Superoxide intermediate in the oxygen reduction on a zinc hydroxide model corrosion product.

The inhibition of the electrochemical oxygen reduction reaction (ORR) by zinc corrosion products plays an important role in the corrosion protection of galvanized steel. Hence, the electrocatalytic mechanism of the ORR on electrodeposited zinc hydroxide-based model corrosion products was investigated by in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy, supplemented by density functional theory (DFT) calculations. Model corrosion products containing flake-like crystalline Zn5(NO3)2(OH)8 were cathodically electrodeposited on germanium(100) electrodes from a zinc nitrate precursor electrolyte. Substantial amounts of the films are non-crystalline, and their surfaces predominantly consist of zinc oxide and hydroxide species, as evidenced by x-ray photoelectron spectroscopy. ATR-IR spectra show a peak at 1180 cm-1 during cathodic currents in O2-saturated NaClO4 solution. This peak is assigned to a surface-bound superoxide, the only ORR intermediate detected. Absorbance from the intermediate increases with increasing cathodic current, indicating an increase in surface concentration of superoxide intermediates at larger ORR current densities. The zinc hydroxide ages in the experiments, most likely by a transformation into zinc oxide, consistent with the observed decrease in absorbance over time of the OH bending mode of zinc hydroxide at 1380 cm-1. This aging is a time-dependent chemical process, implying that pure chemical aging is important in actual corrosion products as well. DFT calculations of adsorbed superoxide yield a Zn-O bond length similar to the bond length in Zn-O, thus enhancing superoxide interaction with undercoordinated tetrahedral Zn2+ sites on the surface. Thus, such active sites catalyze the first reduction step in the ORR.

Water adsorbate phases on ZnO and impact of vapor pressure on the equilibrium shape of nanoparticles.

ZnO nanoparticles are used as catalysts and have potential applications in gas-sensing and solar energy conversion. A fundamental understanding of the exposed crystal facets, their surface chemistry, and stability as a function of environmental conditions is essential for rational design and improvement of synthesis and properties. We study the stability of water adsorbate phases on the non-polar low-index (101¯0) and (112¯0) surfaces from low coverage to multilayers using ab initio thermodynamics. We show that phonon contributions and the entropies due to a 2D lattice gas at low coverage and multiple adsorbate configurations at higher coverage have an important impact on the stability range of water adsorbate phases in the (T,p) phase diagram. Based on this insight, we compute and analyze the possible growth mode of water films for pressures ranging from UHV via ambient conditions to high pressures and the impact of water adsorption on the equilibrium shape of nanoparticles in a humid environment. A 2D variant of the Wulff construction shows that the (101¯0) and (112¯0) surfaces coexist on 12-faceted prismatic ZnO nanoparticles in dry conditions, while in humid environment, the (101¯0) surface is selectively stabilized by water adsorption resulting in hexagonal prisms.

Water aggregation and dissociation on the ZnO(101[combining macron]0) surface.

A comprehensive search for stable structures in the low coverage regime (0-1 ML) and at 2 ML and 3 ML using DFT revealed several new aggregation states of water on the non-polar ZnO(101[combining macron]0) surface. Ladder-like structures consisting of half-dissociated dimers, arranged side-by-side along the polar axis, constitute the most stable aggregate at low coverages (≤1 ML) with a binding energy exceeding that of the monolayer. At coverages beyond the monolayer - a regime that has hardly been studied previously - a novel type of structure with a continuous honeycomb-like 2D network of hydrogen bonds was discovered, where each surface oxygen atom is coordinated by additional H-bonding water molecules. This flat double-monolayer has a relatively high adsorption energy, every zinc and oxygen atom is 4-fold coordinated and every hydrogen atom is engaged in a hydrogen bond. Hence this honeycomb double monolayer offers no H-bond donor or acceptor sites for further growth of the water film. At 3 ML coverage, the interface restructures forming a contact layer of half-dissociated water dimers and a liquid-like overlayer of water attached by hydrogen bonds. The structures and their adsorption energies are analysed to understand the driving forces for aggregation and dissociation of water on the surface. We apply a decomposition scheme based on a Born-Haber cycle, discussing difficulties that may occur in applying such an analysis to the adsorption of dissociated molecules and point out alternatives to circumvent the bias against severely stretched bonds. Water aggregation on the ZnO surface is favoured by direct water-water interactions including H-bonds and dipole-dipole interactions and surface- or adsorption-mediated interactions including enhanced water-surface interactions and reduced relaxations of the water molecules and surface. While dissociation of isolated adsorbed molecules is unfavourable, partial or even full dissociation is preferred for aggregates. Nevertheless, direct water-water interactions change very little in the dissociation reaction. Dissociation is governed by a subtle balance between strongly enhanced water-surface interactions and the large energies required for the geometric changes of the water molecule(s) and the surface. Our conclusions are discussed on the background of the current knowledge on water adsorption at metals and non-metallic surfaces.

Self-Assembled Monolayers: Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects (Adv. Mater. 33/2015).

On page 4877, F. U. Renner, A. Bashir, M. Valtiner, and co-workers describe a star-like dealloying corrosion morphology that appears during the localized attack of smooth well-prepared Cu-Au surfaces. The surfaces are initially protected by thiol or selenol inhibitior films. Localized dealloying of Cu-Au produces nanoporous gold under stress and crystallographic cracks - thereby opening a new approach combining surface science with nanoscale mechanical testing.

Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects.

On self-assembled monolayer-covered Cu-Au substrates, localized volume shrinkage at initial dealloying sites leads to cracks within the attacked regions. It is started from well-controlled surface structures to gain fundamental insights in the driving mechanisms of localized corrosion and crack formation. Both the crack density and the crack morphology are critically dependent on surface orientation, crystallography, and inhibitor molecule species.

Stereochemistry of bistricyclic aromatic enes and related polycyclic systems.

Bistricyclic aromatic enes (BAEs) and related polycyclic systems are a class of molecular materials that display a rich variety of conformations, dynamic stereochemistry and switchable chirality, color, and spectroscopic properties. This is due to the a subtle interplay of the inherent preference for planarity of aromatic systems and the competing necessity of non-planarity due to intramolecular overcrowding in the fjord regions built into the general molecular structure of BAEs. The conformational, dynamic, and spectroscopic properties may be designed and fine-tuned, e.g., by variation of the bridging groups X and Y, the overcrowding in the fjord regions, extensions of the aromatic system, or other modifications of the general BAE structure, based on the fundamental understanding of the structure-property relationships (SPR). The present review provides an analysis of the conformational spaces and the dynamic stereochemistry of overcrowded bistricyclic aromatic enes applying fundamental symmetry considerations. The symmetry analysis presented here allows deeper insight into the conformations, chirality, and the mechanisms of the dynamic stereochemistry, and will be instrumental in future computational studies.

The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters.

The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.

The impact of spectator species on the interaction of H2O2 with platinum--implications for the oxygen reduction reaction pathways.

The impact of electrolyte constituents on the interaction of hydrogen peroxide with polycrystalline platinum is decisive for the understanding of the selectivity of the oxygen reduction reaction (ORR). Hydrodynamic voltammetry measurements show that while the hydrogen peroxide reduction (PRR) is diffusion-limited in perchlorate- or fluoride-containing solutions, kinetic limitations are introduced by the addition of more strongly adsorbing anions. The strength of the inhibition of the PRR increases in the order ClO4(-)≈ F(-) < HSO4(-) < Cl(-) < Br(-) < I(-) as well as with the increase of the concentration of the strongly adsorbing anions. Electronic structure calculations indicate that the dissociation of H2O2 on Pt(111) is always possible, regardless of the coverage of spectator species. However, the adsorption of H2O2 becomes strongly endothermic at high coverage with adsorbing anions. A comparison of our observations on the inhibition of the PRR by spectators with previous studies on the selectivity of the ORR shows that oxygen is reduced to H2O2 only under conditions at which the PRR kinetics is significantly limited, while the ORR proceeds with a complete four-electron reduction only when the PRR is sufficiently fast. Therefore, only a H2O2-mediated pathway that includes a competition between the dissociation and the spectator coverage-dependent desorption of the H2O2 intermediate is enough to explain and unify all the observations that have been made so far on the selectivity of the ORR.

A mechanistic study of the electrochemical oxygen reduction on the model semiconductor n-Ge(100) by ATR-IR and DFT.

The electrochemical oxygen reduction reaction (ORR) on a n-Ge(100) surface in 0.1 M HClO4 was investigated in situ and operando using a combination of attenuated total reflection infrared (ATR-IR) spectroscopy and density functional (DFT) calculations. The vibrational modes of the detected intermediates were assigned based on DFT calculations of solvated model clusters such as Ge-bound superoxides and peroxides. ATR-IR shows the Ge-bound superoxide with a transition dipole moment oriented at (28 ± 10)° with respect to the surface normal. At slightly negative potentials, the surface-bound peroxide is identified by an OOH bending mode as a further intermediate, oriented at a similar angle. At strongly negative potentials, a surface-bound perchlorate is found. The findings indicate a multistep mechanism of the ORR. The reaction is furthermore coupled with the hydrogen evolution reaction (HER).

Formation of highly ordered and orientated gold islands: effect of immersion time on the molecular adlayer structure of pentafluorobenzenethiols (PFBT) SAMs on Au(111).

Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the [11(-)0] substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, ß, γ, δ, and ε were found. The α-, ß-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→ß→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.

Hydrogen peroxide electrochemistry on platinum: towards understanding the oxygen reduction reaction mechanism.

Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).

Near-surface ion distribution and buffer effects during electrochemical reactions.

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.

Steric and chain length effects in the (√(3)×√(3))R30° structures of alkanethiol self-assembled monolayers on Au(111).

The translational and orientational potential energy surfaces (PESs) of n-alkanethiols with up to four carbon atoms are studied for (√(3)×√(3))R30° self-assembled monolayers (SAMs). The PESs indicate that methanethiol may form SAM structures that are not accessible for long-chain thiols. The tilt of the thiol molecules is determined by a compromise between the preferred binding geometry at the sulfur atom and the steric requirements of the alkane chains. The Au-S bond lengths, offset from the bridge position (brg), and the Au-S-C bond angles result in tilt angles of the S-C bond in the range of 55-60°. As DFT/generalized gradient approximation systematically underestimates chain-chain interactions, the binding energies are corrected by comparison to MP2 interaction energies of alkane dimers in SAM-like configurations. The resulting thiol binding energies increase by approximately 1 kcal mol(-1) per CH(2) group, which results in a substantial stabilization of long-chain SAMs due to chain-chain interactions. Furthermore, as the chain length increases, the accessible range of backbone tilt angles is constrained due to steric effects. The combination of these two effects may explain why SAM structures with long-chain thiols exhibit higher order in experiments. For each thiol two favorable SAM structures are found with the sulfur head group at the fcc-brg and hcp-brg positions, respectively. These domains may coexist in thermal equilibrium. In combination with the symmetry of the gold (111) surface, this raises the possibility of up to six different domains on single-crystal terraces. Reconstructions by an adatom or vacancy of ethanethiol SAMs with (√(3)×√(3))R30° lattice are also studied using PES scans. The results indicate that adsorption of thiols next to a vacancy is favorable and may lead to point defects inside SAMs.

Electrostatic chiral distinction: tetrahedral model dimers.

Although chiral distinction plays a pervasive role in chemistry, a complete understanding of how this takes place is still lacking. In this work, we expand the earlier described minimal requirement of so called four-point interactions (vide infra). We focus on chiral point charge model systems as a means to aid in the dissection of the underlying, operative principles. We also construct models with defined symmetry characteristics. By considering extensive constellations of diastereomeric complexes, we are able to identify emerging principles for chiral distinction. As previously postulated, all the diastereomeric complexes, regardless of their nominal contact-points, possess a chiral distinction energy. In the comparison of complexes, we find that, contrary to chemical intuition, the magnitude of chiral distinction does not correlate with the stability of the complexes, i.e., consideration of low energy complexes may not be an effective way to evaluate chiral distinction. Similarly, we do not find a correlation between the number of contact-points and chiral distinction. Moreover, favorable interactions and facile chiral distinction appear to be unrelated. We also see some tendency for greater chiral distinction in less symmetric systems, although this may not be general. These studies can now form the basis to fold in higher levels of complexity into the models so as to gain further insights into the nature of chiral distinction.

Pyramidal inversion mechanism of simple chiral and achiral sulfoxides: a theoretical study.

The pyramidal inversion mechanism of simple sulfoxides was studied, employing ab initio and DFT methods. The convergence of the geometrical and energetic parameters of H2SO and DMSO with respect to the Hamiltonian and basis set was analyzed in order to determine a computational level suitable for methyl phenyl sulfoxide (3), methyl 4-cyanophenyl sulfoxide (4), diphenyl sulfoxide (5), 4,4'-dicyanodiphenyl sulfoxide (6), benzyl methyl sulfoxide (7) and benzyl phenyl sulfoxide (8). The DFT B3LYP/6-311G(d,p) level was chosen for further calculations of larger sulfoxides. The barriers DeltaE calculated for the pyramidal inversion mechanism of sulfoxides 3-8 are in the range of 38.7-47.1 kcal/mol. These values are in good agreement with the experimental barriers for racemization via the pyramidal inversion mechanism. A resonance effect of a phenyl ring selectively stabilizes the transition state conformations, decreasing the energy barrier for pyramidal inversion by about 3 kcal/mol, compared to a similar molecule without a phenyl substituent. Introducing electron withdrawing groups (cyano) at the para positions of the phenyl ring(s) causes a further decrease of the energy barrier.

Polymorphism versus thermochromism: interrelation of color and conformation in overcrowded bistricyclic aromatic enes.

The nature of the thermochromic form of overcrowded bistricyclic aromatic enes (BAEs) has been controversial for a century. We report the single-crystal X-ray structure analysis of the deep-purple and yellow polymorphs of 9-(2,7-dimethyl-9H-fluoren-9-ylidene)-9H-xanthene (11), which revealed the molecules in a twisted and a folded conformation, respectively. Therefore, the deeply colored thermochromic form B of BAEs is identified as having a twisted conformation and the ambient-temperature form A as having a folded conformation. This relationship between the color and the conformation is further supported by the X-ray structures of the deep-purple crystals of the twisted 9-(9H-fluoren-9-ylidene)-9H-xanthene (10), and of the yellow crystals of the folded 9-(11H-benzo[b]fluoren-11-ylidene)-9H-xanthene (12). Based on this conclusive crystallographic evidence, eleven previously proposed rationales of thermochromism in BAEs are refuted. In the twisted structures, the tricyclic moieties are nearly planar and the central double bond is elongated to 1.40 A and twisted by 42 degrees . In the folded structures, the xanthylidene moieties are folded by 45 degrees and the fluorenylidene moieties by 18-20 degrees . Factors stabilizing the twisted and folded conformations are discussed, including twisting of formal single or double bonds, intramolecular overcrowding, and the significance of a dipolar aromatic "xanthenylium-fluorenide" push-pull structure.

Two contact-point chiral distinction: model CHFClBr dimers.

Dimers of the simple chiral molecule CHFClBr have been studied using a variety of computational approaches, including HF, MP2, and DFT B3LYP and the 6-31G*, 6-31++G**, and 6-311++G** basis sets. Both heterochiral and homochiral dimers were studied to allow analysis of the chiral distinction in these systems. The dimers were arranged in edge-to-edge orientations with assorted combinations of two contact-points ("2:2e") between the dimers. The monomers were constrained to tetrahedral symmetry. We demonstrate that chiral distinction does indeed occur in these two contact-point models. While the stabilization energies are driven by the interactions of the nearest atoms (contacts) in the complexes, the degree of chiral distinction is driven by the profile of changing atoms, which, in the present systems, are often the distal atoms of the complexes. Moreover, the chiral distinction does not correlate with the stabilization energies. The terms contact-points and interactions are defined.

Conformational spaces of Cinchona alkaloids.

A systematic and comprehensive study of the conformational spaces of the Cinchona alkaloids quinine, quinidine, cinchonine, cinchonidine, epiquinine, epiquinidine, epicinchonine, and epicinchonidine using the semiempirical PM3 method is described. The results were analyzed in terms of syn/anti and open/closed/hindered and alpha/beta/gamma conformations. Special emphasis was given to the torsion angles T(1) (C(4a')-C(4')-C(9)-C(8)), T(2) (C(4')-C(9)-C(8)-N(1)) and T(3) (H-O(9)-C(9)-C(8)) that define the backbone and the hydroxy conformation, respectively. The results reveal the quasi-enantiomeric relationships between quinine and quinidine and between epiquinine and epiquinidine, and the main structural differences that exist between the therapeutically active Cinchona alkaloids, quinine and quinidine, and their inactive epimers, epiquinine and epiquinidine. The lowest energy conformation of quinine and quinidine is anti-closed-alpha. The lowest energy conformations of epiquinine and epiquinidine are anti-open-beta and anti-open-alpha, respectively. Low energy conformations with an intramolecular hydrogen bond (N(1.)H(.)O(9)) were found in epiquinine (the global minimum) and epiquinidine, but not in quinine and quinidine.

Stereochemistry of (E)- and (Z)-1,1'-dichlorobifluorenylidenes, substituted overcrowded fullerene fragments.

X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9H-fluoren-9-ylidene (5) are reported. The X-ray molecular structure of (Z)-5 indicated an approximately C2 symmetric conformation with pure twist around C9 = C9' of 40.4 degrees. The fjord regions are somewhat overcrowded: r(C8...C8') = 315.3 pm, r(Cl(1)...Cl(1') = 341.7 pm, r(C(8)...H(8')) = 259.0 pm. The four chlorine atoms of two neighboring molecules of (Z)-5 form a chain. The PM3 calculations showed that the global minimum of 5 is the C2 symmetric twisted conformation t(E)-5, which is 2.4 kJ/mol more stable than its diastereomer C2-t(Z)-5. The corresponding AM1 relative stability is reversed: C2-t(Z)-5 is 1.1 kJ/mol more stable than C2-t(E)-5. The pure twists of t(Z)-5 and t(E)-5 are 37.0 degrees and 37.2 degrees (PM3) and 40.5 degrees and 39.1 degrees (AM1). The corresponding (E) --> (Z) (PM3) and (Z) --> (E) (AM1) energy barriers of diastereomerization are 80.6 kJ/mol (PM3) and 75.8 kJ/mol (AM1). Two anti-folded local minima conformations C2-a(Z)-5 and C(i)-a(E)-5 were found to be 21.2 and 29.5 kJ/mol (PM3) and 25.8 and 35.2 kJ/mol (AM1) less stable than t(E)-5. The syn-folded conformations C(S)-s(Z)-5 and C2-s(E)-5 are transition states for the enantiomerization processes of C2-tz-5 and C2-tE-5, respectively, and lay 79.8 and 94.1 kJ/mol (PM3) and 108.3 and 107.4 kJ/mol (AM1) higher in energy than their corresponding twisted conformations. An alternative pathway for enantiomerization of C2-t(E)-5 via the anti-folded achiral intermediate C(i)-a(E) has a barrier of 56.0 kJ/mol (PM3) and 68.5 (AM1). An alternative pathway for enantiomerization of C2-t(Z)-5 via C2-t(E) and C(i)-a(E) has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.

Two-point contact chiral distinction--a theoretical appraisal.

Ab initio calculations reveal chiral distinction in two-point contact CHFCIBr dimers, with chiral distinction energy of 1.5 kJ mol-1 between the SR and SS dimers fully optimized at the MP2/6-311++G** level.