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...

Autoionization of water which gives rise to its pH is one of the key properties of aqueous systems. Surfaces of water and aqueous electrolyte solutions are traditionally viewed as devoid of inorganic ions; however, recent molecular simulations and spectroscopic experiments show the presence of certain ions including hydronium in the topmost layer. This raises the question of what is the pH (defined using proton concentration in the topmost layer) of the surface of neat water. Microscopic simulations and measurements with atomistic resolution show that the water surface is acidic due to a strong propensity of hydronium (but not of hydroxide) for the surface. In contrast, macroscopic experiments, such as zeta potential and titration measurements, indicate a negatively charged water surface interpreted in terms of preferential adsorption of OH(-). Here we review recent simulations and experiments characterizing autoionization at the surface of liquid water and ice crystals in an attempt to present and discuss in detail, if not fully resolve, this controversy.

Reaction mechanisms for the acidic dissociation of HCl in water clusters are considered. Intermediates in the reaction are obtained from stationary points on the potential energy surface of the systems HCl-͑H 2 O͒ n with nϭ4 and 5. These points have been determined by the B3LYP density functional method in an aug-cc-pVDZ atomic orbital ͑AO͒ basis. The total energies of the stationary points are checked by the coupled cluster single-double-triple ͓CCSD͑T͔͒ method in the same AO basis. For the case of nϭ4 a multibody analysis of the interaction energies is performed by the CCSD͑T͒ method as well as by symmetry adapted perturbation theory. The clusters have a completely dissociated form as their energetically lowest minimum.

This paper contains a study of the pair and many-body interactions in cyclic water clusters: trimer, tetramer, and pentamer. Symmetry-adapted perturbation theory (SAPT) is applied to compute the pair-and three-body interactions directly and to analyze the individual electrostatic, induction, dispersion, and exchange contributions. The total interaction energies are also obtained by supermolecule coupled-cluster calculations including single, double, and noniterative triple excitations, CCSD(T). The three-body interactions contribute up to 28% of the total interaction energy in these water clusters in their equilibrium geometries and up to 50% of the barriers for different tunneling processes investigated in the trimer. The main three-body contribution is due to secondand third-order induction effects, but also three-body exchange effects are substantial. Dispersion contributions are only significant in the pair energy. The four-body effects are relatively small, and the five-body effects were found to be negligible. Furthermore, we tested the quality of various density functional methods for describing these many-body interactions.

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The platinum(II) tris (pyrazol-1-yl)borate complex [PtMe 2 {(pz) 3 BH-N,N′}] -reacts with phenol or aqueous HBF 4 in acetone to form the dimethylhydridoplatinum(IV) complex PtHMe 2 {(pz) 3 BH-N,N′,N′′}. The complex decomposes above ∼140 °C in toluene-d 8 to give methane. Theoretical calculations at the SCF and MP2 levels for the species PtXMe 2 {(H 2 CdNsNH) 3 BH-N,N′,N′′} (where X ) H, OH, Me and the fragment [(H 2 Cd-NsNH) 3 BH] -is a model for [(pz) 3 BH] -) yield geometries that compare well with structural reports for Pt(OH)-Me 2 {(pz) 3 BH} and PdMe 3 {(pz) 3 BH}.

Articles you may be interested inPerformance of dispersion-corrected double hybrid density functional theory: A computational study of OCShydrocarbon van der Waals complexes Computational and experimental investigation of intermolecular states and forces in the benzene-helium van der Waals complex Comment on "Anisotropic intermolecular interactions in van der Waals and hydrogen-bonded complexes: What can we get from density-functional calculations?" [J.The applicability of various density functional theory ͑DFT͒ methods to describe the anisotropy of the intermolecular potential energy surfaces of hydrogen-bonded ͓OH Ϫ -H 2 O, (H 2 O͒ 2 ͔ and van der Waals ͓CO-H 2 O, He-CO 2 ] complexes has been tested by comparison with supermolecule CCSD͑T͒ ͑coupled-cluster method restricted to single, double, and noniterative triple excitations͒ and perturbational SAPT ͑symmetry-adapted perturbation theory͒ results computed for the same geometries and with the same basis sets. It is shown that for strongly bound ionic hydrogen-bonded complexes, like OH Ϫ -H 2 O, hybrid approaches provide accurate results. For other systems, including the water dimer, the DFT calculations fail to reproduce the correct angular dependence of the potential surfaces. It is also shown that a hybrid functional adjusted to reproduce the CCSD͑T͒ value of the binding energy for the water dimer produces results worse than the standard hybrid functionals for OH Ϫ -H 2 O, and fails to describe the correct anisotropy of the CO-H 2 O interaction.

An efficient and versatile tandem process of acetalization and cycloisomerization reactions has been developed for the reactions of 1-alkynyl-2-carbonylquinoline substrates. The reaction occurs thanks to Au(I) and Ag(I) catalysis. Silver(I) catalysis has been extensively studied (11 different silver species) on a broad range of quinoline derivatives (variation of alkyne substituent, of carbonyl function and of nucleophiles), leading to a variety of furoquinoline and pyranoquinoline moieties. An insight is given for the presumed mechanism along with DFT-B3 LYP/6-31G** calculations to address the 6-endo and 5-exo regioselectivities observed.