Victoria Buch

Water surface is acidic

Water autoionization reaction 2H2O → 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.

Discrete stages in the solvation and ionization of hydrogen chloride adsorbed on ice particles

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 (H3O+ or H5O2+). 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 H3O+ and Cl-, and an ionic surface phase rich in Zundel ions, H5O2+.

Water in Confining Geometries

I Water Clusters.- 1 Nature of Many-Body Forces in Water Clusters and Bulk.- 2 Thermochemistry and Kinetics of Evaporation and Condensation for Small Water Clusters.- 3 Vibrational Spectroscopy and Reactions of Water Clusters.- 4 Solvent Effects of Individual Water Molecules.- 5 Solvation Effects on the Properties and Reactivities of Ionic and Neutral Water Clusters.- II Water at Interfaces.- 6 Properties of Water Clusters on a Graphite Sheet.- 7 Phase Equilibria and Transitions of Confined Systems in Hydrophobic and Aqueous Environments.- 8 Thin Film Water on Insulator Surfaces.- 9 Protein Hydration Water.- 10 Computational Studies of Liquid Water Interfaces.- 11 Water Confined at the Liquid-Air Interface.- III Atmospheric and Astrophysical Water and Ice.- 12 Physical Properties and Atmospheric Reactivity of Aqueous Sea Salt Micro-Aerosols.- 13 Interactions and Photochemistry of Small Molecules on Ice Surfaces: From Atmospheric Chemistry to Astrophysics.- IV Amorphous Solid Water (ASW) Films.- 14 Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water.- 15 Microporous Amorphous Water Ice Thin Films: Properties and Their Astronomical Implications.- V Ice Particles.- 16 Nucleation of Ice in Large Water Clusters: Experiment and Simulation.- 17 Ice Nanoparticles and Ice Adsorbate Interactions: FTIR Spectroscopy and Computer Simulations.

Proton order in the ice crystal surface.

The physics of the ice crystal surface and its interaction with adsorbates are not only of fundamental interest but also of considerable importance to terrestrial and planetary chemistry. Yet the atomic-level structure of even the pristine ice surface at low temperature is still far from well understood. This computational study focuses on the pattern of dangling H and dangling O (lone pairs) atoms at the basal ice surface. Dangling atoms serve as binding sites for adsorbates capable of hydrogen- and electrostatic bonding. Extension of the well known orientational disorder (“proton disorder”) of bulk crystal ice to the surface would naturally suggest a disordered dangling atom pattern; however, extensive computer simulations employing two different empirical potentials indicate significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms, as suggested long ago by Fletcher [Fletcher NH (1992) Philos Mag 66:109–115]. The presence of striped phase domains within the basal surface is consistent with the hitherto unexplained minor fractional peaks in the helium diffraction pattern observed 10 years ago. Compared with the disordered model, the striped model yields improved agreement between computations and experimental ppp-polarized sum frequency generation spectra.

The semiclassical self-consistent-field (SC-SCF) approach to energy levels of coupled vibrational modes. II. The semiclassical state-interaction procedure

Abstract A method for obtaining energy levels of coupled vibrational modes is described, that utilizes a state-interaction approach combined with semiclassical approximations. The method starts with a semiclassical self-consistent-field calculation of the coupled problem, and uses the eigenstates of the resulting Hartree-like separable SCF vibrational hamiltonian to define a basis set of Hartree products in which the full vibrational hamiltonian is represented and diagonalized. Matrix elements of any interaction potential between single-mode states are approximated semiclassically as the Fourier component of the interaction at the frequency corresponding to the SCF eigenvalue difference. A Fourier-component expression can also be given for the overlap between non-orthogonal single mode states. Thus no wavefunctions ever need to be defined. Application to a sample two-mode problem shows that the method is highly accurate. Further possible applications, in particular to intramolecular rate calculations are noted.

Vibrational spectra of α-glucose, β-glucose, and sucrose: anharmonic calculations and experiment.

The anharmonic vibrational spectra of α-D-glucose, β-D-glucose, and sucrose are computed by the vibrational self-consistent field (VSCF) method, using potential energy surfaces from electronic structure theory, for the lowest energy conformers that correspond to the gas phase and to the crystalline phase, respectively. The results are compared with ultraviolet-infrared (UV-IR) spectra of phenyl β-D-glucopyranoside in a molecular beam, with literature results for sugars in matrices and with new experimental data for the crystalline state. Car-Parrinello dynamics simulations are also used to study temperature effects on the spectra of α-D-glucose and β-D-glucose and to predict their vibrational spectra at 50, 150, and 300 K. The effects of temperature on the spectral features are analyzed and compared with results of the VSCF calculations conducted at 0 K. The main results include: (i) new potential surfaces, constructed from Hartree-Fock, adjusted to fit harmonic frequencies from Møller-Plesset (MP2) calculations, that give very good agreement with gas phase, matrix, and solid state spectra; (ii) computed infrared spectra of the crystalline solid of α-glucose, which are substantially improved by including mimic groups that represent the effect of the solid environment on the sugar; and (iii) identification of a small number of combination-mode transitions, which are predicted to be strong enough for experimental observation. The results are used to assess the role of anharmonic effects in the spectra of the sugars in isolation and in the solid state and to discuss the spectroscopic accuracy of potentials from different electronic structure methods.

The single-crystal, basal face of ice Ih investigated with sum frequency generation

Sum frequency generation spectroscopy has been used to investigate the hydrogen-bonded region of single-crystal, hexagonal ice in the temperature range of 113-178 K. The temperature and polarization dependences of the signal are used in conjunction with a recent theoretical model to suggest an interpretation of the bluest and reddest of the hydrogen-bonded peaks. The reddest feature is associated with strong hydrogen bonding; the dynamic polarizability of this feature is primarily parallel to the surface. It is assigned to a cooperative motion among the companion to the free-OH and four-coordinate oscillators hydrogen bonded to dangling lone-pair molecules on the surface. The bluest hydrogen-bonded feature is similarly assigned to a cooperative motion of the OH stretch of dangling lone-pair molecules and of four-coordinate molecules in the lower half bilayer that are hydrogen bonded to free-OH molecules. Reconstruction induced strain is present at as low as 113 K. These results provide a richer picture of the ice surface than has heretofore been possible.

Detection of the book isomer from the OH-stretch spectroscopy of size selected water hexamers

The vibrational OH-stretch spectrum of size selected water hexamer clusters has been measured for cluster temperatures between 40 and 60 K. By comparison with temperature dependent calculations the spectra were identified to be those of the book isomer. This result is in good agreement with recent predictions of the equilibrium isomer distributions in this temperature range.

Quantum Monte Carlo simulation of intermolecular excited vibrational states in the cage water hexamer

Rigid-body diffusion Monte Carlo simulations of the ground state and ten low-lying intermolecular excited vibrational states for the cage form of (H2O)6 are reported. The excited states are found by a nodal optimization procedure in which the fundamental excited-state nodes are constructed from the harmonic normal coordinates. The anharmonic effects in the excited states are found to be large. One of the states with relatively large transition intensity involves primarily flipping motions of the free OH bonds on the doubly bound monomers, and is assigned to the vibration–rotation–tunnelling band observed experimentally by Liu et al. [Nature 301, 501–503 (1996)].

Evidence for the surface origin of point defects in ice : Control of interior proton activity by adsorbates

Spectroscopic studies are presented of H-D isotopic exchange in the interior of ice nanocrystals. The exchange process is dominated by ionic and orientational defects long viewed as governing the electrical properties of ice. A new finding that interior exchange rates can be controlled by acidic and basic adsorbates is evidence that the defects originate at the ice surface. In particular, it is argued that interior isotopic exchange is a reflection of proton concentrations equilibrated at the ice surface.