Electron solvation by highly polar molecules: Density functional theory study of atomic sodium interaction with water, ammonia, and methanol

Aufbau Rules for Solvated Electron Precursors: Be(NH3)40,± Complexes and Beyond

Tetra-amino beryllium complexes and ions, Be(NH₃)₄^0,±, have a tetrahedral Be(NH₃)₄²⁺ core with one, two, or three outer electrons orbiting its periphery. Our calculations reveal a new class of molecular entities, solvated electron precursors, with Aufbau rules (1s, 1p, 1d, 2s, 1f, 2p, 2d) that differ from their familiar hydrogenic counterparts and resemble those of jellium or nuclear-shell models. The core's radial electrostatic potential suffices to reproduce the chief features of the ab initio results. Wave function and electron-propagator methods combined with diffuse basis sets are employed to calculate accurate geometries, ionization energies, electron affinities, and excitation energies.

Molecules mimicking atoms: monomers and dimers of alkali metal solvated electron precursors

Tetra-amino lithium and sodium complexes M(NH₃)0,−4 (M = Li, Na) have one or two electrons that occupy diffuse hydrogenic type orbitals distributed chiefly outside the M(NH₃)₄⁺ core. Two such neutral species can bind to form a dimer which can be seen as the analogue of molecular hydrogen.

Some fundamental properties and reactions of ice surfaces at low temperatures

Ice surfaces offer a unique chemical environment in which reactions occur quite differently from those in liquid water or gas phases. In this article, we examine the basic properties of ice surfaces below the surface premelting temperature and discuss some of the recent investigations carried out on reactions at the ice surfaces. The static and dynamic properties of an ice surface as a reaction medium, such as its structure, molecule diffusion and proton transfer dynamics, and the surface preference of hydronium and hydroxide ions, are discussed in relation to the reactivity of the surface.

Experimental and theoretical investigation of HC5N adsorption on amorphous ice surface: simulation of the interstellar chemistry

HC 5N adsorbed on amorphous water ice at 10 K presents an interaction with the ice surface and induces the restructuring of the ice amorphous bulk. Warming up the sample induces the HC 5N desorption from the H 2O ice film, between 120 and 160 K, and the associated desorption energy is 90 kJ/mol. This value is in good agreement with that calculated E d (80 kJ/mol) and gives evidence that the amorphous ice surface is essentially dynamic. From theoretical calculations, it is shown that the HC 5N moiety presents a curvature and is no more linear and stabilized by two strong N...H bonds (2.09 and 2.29 A) and one H...O bond (1.84 A).

Ammoniated electron as a solvent stabilized multimer radical anion

The excess electron in liquid ammonia ("ammoniated electron") is commonly viewed as a cavity electron in which the s-type wave function fills the interstitial void between 6 and 9 ammonia molecules. Here we examine an alternative model in which the ammoniated electron is regarded as a solvent stabilized multimer radical anion in which most of the excess electron density resides in the frontier orbitals of N atoms in the ammonia molecules forming the solvation cavity. The cavity is formed due to the repulsion between negatively charged solvent molecules. Using density functional theory calculations, we demonstrate that such core anions would semiquantitatively account for the observed pattern of Knight shifts for 1H and 14N nuclei observed by NMR spectroscopy and the downshifted stretching and bending modes observed by infrared spectroscopy. We speculate that the excess electrons in other aprotic solvents might be, in this respect, analogous to the ammoniated electron, with substantial transfer of the spin density into the frontier N and C orbitals of methyl, amino, and amide groups.

Electron solvation in water-ammonia mixed clusters: Structure, energetics, and the nature of localization states of the excess electron

The structure and energetics of water-ammonia mixed clusters with an excess electron, [(H2O)n(NH3)m]- with m=1, n=2-6 and m=2, n=2, and also the corresponding neutral clusters are investigated in detail by means of ab initio quantum chemical calculations. The authors focus on the localization structure of the excess electron with respect to its surface versus interiorlike states, its binding to ammonia versus water molecules, the spatial and orientational arrangement of solvent molecules around the excess electron, the changes of the overall hydrogen-bonded structure of the clusters as compared to those of the neutral ones and associated dipole moment changes, vertical detachment energies of the anionic clusters, and also the vertical attachment energies of the neutral clusters. It is found that the hydrogen-bonded structure of the anionic clusters are very different from those of the neutral clusters unlike the case of water-ammonia dimer anion, and these changes in structural arrangements lead to drastically different dipole moments of the anionic and the neutral clusters. The spatial distribution of the singly occupied molecular orbital holding the excess electron shows only surface states for the smaller clusters. However, for n=5 and 6, both surface and interiorlike binding states are found to exist for the excess electron. For the surface states, the excess electron can be bound to the dangling hydrogens of either an ammonia or a water molecule with different degrees of stability and vertical detachment energies. The interiorlike states, wherever they exist, are found to have a higher vertical detachment energy than any of the surface states of the same cluster. Also, for interiorlike states, the ammonia molecule with its dangling hydrogens is always found to stay on top or on a far side of the charge density of the excess electron without participating in the hydrogen bond network of the cluster; the intermolecular hydrogen bonds are formed by the water molecules only which add to the overall stability of these anionic clusters.

Computational studies of aqueous-phase photochemistry and the hydrated electron in finite-size clusters

A survey of recent ab initio calculations on excited electronic states of water clusters and various chromophore-water clusters is given. Electron and proton transfer processes in these systems have been characterized by the determination of electronic wave functions, minimum-energy reaction paths and potential-energy profiles. It is pointed out that the transfer of a neutral hydrogen atom (leading to biradicals) rather than the transfer of a proton (leading to ion pairs) is the generic excited-state reaction mechanism in these systems. The hydrated hydronium radical, (H3O)(aq), plays a central role in this scenario. The electronic and vibrational spectra of H3O(H2O)(n) clusters and the decay mechanism of these metastable species have been investigated in some detail. The results suggest that (H3O)(aq) could be the carrier of the characteristic spectroscopic properties of the hydrated electron in liquid water.

Interaction of NaI with Solid Water and Methanol

The interaction of NaI with amorphous solid water (ASW) and methanol (MeOH) has been investigated with metastable impact electron spectroscopy (MIES), UPS(HeI), and temperature programmed desorption (TPD). We have studied the electron emission from the ionization of the highest-lying states of H(2)O, CH(3)OH, and of 5pI. We have prepared NaI layers on ASW (MeOH) films at about 105 K and annealed them up to about 200 K. Surface segregation of iodide is observed in ASW, as predicted for NaI aqueous solutions. On the other hand, surface segregation is not observed in MeOH, again as predicted for the interaction of NaI with liquid methanol. Electronic properties (ionization potentials, optical band gaps) and water binding energies are reported and are analyzed on the basis of available DFT results for hydrated NaI clusters.

Acetic Acid−Water Interaction in Solid Interfaces

The adsorption of acetic acid on a proton-ordered water ice surface is modeled using periodic plane-waves density-functional theory. The structures of acetic acid adsorbed as a monomer or oligomers, hydrated or not, are calculated through gradient optimization. The resulting quantum electronic density of states are compared to metastable impact electron spectroscopy (MIES) results and lead to selection of the most plausible structures of acetic acid on water ice. Hypotheses are formulated for the structure of the acid film growing on the ice surface including mainly cyclic dimers and hydrated forms. Adsorptions of single water molecules on acetic acid crystal surfaces are also studied after optimization of the acetic acid crystal bulk and surface structure. More comparisons with spectroscopic studies are proposed in the accompanying paper.

Photoemission from Sodium on Ice: A Mechanism for Positive and Negative Charge Coexistence in the Mesosphere

Photoemission from sodium deposited on ice films is described. Deposition of 0.02 ML of sodium is found to dramatically reduce the threshold for photoemission from the ice film to (2.3+/-0.2) eV. Thus, the cross-section for photoemission reaches >10(-18) cm2 in the visible region of the spectrum. It is proposed that the initial state is a solvated electron on the ice surface, which is supported by optical transmission spectroscopy. The potential significance of these results in understanding unexplained charging phenomena in the mesosphere is discussed.

The elimination of a hydrogen atom in Na(H2O)n

By a systematic examination on Na(H2O)n, with n = 4-7, 9, 10, and 15, we demonstrate that a hydrogen loss reaction can be initiated by a single sodium atom with water molecules. This reaction is similar to the well-known size-dependent intracluster hydrogen loss in Mg+(H2O)n, which is isoelectronic to Na(H2O)n. However, with one less charge on Na(H2O)n than that on Mg+(H2O)n, the hydrogen loss for Na(H2O)n is characterized by a higher barrier and a more flexible solvation shell around the metal ion, although the reaction should be accessible, as the lowest barrier is around 8 kcal/mol. Interestingly, the hydroxide ion OH- produced in the process is stabilized by the solvation of H2O molecules and the formation of an ion pair Na+(H2O)4(H2O)n-l-4[OH-(H2O)l]. The activation barrier is reduced as the unpaired electron in Na(H2O)n moves to higher solvation shells with increasing cluster size, and the reaction is not switched off for larger clusters. This is in sharp contrast to the reaction for Mg+(H2O)n, in which the OH- ion is stabilized by direct coordination with Mg2+ and the reaction is switched off for n > 17, as the unpaired electron moved to higher solvation shells. Such a contrast illustrates the important link between microsolvation environment and chemical reactivity in solvation clusters.

Sodium Interacting with Amorphous Water Films at 10 and 100 K

In the present study, we compare the adsorption of Na on amorphous D(2)O ice films, held at 10 and 100 K. OH, D(2)O, and Na are easily distinguished by their characteristic signatures in metastable impact electron spectroscopy (MIES). It is found that at 10 K substrate temperature the donation of 3sNa charge to the ice film, which is regarded as a precursor for water deprotonation, is significantly reduced relative to 100 K. This observation is discussed on the basis of recent theoretical work, suggesting that a rearrangement of the water molecules at the outermost water surface is the prerequisite for hydration/solvation of the 3sNa electron in the water ice bulk. The MIES spectra, showing spectral features from both OH and D(2)O, can be interpreted as reflecting the composition of the Na-water complexes in the near surface region. The relative intensity of the OH and D(2)O features is the same for 10 and 100 K. This finding suggests that two different sites for Na adsorption exist, one on the perfect water network and the other at OH dangling bond sites whereby, at 10 K, only the latter one leads to deprotonation of D(2)O. Finally, charge exchange phenomena observed when applying electron spectroscopies to ice films are discussed.

Quantum studies of hydrogen bonding in formic acid and water ice surface

The structure and spectroscopy (electronic and vibrational) of formic acid (HCOOH) dimers and trimers are investigated by means of the hybrid (B3LYP) density-functional theory. Adsorption of single and dimer HCOOH on amorphous water ice surface is modeled using two different water clusters. Particular attention has been given to spectroscopic consequences. Several hypotheses on formic acid film growing on ice and incorporation of a single water molecule in the formic acid film are proposed.

Interaction of NaCl with solid water

The interaction of NaCl with solid water, deposited on tungsten at 80 K, was investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) (He I). We have studied the ionization of Cl(3p) and the 1b(1), 3a(1), and 1b(2) bands of molecular water. The results are supplemented by first-principles density functional theory (DFT) calculations of the electronic structure of solvated Cl(-) ions. We have prepared NaCl/water interfaces at 80 K, NaCl layers on thin films of solid water, and H(2)O ad-layers on thin NaCl films; they were annealed between 80 and 300 K. At 80 K, closed layers of NaCl on H(2)O, and vice versa, are obtained; no interpenetration of the two components H(2)O and NaCl was observed. However, ionic dissociation of NaCl takes place when H(2)O and NaCl are in direct contact. Above 115 K solvation of the ionic species Cl(-) becomes significant. Our results are compatible with a transition of Cl(-) species from an interface site (Cl in direct contact with the NaCl lattice) to an energetically favored configuration, where Cl species are solvated. The DFT calculations show that Cl(-) species, surrounded by their solvation shell, are nevertheless by some extent accessed by MIES because the Cl(3p)-charge cloud extends through the solvation shell. Water desorption is noticeable around 145 K, but is not complete before 170 K, about 15 K higher than for pure solid water. Above 150 K the NaCl-induced modification of the water network gives rise to gas phase like structures in the water spectra. In particular, the 3a(1) emission turns into a well-defined peak. This suggests that under these conditions water molecules interact mainly with Cl(-) rather than among themselves. Above 170 K only Cl is detected on the surface and desorbs around 450 K.

Electron delocalization by polar molecules: Interaction of Na atoms with solid ammonia films studied with MIES and density functional theory

The interaction of Na and NH(3) on tungsten was studied with metastable impact electron spectroscopy under UHV conditions. NH(3)(Na) films were grown at 90(+/-10) K on tungsten substrates and exposed to Na(NH(3)). No Na-induced reaction involving NH(3) takes place. At small Na exposures a Na-induced shift of the NH(3) spectral features is seen, in parallel with a decrease of the surface work function. At larger exposures three 3sNa-related spectral structures are seen, two of them at energetic positions different from that found for Na on metals or semiconductors. The main additional peak is attributed to delocalized Na species. A small additional feature is attributed to simultaneous ionization and excitation of partially ammoniated Na(2) species. The results are compared with density functional theory calculations which suggest that the 3sNa emission at small exposures appears to originate mainly from delocalized 3sNa electrons; they are located far from the Na species and become stabilized by solvent molecules. When depositing NH(3) molecules onto Na films, metalliclike Na patches and delocalized Na species coexist. The delocalization of 3sNa is seen up to T=130 K where the NH(3) species desorb.

Electron solvation by polar molecules: The interaction of Na atoms with solid methanol films studied with MIES and density functional theory calculations

The interaction of Na atoms with CH(3)OH films was studied with metastable impact electron spectroscopy (MIES) under UHV conditions. The films were grown at 90(+/-10) K on tungsten substrates and exposed to Na. Na-induced formation of methoxy (CH(3)O) species takes place, and Na atoms become ionized. At small Na exposures the outermost solvent layer remains largely intact as concluded from the absence of MIES signals caused by the reaction products. However, emission from CH(3)O, located at the film surface, occurs at larger exposures. In the same exposure range also Na species can be detected at the surface. The spectral feature from 3s Na ionization occurs at an energetic position different from that found for metals or semiconductors. The results are compared with density functional theory calculations [see Y. Ferro, A. Allouche, and V. Kempter, J. Chem. Phys. 120, 8683 (2004), preceding paper]. Experiment and theory agree in the energetic positions of the main spectral features from the methanol and sodium ionization. The calculations suggest that the 3s Na emission observed experimentally originates from solvated 3s electrons which are located far from the Na core and become stabilized by solvent molecules. The simultaneous emergence of emission from CH(3)O and from solvated 3s electrons suggests that the delocalization and, consequently, the solvation play an important role in the Na-induced formation of CH(3)O from CH(3)OH.