] and infrared [3] laser spectroscopic studies demonstrated conclusively a cyclic structure for n 3 5, and a transition towards a three-dimensional cage structure at n 6 [4]. Similar conclusions were drawn from the double resonance ion-dip infrared experiments on water clusters connected to a benzene molecule [5]. At present, the challenge is to understand the evolution in the "cage" regime n $ 7. A number of theoretical studies addressed this problem [6-9]. The only pertinent experimental results focus on the OH stretch spectroscopy of larger clusters without size selection [10,11] and n 7, 8 clusters attached to benzene [5,9].The present Letter reports the first measurement of the infrared spectroscopy of the OH stretch mode of pure water clusters in the size range n 8 10. The OH stretch spectra of hydrogen bonded H 2 O, which are redshifted by hundreds of cm 21 with respect to gaseous H 2 O, are known to be strikingly sensitive to hydrogen bond coordination and to bonding geometry [7,12]. Thus they can be used to probe the cluster structure. Moreover, the spectroscopy of clusters can serve as a benchmark for the calibration of flexible force fields for studies of condensed H 2 O and H 2 O surfaces.The experimental method which we apply is a combination of size selection by momentum transfer in a scattering experiment with atoms, with the infrared depletion technique [13,14]. This technique has been developed in our laboratory in Göttingen and mainly applied to cluster sizes n # 6. In the first step the different clusters are dispersed into different angles according to their masses and detected by a mass spectrometer. Then the OH stretch vibrational mode of the water molecules is excited by infrared laser radiation. The detector records the depletion in the cluster signal caused by the clusters which are dissociated by the absorbed radiation.The experimental setup consists of a crossed molecular beam apparatus with an angular dependent detection of the scattered beam with a resol...

2004) Solid water clusters in the size range of tens-thousands of H 2 O: a combined computational/spectroscopic outlook, A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n ¼ 20-931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasispherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mostly crystalline subsurface layer are stretched by the interaction with the disordered component. The emergence of the (strained) crystal core occurs at a size of a few hundred H 2 O. Smaller clusters are described as compactamorphous.

Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl(-)) and hydrated protons (H(3)O(+) or H(5)O(2)(+)). This acid ionization process also occurs in small water clusters and on ice surfaces, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H(3)O(+) and Cl(-), and an ionic surface phase rich in Zundel ions, H(5)O(2)(+).

The study focuses on acid adsorption on cold ice particle surfaces. The investigation encompasses HCl, DCl, and HBr adsorbate spectroscopy, Monte Carlo simulations of molecular HCl adsorbate on a model ice particle, and ab initio studies of HCl solvation and ionization in mixed acid−water clusters. It is shown that ice nanocrystal surfaces offer a range of adsorption sites, in which HCl freezes in different recognizable solvation stages. These stages were identified spectroscopically and assigned, with the help of calculations, to weakly and strongly stretched HCl molecules, and to Zundel and hydronium ions that are products of proton transfer. At moderate submonolayer coverages in the 50−60 K range, the acid adsorbate is predominantly molecular. Heating promotes formation of contact hydronium − chloride ion pairs. Near 90 K, an ionization burst is observed, resulting in an ionic surface hydrate layer rich in Zundel cations.

Simulations are presented of H2O ice, liquid, and clusters (H2O) n n ≤ 7. The first part is devoted to orientational energetics of ice. Ordinary hexagonal ice is orientationally disordered; a transition to an ordered form (ice XI) can be induced at 72 K. The recently demonstrated ferroelectric structure of ice XI (Jackson, S. M.; Wield, V. M.; Whitworth, R. W.; Oguro, M.; Wilson, C. J. Phys. Chem. B 1997, 101, 6142) seems to contradict our understanding of H2O···H2O interactions. A polarizable water potential is proposed that accounts qualitatively for the existence of ferroelectric ice XI; its crucial ingredient is the location of the polarizability center. This potential is then tested in classical trajectory simulations of water structure, energetics, self-diffusion, and dielectric constant. Cluster energetics and rotational constants are calculated using the rigid body diffusion Monte Carlo technique.

Clathrate hydrates (CHs) are inclusion compounds in which "tetrahedrally" bonded H(2)O forms a crystalline host lattice composed of a periodic array of cages. The structure is stabilized by guest particles which occupy the cages and interact with cage walls via van der Waals interactions. A host of atoms or small molecules can act as guests; here the focus is on guests that are capable of strong to intermediate H-bonding to water (small ethers, H(2)S, etc.) but nevertheless "choose" this hydrate crystal form in which H-bonding is absent from the equilibrium crystal structure. These CHs can form by exposure of ice to guest molecules at temperatures as low as 100-150 K, at the (low) guest saturation pressure. This is in contrast to the "normal" CHs whose formation typically requires temperatures well above 200 K and at least moderate pressures. The experimental part of this study addresses formation kinetics of CHs with H-bonding guests, as well as transformation kinetics between different CH forms, studied by CH infrared spectroscopy. The accompanying computational study suggests that the unique properties of this family of CHs are due to exceptional richness of the host lattice in point defects, caused by defect stabilization by H-bonding of water to the guests.

The structure of the ice surface and its interaction with adsorbates are investigated by several experimental tools, combined with computer modeling. Spectroscopic features characteristic of icy surfaces were identified and assigned. Adsorbate spectroscopy is used to probe both the adsorbate layer and the ice surface structure. These results are potentially informative of basic questions, such as cooperative aspects of H-bonding and the mechanism of ice vaporization, and of diverse practical questions, such as the role of icy particles in atmospheric chemistry and physics as well as the chemistry of interstellar space. Methods are described for the preparation and spectroscopic study of microporous amorphous ice and cubic ice nanocrystals with surface to volume ratios that make it possible to obtain low-noise infrared and Raman spectra of the vibrational modes localized near the surfaces and of the fundamental modes of small molecule adsorbates. The assignment of the bands of several of the surface-localized modes is reported, on the basis of primarily the calculated vibrational excitations for simulated structures of both amorphous and crystalline ice. The usefulness of these spectra is enhanced by conversion to difference spectra that compare high surface area and low surface area samples. Bands have been assigned to each of the three important types of surface water molecules, as revealed by the simulated structures and spectra: molecules with non-H-bonded or dangling-H(D) atoms, molecules with a dangling-0 coordination, and 4-coordinated surface water molecules. The experimental difference spectra have also been used to display the influence of small adsorbate molecules on surfacelocalized vibrations of each type of water molecule. This influence is apparent through the shifting and enhancement of bands of surface-localized modes, the response of the modes of the adsorbate molecules, and the determination of site-selective heats of adsorption of small molecules using the assigned ice modes. Computer modeling in conjunction with ab initio calculations was used to analyze and interpret adsorbate spectra and to elucidate the influence of factors such as the extent of surface disorder on gas-surface interactions. The results suggest significant modification of the ice surface structure with respect to the cubic crystalline interior, toward loss of lateral order.