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We report the high-pressure behavior of two perovskite-like metal formate frameworks with the ethylammonium cation (EtAKCr and EtANaAl) and compare them to previously reported data for EtANaCr. High-pressure single-crystal X-ray diffraction and Raman data for EtAKCr show the occurrence of two high-pressure phase transitions observed at 0.75(16) and 2.4(2) GPa. The first phase transition involves strong compression and distortion of the KO6 subnetwork followed by rearrangement of the −CH2CH3 groups from the ethylammonium cations, while the second involves octahedral tilting to further reduce pore volume, accompanied by further configurational changes of the alkyl chains. Both transitions retain the ambient P21/n symmetry. We also correlate and discuss the influence of structural properties (distortion parameters, bulk modulus, tolerance factors, and compressibility) and parameters calculated by using density functional theory (vibrational entropy, site-projected phonon density of states, and hydrogen bonding energy) on the occurrence and properties of structural phase transitions observed in this class of metal formates.

Metal and nitrogen co-doped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). Here, density functional theory calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modelled as an M-N4 motif embedded in a planar carbon sheet (M= Cr , Mn, Fe , Co). The two-dimensional catalyst structure implies that each metal atom has two potential active sites, one on each side of the sheet. Adsorption of anions or impurities on both sites results in poisoning, but adsorption on one of the sites leads to a modified ORR activity on the remaining site. The calculated adsorption energies show that a number of species adsorb on one of the two sites only under realistic experimental conditions. A few of these adsorbates furthermore modify the adsorption energies of the ORR intermediates on the remaining site in such a way that the limiting potential is improved.

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high-quality crystals with tunable bandgaps in the visible and near-infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin-film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

The solid-electrolyte interphase (SEI) is of crucial importance for the performance of Li-ion batteries. Here, density functional theory (DFT) calculations are used to study the formation of one of the simplest and early appearing components of the SEI layer, namely, LiF, which is produced by splitting HF impurities. The process is investigated on different models representing the basal and edge-planes of a graphitic anode, and on covalently connected carbon nanotubes and graphene sheets, known as pillared graphene. The results show that 2 Li atoms are required to bind F in the final state in order to make the reaction energetically favorable, or alternatively, a H atom must be preadsorbed. The Li adsorption energy, and thereby the Li coverage at a given potential, varies for the different carbon structures, demonstrating that the artificial nanostructure of the carbon can influence the formation of the SEI.

Metal and nitrogen co-doped carbons (M-N/Cs) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). Here, density functional theory calculations are used to compare the ORR activity of Co single and double sites embedded in N-doped carbon. Two different models of a Co double site are investigated, one in which two single sites are stacked on top of each other and one which has two Co atoms next to each other in a single graphene sheet. For both it is found that the ORR can proceed via a dissociative mechanism that splits the ∗OOH intermediate into ∗O and ∗OH, but only for the double site in the graphene sheet does it result in a significant deviation from the scaling relations. The adsorption energies of the ORR intermediates are investigated using different implicit and explicit solvent models, showing some variation in the results. In particular, the addition of explicit water on the same side of the catalyst as the ORR intermediate can result in stabilisation due to hydrogen bonding, while an explicit water molecule adsorbed on the opposite side of the Co atom can have an effect due to the change in coordination which affects the splitting of the Co d orbitals.

We report the high-pressure properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) templated by dimethylammonium (NH2(CH3)2, DMA+) with the general formula [DMA]MI0.5CrIII0.5(HCOO)3, where MI = Na+ (DMANaCr) and K+ (DMAKCr). The high-pressure Raman scattering studies show crystal instabilities in the 4.0-4.4 GPa and 2.0-2.5 GPa ranges for DMANaCr and DMAKCr, respectively. The mechanism is similar in the two compounds and involves strong deformation of the metal-formate framework, especially pronounced for the subnetwork of CrO6 octahedra, accompanied by substantial compressibility of the DMA+ cations. Comparison with previous high-pressure Raman studies of sodium-chromium heterometallic MOFs show that the stability depends on the templated cation and increases as follows: ammonium < imidazolium < DMA+. Density functional theory (DFT) calculations are performed to get a better understanding of the structural properties leading to the existence of phase transitions. We calculate the energy of the hydrogen bonds (HBs) between the DMA+ cation and the metal formate cage, revealing a stronger interaction in the DMAKCr compound due to a HB arrangement that primarily involves the energetically preferred bonding to KO6 octahedra. This material however also has a smaller structural tolerance factor (TF) and a higher vibrational entropy than DMANaCr. This indicates a more flexible crystal structure, explaining the lower phase transition pressure, as well as the previously observed phase transition at 190 K, which is absent in the DMANaCr compound. The DFT high-pressure simulations show the largest contraction to be along the trigonal axis, leading to a minimal distortion of the HBs formed between the DMA+ cations and the metal-formate sublattice.

The energetics and electronic structure of defects in the metal–organic framework UiO-66 is investigated using density functional theory., Vacancy lattice sites in the metal–organic framework UiO-66 are known to have a profound effect on the material properties. Here we use density functional theory to compare the energies of defect arrangements containing missing linkers and missing metal clusters for different choices of charge compensation. Our results show that the preference for missing metal clusters or missing linker defects depends on the charge compensation as well as the overall concentration of defects in the crystal. Both regimes can be experimentally accessible depending on the synthesis conditions. We investigate the electronic structure of the different types of defects, showing that, despite some changes in the localisation of the frontier orbitals, the electronic energy levels are only weakly affected by the presence of point defects.

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li2IrO3 which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts., Renewable hydrogen production from water will require understanding and improving the oxygen evolution reaction (OER) on catalyst surfaces. Here, authors report α-Li2IrO3 to transform into a hydrated birnessite phase under OER conditions that exhibits enhanced OER performances and durabilities.

Metal and nitrogen codoped carbons (M-N/Cs) have emerged as promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). DFT calculations are used to investigate the adsorption of anions and impurities from the electrolyte on the active site, modeled as an M-N

We report a comprehensive model surface science study, using scanning tunneling microscopy (STM), of the regioselective and stereospecific complexation of a prochiral substrate molecule to a chiral modifier molecule on a metal surface. The system is chosen so as to compare the prochiral ratio (pr) measured directly by STM for the model system with the reported enantiomeric ratio (er) for the hydrogenation of the prochiral molecule using the same chiral modifier and metal under true reaction conditions. Specifically, diastereomeric complexes formed between ketopantolactone (KPL) and (R)-1-(1-naphthyl)ethylamine ((R)-NEA) on Pt(111) were studied as a function of the ratio of KPL to (R)-NEA. Only KPL molecules in complexes are detected in the STM experiments, performed between 237 and 250 K, due to rapid diffusion of free KPL on the surface. The prochirality of KPL in almost all bimolecular and termolecular, (KPL)2/(R)-NEA, complexation configurations is assigned as pro-R or pro-S using the contrast within t...

We report the high-pressure structure-related properties of a ferroelectric heterometallic formate perovskite framework templated by the ethylammonium cation (CH3CH2NH3+, EtA+). High-pressure X-ray diffraction studies show a first-order structural phase transition at 3.6(2) GPa from the polar Pn ambient phase to a centrosymmetric P21/n high-pressure phase. A high-pressure Raman scattering experiment indicates the same transition in the 4.0–4.4 GPa range. The mechanism of the phase transition involves strong compression and distortion of the NaO6 subnetwork and a decrease in the space available for the accommodated EtA+ cations, resulting in a change in their configuration within the pores at 3.7 GPa. Using density functional theory the value of the ferroelectric polarisation within the ac plane is calculated to be 0.9 μC cm−2 at ambient pressure, increasing in magnitude to a value of 1.1 μC cm−2 at a pressure of 3 GPa before vanishing at the phase transition.

The enormous research efforts dedicated to hybrid organic−inorganic perovskites have led to a deep understanding of these materials; however, the role of entropy and its ramifications for the properties of the materials havebeen only sparsely explored. In this study, we quantify the phase transition mechanism in the hybrid organic−inorganic perovskite [CH3NH3]PbBr3 by studying low-energy collective phonon modes using a combination of inelastic neutron scattering and ab initio lattice dynamics. We demonstrate that a delicate interplay among hydrogen bonding interactions, lattice vibrational entropy, and configurational disorder determines the thermodynamics and results in the richphase evolution of [CH3NH3]PbBr3 as a function of temperature. Our results have important implications for the manipulation of macroscopic properties and provide a blueprint for future studies that will focus on unravelling phase transition mechanisms in hybrid perovskites and related materials such asdense and porous coordination polymers.

We present an extended metal-coordinated structure obtained by deposition of trimesic acid (TMA) onto the Ag(111) surface under ultra-high vacuum conditions followed by annealing to 510 K. Scanning tunneling microscopy and density functional theory calculations reveal the structure to consist of metal clusters containing seven Ag atoms each, coordinated by six dehydrogenated TMA molecules. The molecules are asymmetrically arranged, resulting in a chiral structure. The calculations confirm that this structure has a lower free energy under the experimental conditions than the hydrogen-bonded structures observed after annealing at lower temperatures. We show that the formation of such large metal clusters is possible due to the low adatom formation energy on silver and the relatively strong Ag-O bond in combination with a good lattice match between the structure and the Ag surface.

We present an extended metal-coordinated structure obtained by deposition of trimesic acid (TMA) onto the Ag(111) surface under ultra-high vacuum conditions followed by annealing to 510 K. Scanning tunneling microscopy and density functional theory calculations reveal the structure to consist of metal clusters containing seven Ag atoms each, coordinated by six dehydrogenated TMA molecules. The molecules are asymmetrically arranged, resulting in a chiral structure. The calculations confirm that this structure has a lower free energy under the experimental conditions than the hydrogen-bonded structures observed after annealing at lower temperatures. We show that the formation of such large metal clusters is possible due to the low adatom formation energy on silver and the relatively strong Ag-O bond in combination with a good lattice match between the structure and the Ag surface. Published by AIP Publishing.

We report the synthesis, crystal structure, vibrational and luminescence properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) containing the ammonium cation (NH4+, Am+): [NH4][Na0.5Cr0.5(HCOO)3] (AmNaCr) and [NH4][Na0.5Al0.475Cr0.025(HCOO)3] (AmNaAlCr) in comparison to the previously reported [NH4][Na0.5Al0.5(HCOO)3] (AmNaAl). The room-temperature crystal structure of AmNaCr and AmNaAlCr was determined to be R3[combining macron]. The hydrogen bonding (HB) energy calculated using density functional theory (DFT) agrees well with experimental data, and confirms the existence of almost identical H-bonding in AmNaCr and AmNaAl, with three short hydrogen bonds and a longer trifurcated H-bond. Temperature-dependent Raman measurements supported by differential scanning calorimetry show that AmNaCr does not undergo any structural phase transitions in the 80-400 K temperature range. The high-pressure Raman spectra of AmNaCr show the onset of two structural instabilities near 0.5 and 1.5 GPa. The first instability involves weak distortion of the framework, while the second leads to irreversible amorphization of the sample. High-pressure DFT simulations show that the unit cell of the AmNaCr compound contracts along the c axis, which leads to a shortening of the trifurcated H-bond. The optical properties show that both studied crystals exhibit Cr3+-based emission characteristic of intermediate ligand field strength.

Hybrid organic–inorganic perovskites represent a special class of metal–organic framework where a molecular cation is encased in an anionic cage. The molecule–cage interaction influences phase stability, phase transformations, and the molecular dynamics. We examine the hydrogen bonding in four AmBX3 formate perovskites: [Am]Zn(HCOO)3, with Am+ = hydrazinium (NH2NH3+), guanidinium (C(NH2)3+), dimethylammonium (CH3)2NH2+, and azetidinium (CH2)3NH2+. We develop a scheme to quantify the strength of hydrogen bonding in these systems from first-principles, which separates the electrostatic interactions between the amine (Am+) and the BX3– cage. The hydrogen-bonding strengths of formate perovskites range from 0.36 to 1.40 eV/cation (8–32 kcalmol–1). Complementary solid-state nuclear magnetic resonance spectroscopy confirms that strong hydrogen bonding hinders cation mobility. Application of the procedure to hybrid lead halide perovskites (X = Cl, Br, I, Am+ = CH3NH3+, CH(NH2)2+) shows that these compounds have significantly weaker hydrogen-bonding energies of 0.09 to 0.27 eV/cation (2–6 kcalmol–1), correlating with lower order–disorder transition temperatures.

The crystallography of mechanically soft materials such as hybrid organic–inorganic compounds often reveals large thermal displacement factors and partially occupied lattice sites, which can arise from static or dynamic disorder. A combination of ab initio lattice dynamics and molecular dynamics simulations reveals the origin of the giant thermal displacements in the biphenyl-4,4′-dicarboxylate (BPDC) linker in the metal–organic framework UiO-67. The dihedral angle between the two phenyl rings has two equivalent minima at ±31°, which cannot be described by harmonic phonons. Instead, anharmonic switching between the minima results in the experimentally observed large thermal ellipsoids. The switching frequency is found to be similar in the topologically distinct framework IRMOF-10, suggesting that dynamic disorder is a general feature of MOFs based on BPDC and structurally similar linkers.

To build on the success of other mineral systems employed in solar cells, including kesterites (Cu2ZnSnS4) and herzenbergite (SnS), as well as mineral-inspired systems such as lead halide perovskites (CH3NH3PbI3), we have searched for photoactive minerals with the additional constraint that a polar crystal structure is adopted. Macroscopic electric fields provide a driving force to separate electrons and holes in semiconductor devices, while spontaneous lattice polarisation in polar semiconductors can facilitate microscopic photo-carrier separation to enhance carrier stability and lifetimes. We identify enargite (Cu3AsS4), stephanite (Ag5SbS4), and bournonite (CuPbSbS3) as candidate materials and explore their chemical bonding and physical properties using a first-principles quantum mechanical approach.

Mittels Koadsorption von Metallatomen und orga- nischen Liganden hergestellte Koordinationsnetzwerke haben interessante Eigenschaften, z. B. in der Katalyse oder fr die Datenspeicherung. Bisher basierten solche Oberflchen-Netz- werke auf Koordination an einzelnen Metallatomen. Hier be- schreiben wir hingegen die Bildung eines neuartigen Metall- organischen Netzwerkes, das auf Kupfertrimeren als Noden basiert. Dieses Netzwerk entsteht nach dem Aufdampfen und anschliesendem Heizen von Tetrahydroxybenzol (THB) auf Cu(111) im Ultrahochvakuum. Mittels STM, XPS und DFT- Rechnungen konnte gezeigt werden, dass bei einer Temperatur von 440 K alle vier Hydroxygruppen des THB dehydriert werden. Der dabei entstehende, hochreaktive Ligand bindet anschliesend an Cu-Trimere, die in hochaufgelcsten STM- Aufnahmen sichtbar sind. Mit den gleichm�sig angeordneten und monodispersen Cu-Trimeren stellt das hier vorgestellte Netzwerk ein zweidimensionales Analogon zu Metall-organi- schen Gersten dar, die in der Regel ebenfalls komplexere Noden als einzelne Metallatome aufweisen.

Thin-film deposition of metal–organic frameworks (MOFs) is now possible, but little is known regarding the microscopic nature of hybrid hetero-interfaces. We first assess optimal substrate combinations for coherent epitaxy of MOFs based on a lattice matching procedure. We then perform a detailed quantum mechanical/molecular mechanical investigation of the growth of (011) MOF-5 on (110) rutile TiO2. The lowest energy interface configuration involves a bidentate connection between two TiO6 polyhedra with deprotonation of terephthalic acid to a bridging oxide site. The epitaxy of MOF-5 on the surface of TiO2 was modelled with a forcefield parameterised to quantum chemical binding energies and bond lengths. The microscopic interface structure and chemical bonding characteristics are expected to be relevant to other hybrid framework-oxide combinations.

The modelling of materials properties and processes from first principles is becoming sufficiently accurate as to facilitate the design and testing of new systems in silico. Computational materials science is both valuable and increasingly necessary for developing novel functional materials and composites that meet the requirements of next-generation technology. A range of simulation techniques are being developed and applied to problems related to materials for energy generation, storage and conversion including solar cells, nuclear reactors, batteries, fuel cells, and catalytic systems. Such techniques may combine crystal-structure prediction (global optimisation), data mining (materials informatics) and high-throughput screening with elements of machine learning. We explore the development process associated with computational materials design, from setting the requirements and descriptors to the development and testing of new materials. As a case study, we critically review progress in the fields of thermoelectrics and photovoltaics, including the simulation of lattice thermal conductivity and the search for Pb-free hybrid halide perovskites. Finally, a number of universal chemical-design principles are advanced.

Self-assembly of a molecule with many distinct conformational states, resulting in eight possible pairs of surface enantiomers, is investigated on a Au(111) surface under UHV conditions. The complex molecule is equipped with alkyl and carboxyl moieties to promote controlled self-assembly of lamellae structures. From statistical analysis of Scanning Tunnelling Microscopy (STM) data we observe a clear selection of specific conformational states after self-assembly. Using Density Functional Theory (DFT) calculations we rationalise how this selection is correlated to the orientation of the alkyl moieties in mirror-image domains of the lamellae structures, leading to selection of three out of the eight possible enantiomeric pairs.

Entropy is a critical, but often overlooked, factor in determining the relative stabilities of crystal phases. The importance of entropy is most pronounced in softer materials, where small changes in free energy can drive phase transitions, which has recently been demonstrated in the case of organic-inorganic hybrid-formate perovskites. In this Rapid Communication we demonstrate the interplay between composition and crystal structure that is responsible for the particularly pronounced role of entropy in determining polymorphism in hybrid organic-inorganic materials. Using ab initio based lattice dynamics, we probe the origins and effects of vibrational entropy of four archetype perovskite $(AB{X}_{3})$ structures. We consider an inorganic material $({\mathrm{SrTiO}}_{3})$, an $A$-site hybrid-halide material $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}){\mathrm{PbI}}_{3}$, a $X$-site hybrid material $\mathrm{KSr}{({\mathrm{BH}}_{4})}_{3}$, and a mixed $A$- and $X$-site hybrid-formate material $({\mathrm{N}}_{2}{\mathrm{H}}_{5})\mathrm{Zn}{({\mathrm{HCO}}_{2})}_{3}$, comparing the differences in entropy between two common polymorphs. The results demonstrate the importance of low-frequency intermolecular modes in determining the phase stability in these materials. The understanding gained allows us to propose a general principle for the relative stability of different polymorphs of hybrid materials as temperature is increased.

We present a comprehensive theoretical investigation of the structures formed by self-assembly of tetrahydroxybenzene (THB)-derivatives on Cu(111). The THB molecule is known to dehydrogenate completely during annealing, forming a reactive radical which assembles into a close-packed structure or a porous metal-coordinated network depending on the coverage of the system. Here, we present details on how the structures are determined by density functional theory calculations, using scanning tunneling microscopy-derived information on the periodicity. The porous network is based on adatom trimers. By analysing the charge distribution of the structure, it is found that this unusual coordination motif is preferred because it simultaneously provides a good coordination of all oxygen atoms and allows for the formation of a two-dimensional network on the surface. (C) 2016 AIP Publishing LLC.

We report the development of a forcefield capable of reproducing accurate lattice dynamics of metal-organic frameworks. Phonon spectra, thermodynamic and mechanical properties, such as free energies, heat capacities and bulk moduli, are calculated using the quasi-harmonic approximation to account for anharmonic behaviour due to thermal expansion. Comparison to density functional theory calculations of properties such as Gr$\mathrm{\ddot{u}}$neisen parameters, bulk moduli and thermal expansion supports the accuracy of the derived forcefield model. Material properties are also reported in a full analysis of the lattice dynamics of an initial subset of structures including: MOF-5, IRMOF-10, UiO-66, UiO-67, NOTT-300, MIL-125, MOF-74 and MOF-650., Comment: 10 figures, 17 pages

Supra-molecular self-assembly on surfaces often involves molecular conformational flexibility which may act to enrich the variation and complexity of the structures formed. However, systematic and explicit investigations of how molecular conformational states are selected in surface self-assembly processes are relatively scarce. Here, we use a combination of high-resolution scanning tunneling microscopy and Density Functional Theory (DFT) calculations to investigate self-assembly for a custom-designed molecule capable of assuming eight distinct surface conformations (four enantiomeric pairs). The conformations result from binary positions of n = 3 naphtalene units on a linear oligo(naphthylene-ethynylene) backbone. On Au(111), inter-molecular interactions involving carboxyl and bulky tert-butyl-phenyl functional groups induce the molecules to form two ordered phases with brick-wall and lamella structure, respectively. These structures each involve molecules in two conformational states, and there is a clear separation between the conformers involved in the two types of structures. On Cu(111), individual molecules isolated by carboxylate-substrate binding show a distribution involving all possible conformational states. Together these observations imply selection and adaptation of conformational states upon molecular self-assembly. From DFT modeling and statistical analysis of the molecular conformations, the observed selection of conformational states is attributed to steric interaction between the naphthalene units. The present study enhances our understanding of how ordering and selection of molecular conformations is controlled by intermolecular interactions in a complex situation with many distinct conformational states for the participating molecules.

Proteins fold into three-dimensional structures, which determine their diverse functions. The conformation of the backbone of each structure is locally at each Cα effectively described by conformational angles resulting in Ramachandran plots. These, however, do not describe the conformations around hydrogen bonds, which can be non-local along the backbone and are of major importance for protein structure. Here, we introduce the spatial rotation between hydrogen bonded peptide planes as a new descriptor for protein structure locally around a hydrogen bond. Strikingly, this rotational descriptor sampled over high-quality structures from the protein data base (PDB) concentrates into 30 localized clusters, some of which correlate to the common secondary structures and others to more special motifs, yet generally providing a unifying systematic classification of local structure around protein hydrogen bonds. It further provides a uniform vocabulary for comparison of protein structure near hydrogen bonds even between bonds in different proteins without alignment.

The formation of bimolecular complexes on metal surfaces through interaction between a single chemisorbed chiral molecule and a single chemisorbed prochiral substrate molecule can be considered as a preorganization step toward chirality transfer. In the case of asymmetric hydrogenation on chirally modified platinum catalysts, the metal surface dissociates H-2 and provides atomic hydrogen for the desymmetrization step. Along the reaction path, the combined chemisorption and intermolecular interactions in the assembly formed between the modifier and the substrate determine which enantiomer is formed in excess. In this study, we use DFT calculations and STM measurements to describe chemisorption and intermolecular interactions in isolable structures formed between single ketopantolactone and single (R)-1-(1-naphthyl)ethylamine molecules on Pt(111). The study reveals several distinct complexation geometries at the sub-molecular level as well as the stereodirecting forces operating in the most abundant bimolecular assemblies. The comparison of theoretical and experimental data strongly suggests that partial hydrogenation of KPL occurs under the experimental conditions and that some of the most abundant complexes are formed by the hydroxy intermediate.

Proteins fold into three-dimensional structures, which determine their diverse functions. The conformation of the backbone of each structure is locally at each C(α) effectively described by conformational angles resulting in Ramachandran plots. These, however, do not describe the conformations around hydrogen bonds, which can be non-local along the backbone and are of major importance for protein structure. Here, we introduce the spatial rotation between hydrogen bonded peptide planes as a new descriptor for protein structure locally around a hydrogen bond. Strikingly, this rotational descriptor sampled over high-quality structures from the protein data base (PDB) concentrates into 30 localized clusters, some of which correlate to the common secondary structures and others to more special motifs, yet generally providing a unifying systematic classification of local structure around protein hydrogen bonds. It further provides a uniform vocabulary for comparison of protein structure near hydrogen bonds even between bonds in different proteins without alignment.

Surface coordination networks formed by co- adsorption of metal atoms and organic ligands have interesting properties, for example regarding catalysis and data storage. Surface coordination networks studied to date have typically been based on single metal atom centers. The formation of a novel surface coordination network is now demonstrated that is based on network nodes in the form of clusters consisting of three Cu adatoms. The network forms by deposition of tetrahydroxybenzene (THB) on Cu(111) under UHV condi- tions. As shown from a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations, all four hydroxy groups of THB dehydrogenate upon thermal activation at 440 K. This highly reactive ligand binds to Cu adatom trimers, which are resolved by high-resolution STM. The network creates an ordered array of mono-dispersed metal clusters constituting a two-dimen- sional analogue of metal-organic frameworks.

The reaction free energy for dehydrogenation of phenol, aniline, thiophenol, benzoic acid, and 1,4-benzenediol on the close packed copper, silver, and gold surfaces has been studied by density functional theory calculations. Dehydrogenation of thiophenol is found to be favourable on all three surfaces while aniline does not dehydrogenate on any of them. For phenol, benzenediol and benzoic acid dehydrogenation is favourable on copper and silver only, following the general trend of an increasing reaction free energy when going form gold to silver to copper. This trend can be correlated with the changes in bond lengths within the molecule upon dehydrogenation. While copper is able to replace hydrogen, leaving small changes in the bond lengths of the aromatic ring, the metal-molecule bond is weaker for silver and gold, resulting in a partial loss of aromaticity. This difference in bond strength leads to pronounced differences in adsorption geometries upon multiple dehydrogenations.

Adsorption of tetrahydroxybenzene (THB) on Cu(111) and Au(111) surfaces is studied using a combination of STM, XPS, and DFT. THB is deposited intact, but on Cu(111) it undergoes gradual dehydrogenation of the hydroxyl groups as a function of substrate temperature, yielding a pure dihydroxy-benzoquinone phase at 370 K. Subtle changes to the adsorption structure upon dehydrogenation are explained from differences in molecule-surface bonding.