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 Zn(NO)(OH) were cathodically electrodeposited on germanium(100) electrodes from a zinc nitrate precursor electrolyte.

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.

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.

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

On page 4877, F. U. Renner, A.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

Inversion Barrier of Corannulene. A Benchmark for Bowl-to-Bowl Inversions in Fullerene Fragments.

In a systematic ab initio study, the Hartree-Fock, B3LYP density functional, and MP2 methods are employed to calculate the bowl-to-bowl inversion barrier of corannulene (1). Basis sets ranging from a minimal basis (STO-3G) to a double polarized valence triple-zeta basis (6-311G(2d,2p)) were used. In comparison with experimental data, it was found that inclusion of dynamic electron correlation (e.

Molecular Structure Parameters and Predictions of Enthalpies of Formation for Catacondensed and Pericondensed Polycyclic Aromatic Hydrocarbons.

Simple procedures that combine calculated ab initio theoretical energies with empirical structural parameters to correlate experimental enthalpies of formation for polycyclic aromatic hydrocarbons are evaluated for predictive potential. The analyzed data set consists of every benzenoid PAH with an experimentally determined DeltaH(f) degrees (g), i.e.

Fulvalenes, Fulvenes, and Related Molecules: An ab Initio Study.

Ab initio calculations using conventional (HF/6-31G and MP2/6-31G) and density functional theory (B-LYP/6-31G) methods have been used to determine the structures of the [n]fulvene and [n,m]fulvalene (n, m = 3, 5, 7) series of molecules, with particular emphasis on heptafulvalene (n = m = 7: 12). Calculations have also been performed on the parent cycloalkenes: cyclopropene, cyclopentadiene, and cycloheptatriene (1-3, respectively). All the fulvenes (n = 3, 5, 7: 4-6, respectively) and the smaller fulvalenes (n = 3, m= 3, 5, 7: 7-9, respectively, and n = m = 5: 10) are found to be planar.