Showing papers on "Chemical bond published in 2005"
TL;DR: Covalent organic frameworks (COFs) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid and hexahydroxytriphenylene to form rigid porous architectures with pore sizes ranging from 7 to 27 angstroms.
Abstract: Covalent organic frameworks (COFs) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid {C6H4[B(OH)2]2} and hexahydroxytriphenylene [C18H6(OH)6]. Powder x-ray diffraction studies of the highly crystalline products (C3H2BO)6.(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed expanded porous graphitic layers that are either staggered (COF-1, P6(3)/mmc) or eclipsed (COF-5, P6/mmm). Their crystal structures are entirely held by strong bonds between B, C, and O atoms to form rigid porous architectures with pore sizes ranging from 7 to 27 angstroms. COF-1 and COF-5 exhibit high thermal stability (to temperatures up to 500 degrees to 600 degrees C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).
4,843 citations
TL;DR: In this article, the authors explored the possibility of the existence of a common scale, which can be used to gauge bond strength between various surfaces and found that the changes in wettability of surfaces owing to various levels of plasma exposure can be a useful parameter to gauge the bond strength.
Abstract: An issue in microfabrication of the fluidic channels in glass/poly (dimethyl siloxane) (PDMS) is the absence of a well-defined study of the bonding strength between the surfaces making up these channels. Although most of the research papers mention the use of oxygen plasma for developing chemical (siloxane) bonds between the participating surfaces, yet they only define a certain set of parameters, tailored to a specific setup. An important requirement of all the microfluidics/biosensors industry is the development of a general regime, which defines a systematic method of gauging the bond strength between the participating surfaces in advance by correlation to a common parameter. This enhances the reliability of the devices and also gives a structured approach to its future large-scale manufacturing. In this paper, we explore the possibility of the existence of a common scale, which can be used to gauge bond strength between various surfaces. We find that the changes in wettability of surfaces owing to various levels of plasma exposure can be a useful parameter to gauge the bond strength. We obtained a good correlation between contact angle of deionized water (a direct measure of wettability) on the PDMS and glass surfaces based on various dosages or oxygen plasma treatment. The exposure was done first in an inductively coupled high-density (ICP) plasma system and then in plasma enhanced chemical vapor deposition (PECVD) system. This was followed by the measurement of bond strength by use or the standardized blister test.
825 citations
TL;DR: The QTAM scheme is intimately linked with the atomistic picture of the chemical bond, not only allowing the separation of different two-body contributions to the interaction between a pair of atoms but also including an effective many-body contribution to the binding due to the deformation of the atoms within the many-electron system as compared to the free atoms.
Abstract: We make use of the Quantum Theory of Atoms in Molecules (QTAM) to partition the total energy of a many-electron system into intra- and interatomic terms, by explicitly computing both the one- and two-electron contributions. While the general scheme is formally equivalent to that by Bader et al., we focus on the separation and computation of the atomic self-energies and all the interaction terms. The partition is ultimately performed within the density matrices, in analogy with McWeeny's Theory of Electronic Separability, and then carried onto the energy. It is intimately linked with the atomistic picture of the chemical bond, not only allowing the separation of different two-body contributions (point-charge-like, multipolar, total Coulomb, exchange, correlation, ...) to the interaction between a pair of atoms but also including an effective many-body contribution to the binding (self-energy, formally one-body) due to the deformation of the atoms within the many-electron system as compared to the free atoms. Many qualitative ideas about the chemical bond can be quantified using this scheme.
584 citations
TL;DR: It is reported that the Kondo effect exerted by a magnetic ion depends on its chemical environment and is attributed to the small on-site Coulomb repulsion and the large half-width of the hybridized d-level.
Abstract: We report that the Kondo effect exerted by a magnetic ion depends on its chemical environment. A cobalt phthalocyanine molecule adsorbed on an Au(111) surface exhibited no Kondo effect. Cutting away eight hydrogen atoms from the molecule with voltage pulses from a scanning tunneling microscope tip allowed the four orbitals of this molecule to chemically bond to the gold substrate. The localized spin was recovered in this artificial molecular structure, and a clear Kondo resonance was observed near the Fermi surface. We attribute the high Kondo temperature (more than 200 kelvin) to the small on-site Coulomb repulsion and the large half-width of the hybridized d-level.
565 citations
TL;DR: In this paper, a discussion of the complementary nature of MO theory and QTAIM is presented, together with the means of bridging the two approaches in a discussion about the bonding in the carbonyl complexes of Cr, Fe and Ni and the metallocene complexes of Fe, Al+ and Ge.
439 citations
TL;DR: Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture of the chemisorption bond was given in this paper.
Abstract: Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure.
327 citations
TL;DR: Although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique, and the molecule contains three electron-pair bonds and four one-electron bonds, and two ferromagnetically coupled electrons localized on one U atom each—so all known covalent bonding types are contributing.
Abstract: Covalent bonding is commonly described by Lewis's theory1, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description2 for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for actinide–actinide bonds is restricted to the matrix-isolation3 of uranium hydrides, including H2U–UH2, and the gas-phase detection4 and preliminary theoretical study5 of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each—so all known covalent bonding types are contributing.
289 citations
TL;DR: In this paper, the pi-stacking interaction between planar organic molecules is investigated within the framework of ab initio calculations, and the adsorption of these molecules on the sidewall of the cylindrical carbon structure induces a small binding energy compared to conventional covalent functionalization.
Abstract: The pi-stacking interaction between various planar organic molecules is investigated within the framework of ab initio calculations. The adsorption of these molecules on the sidewall of the cylindrical carbon structure induces a small binding energy compared to conventional covalent functionalization. Such a weak interaction is found to be only physisorption and leads to minor and predictable modifications of the electronic structure. These changes in the electronic behavior of the host carbon nanotube are ruled by the relative positions of the molecular levels of the isolated molecule and both the valence and conduction bands of the perfect tube.
282 citations
TL;DR: In this article, the interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes SWNT was investigated using first-principles plane wave method, and it was found that the binding energy of the H2 physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic.
Abstract: Interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes SWNT is investigated using first-principles plane wave method. It is found that the binding energy of the H2 physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic. Although the bonding is strengthened upon adsorption directly to the Li atom, its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of a Pt atom. A single H2 is chemisorbed to the Pt atom on the SWNT either dissociatively or molecularly. The dissociative adsorption is favorable energetically and is followed by the weakening of the Pt-SWNT bond. Out of two adsorbed H2, the first one can be adsorbed dissociatively and the second one is chemisorbed molecularly. The nature of bonding is a very weak physisorption for the third adsorbed H2. Palladium also promotes the chemisorption of H2 with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and these results advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.
264 citations
TL;DR: The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces and finds no chemical bonding due to ionic or covalent interactions.
Abstract: The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.
260 citations
TL;DR: In this paper, the adsorption of a benzene molecule on carbon nanotubes (CNTs) with various diameters and chiral angles is investigated within the ab initio framework.
Abstract: The adsorption of a benzene molecule on carbon nanotubes (CNTs) with various diameters and chiral angles is investigated within the ab initio framework. The physisorption of such an organic molecule is an example of noncovalent functionalization involving pi-stacking interactions and corresponding to a weak binding energy. Our calculations show that for small diameter tubes, the most favorable adsorption site is one type of C-C bond. The disparities between the inequivalent bonds of a CNT are discussed in terms of the pi orbital axis vector misalignment. Moreover, the curvature and the chirality effect on benzene adsorption are analyzed, showing that large diameter nanotubes are the most reactive ones.
TL;DR: Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration.
Abstract: Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are $0.18\ifmmode\pm\else\textpm\fi{}0.03\text{ }\AA{}$ closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled---through charge transfer into the former lowest unoccupied molecular orbital---to the primary, extended chemisorption bond via the perylene skeleton.
TL;DR: Valence bond theory and electron-localization function (ELF) calculations demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair density plays a dominant role.
Abstract: This paper deals with a cen- tral paradigm of chemistry, the elec- tron-pair bond. Valence bond (VB) theory and electron-localization func- tion (ELF) calculations of 21 single bonds demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair den- sity plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RECS, and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contribu- tions to the bond energy. The paper de- fines the atomic propensity and territo- ry for CS bonding: Atoms (fragments) that are prone to CS bonding are com- pact electronegative and/or lone-pair- rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and in- volves: a) homopolar bonds of heteroa- toms with zero static ionicity, b) heter- opolar s and p bonds of the electro- negative and/or electron-pair-rich ele- ments among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental mani- festations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reac- tions as compared to hydrogen-trans- fers.
TL;DR: Two imido analogs of the uranyl ion, UO2+2, are synthesised, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U (NPh)2 I2( THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran).
Abstract: Here we describe the synthesis of two imido analogs of the uranyl ion, UO2+2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.
TL;DR: In this article, the authors summarize recent theoretical studies of Fischer and Schrock-type carbene complexes and show that with the help of charge and energy partitioning techniques, it is possible to build a bridge between heuristic bonding models and the physical mechanism which leads to a chemical bond.
TL;DR: A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides by first-principles methods shows that in these materials the chemical bonding is strongly covalent.
Abstract: A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM=Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced...
TL;DR: In this paper, a comparative study on the structure and stability of oxygen defects in ZnO is presented by means of first-principles calculations based on local density functional theory.
Abstract: A comparative study on the structure and stability of oxygen defects in ZnO is presented. By means of first-principles calculations based on local density functional theory we investigate the oxygen vacancy and different interstitial configurations of oxygen in various charge states. Our results reveal that dumbbell-like structures are thermodynamically the most stable interstitial configurations for neutral and positive charge states due to the formation of a strongly covalent oxygen--oxygen bond. For negative charge states the system prefers a split-interstitial configuration with two oxygen atoms in almost symmetric positions with respect to the associated perfect lattice site. The calculated defect formation energies imply that interstitial oxygen atoms may provide both donor- and acceptor-like defects.
TL;DR: The adsorption of phenylthiol on the Au(111) surface is modeled using Perdew and Wang density-functional calculations, indicating that, at all sulfur locations, the adsorbate can be regarded as a thiyl species that forms a net single covalent bond to the surface of strength 31 kcal mol(-1).
Abstract: The adsorption of phenylthiol on the Au(111) surface is modeled using Perdew and Wang density-functional calculations. Both direct molecular physisorption and dissociative chemisorption via S–H bond cleavage are considered as well as dimerization to form disulfides. For the major observed product, the chemisorbed thiol, an extensive potential-energy surface is produced as a function of both the azimuthal orientation of the adsorbate and the linear translation of the adsorbate through the key fcc, hcp, bridge, and top binding sites. Key structures are characterized, the lowest-energy one being a broad minimum of tilted orientation ranging from the bridge structure halfway towards the fcc one. The vertically oriented threefold binding sites, often assumed to dominate molecular electronics measurements, are identified as transition states at low coverage but become favored in dense monolayers. A similar surface is also produced for chemisorption of phenylthiol on Ag(111); this displays significant qualitativ...
TL;DR: It is shown that nonpolar bonds between main-group elements of the first and higher octal rows of the periodic system, which are prototypical covalent bonds, have large attractive contributions from classical electrostatic interactions, which may even be stronger than the attractive orbital interactions.
Abstract: The nature of the chemical bond in nonpolar molecules has been investigated by energy-partitioning analysis (EPA) of the ADF program using DFT calculations. The EPA divides the bonding interactions into three major components, that is, the repulsive Pauli term, quasiclassical electrostatic interactions, and orbital interactions. The electrostatic and orbital terms are used to define the nature of the chemical bond. It is shown that nonpolar bonds between main-group elements of the first and higher octal rows of the periodic system, which are prototypical covalent bonds, have large attractive contributions from classical electrostatic interactions, which may even be stronger than the attractive orbital interactions. Fragments of molecules with totally symmetrical electron-density distributions, like the nitrogen atoms in N(2), may strongly attract each other through classical electrostatic forces, which constitute 30.0 % of the total attractive interactions. The electrostatic attraction can be enhanced by anisotropic charge distribution of the valence electrons of the atoms that have local areas of (negative) charge concentration. It is shown that the use of atomic partial charges in the analysis of the nature of the interatomic interactions may be misleading because they do not reveal the topography of the electronic charge distribution. Besides dinitrogen, four groups of molecules have been studied. The attractive binding interactions in H(n)E-EH(n) (E=Li to F; n=0-3) have between 20.7 (E=F) and 58.4 % (E=Be) electrostatic character. The substitution of hydrogen by fluorine does not lead to significant changes in the nature of the binding interactions in F(n)E-EF(n) (E=Be to O). The electrostatic contributions to the attractive interactions in F(n)E-EF(n) are between 29.8 (E=O) and 55.3 % (E=Be). The fluorine substituents have a significant effect on the Pauli repulsion in the nitrogen and oxygen compounds. This explains why F(2)N-NF(2) has a much weaker bond than H(2)N-NH(2), whereas the interaction energy in FO-OF is much stronger than in HO-OH. The orbital interactions make larger contributions to the double bonds in HB=BH, H(2)C=CH(2), and HN=NH (between 59.9 % in B(2)H(2) and 65.4 % in N(2)H(2)) than to the corresponding single bonds in H(n)E-EH(n). The orbital term Delta E(orb) (72.4 %) makes an even greater contribution to the HC triple bond CH triple bond. The contribution of Delta E(orb) to the H(n)E=EH(n) bond increases and the relative contribution of the pi bonding decreases as E becomes more electronegative. The pi-bonding interactions in HC triple bond CH amount to 44.4 % of the total orbital interactions. The interaction energy in H(3)E-EH(3) (E=C to Pb) decreases monotonically as the element E becomes heavier. The electrostatic contributions to the E-E bond increases from E=C (41.4 %) to E=Sn (55.1 %) but then decreases when E=Pb (51.7 %). A true understanding of the strength and trends of the chemical bonds can only be achieved when the Pauli repulsion is considered. In an absolute sense the repulsive Delta E(Pauli) term is in most cases the largest term in the EPA.
TL;DR: The abrupt modulation of the extent of slippage in the [Ni(mnt)(2)](-) complexes by external perturbations provided new possibilities for the design of molecular magnetic switching devices.
Abstract: Magnetic switchable maleonitriledithiolate (mnt) complexes were studied by density functional theory. The calculations were performed for anion dimers of [RBzPyR‘][Ni(mnt)2] (RBzPyR‘ = derivatives of benzylpyridinium) to elucidate magnetostructural correlations and the nature of the weak intermolecular chemical bonding. The calculated results showed that the spin delocalization, favored by the eclipsed stacking and the shorter interlayer distance, was responsible for the diamagnetic character of [1-benzyl-4-aminopyridinium][Ni(mnt)2] at low temperature. The weak antiferromagnetic and ferromagnetic interactions were also reproduced for [1-benzyl-4-aminopyridinium][Ni(mnt)2] and [1-(4‘-fluorobenzyl)pyridinium][Ni(mnt)2] at high temperature, respectively. The natural bond orbital analysis suggested that the cooperative effect of the weak intermolecular bondings may be the intrinsic driving force resulting in the switchable property, which is essentially similar to those in organic radicals exhibiting magneti...
TL;DR: When electron-withdrawing substituents are present, aryl C-H groups become powerful hydrogen bond donors, forming stronger complexes than obtained with conventional O-H and N-H Groups.
TL;DR: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet, consistent with the distribution of hydrogen bond strengths obtained for the "overstructured" ST2 model of water.
Abstract: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (-22 degrees to +15 degrees C) yields a value of 1.5 +/- 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated (\"ice-like\") and distorted (\"broken-donor\") local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the \"overstructured\" ST2 model of water.
TL;DR: In this paper, the ideal crystal morphologies and interactions between crystal surfaces and ethanol molecules are comprehensively studied on the basis of calculations of the bond strength and expanded capacity of low Miller index planes in KDP and NH4H2PO4 (ADP) crystals.
TL;DR: In this article, the authors propose that resistance is governed by the competition between the short-range covalent and long-range ionic forces, and quantify this picture using quantum-mechanical calculations, and calculate the Voronoi deformation density charges and Mulliken overlap populations of 36 materials, representative of different families, including complex oxides.
Abstract: We discuss what defines a material's resistance to amorphization by radiation damage. We propose that resistance is generally governed by the competition between the short-range covalent and long-range ionic forces, and we quantify this picture using quantum-mechanical calculations. We calculate the Voronoi deformation density charges and Mulliken overlap populations of 36 materials, representative of different families, including complex oxides. We find that the computed numbers generally follow the trends of experimental resistance in several distinct families of materials: the increase (decrease) of the short-range covalent component in material's total force field decreases (increases) its resistance.
TL;DR: In this article, the density functional theory within the ultrasoft pseudopotentials and generalized gradient approximation was used to investigate water adsorption on ordered hydroxylated silica surfaces, which can be inferred from the shortened hydrogen-bond length, the vibrational spectra from the molecular dynamics simulation and the redistribution of electron density.
Abstract: We present an ab initio investigation of water adsorption on ordered hydroxylated silica surfaces, using the density functional theory within the ultrasoft pseudopotentials and generalized-gradient approximation. The (100) and (111) surfaces of the hydroxylated cristobalite are used as substrates to adsorb water clusters and overlayers. Water adsorbs through hydrogen bonds formed between water and surface hydroxyl groups on the beta(alpha)-cristobalite (100) surface. A large enhancement of the hydrogen bonding in the adsorbed water dimer is observed, which can be inferred from the shortened hydrogen-bond (H bond) length, the vibrational spectra from the molecular dynamics simulation and the redistribution of electron density. At one monolayer (ML) coverage, a "tessellation ice," with characteristic quadrangular and octagonal hydrogen-bonded water rings, is formed. It has two types of H bonds and can exist on two different adsorption sites with two different OH orderings in a surface supercell. Our study is further extended to the beta-cristobalite (111) surface. Based on these studies, we find that the water-silica bond, which comprises several H bonds, is usually stronger than other associative water-surface interactions. The H bonds between water and surface usually differ in strength-and hence, in vibrational spectra-from those between adsorbed water molecules. Because the (100) and (111) surfaces sustain different silanol groups (geminal and isolated silanols), a well-defined two-dimensional tessellation ice phase can be observed only on the cristobalite (100) surface. On beta-cristobalite (111) surface, however, isolated water molecules, hydrogen-bonded to the surface hydroxyls, are formed, even at 1 ML coverage.
TL;DR: Nguyen et al. as mentioned in this paper used functional groups that bond with chromium in such a way that the chromium atoms can form quintuple bonds with each other, which opens the way for more novel multiple bond chemistry and will provide plenty of food for thought for experimentalists and theoreticians alike.
Abstract: Since 1964, chemists have tried without success to synthesize molecules in which the constituent atoms have bond order (that is, the number of bonds between a pair of atoms) greater than four. In his Perspective, [Frenking][1] discusses results reported in the same issue by [ Nguyen et al. ][2] in which fivefold bonding between chromium atoms has been achieved. The synthesis was made possible through the use of functional groups that bond with chromium in such a way that the chromium atoms can form quintuple bonds with each other. This opens the way for more novel multiple bond chemistry and will provide plenty of food for thought for experimentalists and theoreticians alike.
[1]: http://www.sciencemag.org/cgi/content/full/310/5749/796
[2]: http://www.sciencemag.org/cgi/content/short/310/5749/844
TL;DR: In this article, the dynamics of aluminum nanoclusters s20 nm diameterd were investigated using a parallel molecular dynamics approach based on variable charge interatomic interactions due to Streitz and Mintmire that include both ionic and covalent effects.
Abstract: The dynamics of oxidation of aluminum nanoclusters s20 nm diameterd is investigated using a parallel molecular dynamics approach based on variable charge interatomic interactions due to Streitz and Mintmire that include both ionic and covalent effects. Simulations are performed for both canonical ensembles for molecular oxygen sO2d environments and microcanonical ensembles for molecular sO2d and atomic sO1d oxygen environments. Structural and dynamic correlations in the oxide region are calculated, as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments due to the enormous energy release associated with Al-O bonding. Local stresses in the oxide scale cause rapid diffusion of aluminum and oxygen atoms. Analyses of the oxide scale reveal significant charge transfer and a variation of local structures from the metal-oxide interface to the oxide-environment interface. In the canonical ensemble, oxide depth grows linearly in time until ,30 ps, followed by saturation of oxide depth as a function of time. An amorphous oxide layer of thickness ,40 A is formed after 466 ps, in good agreement with experiments. The average mass density in the oxide scale is 75% of the bulk alumina density. Evolution of structural correlation in the oxide is analyzed through radial distribution and bond angles. Through detailed analyses of the trajectories of O atoms and their formation of OAln structures, we propose a three-step process of oxidative percolation that explains deceleration of oxide growth in the canonical ensemble.
TL;DR: The results on the local hydrogen- bonding environment with mainly two disparate hydrogen-bonding configurations are consistent with an extended structural model of supercritical water as a heterogeneous system with small patches of bonded molecules in various tetrahedral configurations and surrounding nonbonded gas-phase-like molecules.
Abstract: The local hydrogen-bonding environment in supercritical water (380 degrees C, 300 bars, density 0.54 gcm3) was studied by x-ray Raman scattering at the oxygen K edge. The spectra are compared to those of the gas phase, liquid surface, bulk liquid, and bulk ice, as well as to calculated spectra. The experimental model systems are used to assign spectral features and to quantify specific local hydrogen-bonding situations in supercritical water. The first coordination shell of the molecules is characterized in more detail with the aid of the calculations. Our analysis suggests that approximately 65% of the molecules in supercritical water are hydrogen bonded in configurations that are distinctly different from those in liquid water and ice. In contrast to liquid water the bonded molecules in supercritical water have four intact hydrogen bonds and in contrast to ice large variations of bond angles and distances are observed. The remaining approximately 35% of the molecules exhibit two free O-H bonds and are thus either not involved in hydrogen bonding at all or have one or two hydrogen bonds on the oxygen side. We determine an average O-O distance of 3.1+/-0.1 A in supercritical water for the H bonded molecules at the conditions studied here. This and the corresponding hydrogen bond lengths are shown to agree with neutron- and x-ray-diffraction data at similar conditions. Our results on the local hydrogen-bonding environment with mainly two disparate hydrogen-bonding configurations are consistent with an extended structural model of supercritical water as a heterogeneous system with small patches of bonded molecules in various tetrahedral configurations and surrounding nonbonded gas-phase-like molecules.
TL;DR: It is found that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O-H antibonding orbitals on neighboring molecules to minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms.
Abstract: We combine photoelectron and x-ray absorption spectroscopy with density functional theory to derive a molecular orbital picture of the hydrogen bond in ice We find that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O-H antibonding orbitals on neighboring molecules Together with internal s-p rehybridization this minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms, which is essential for a strong attractive electrostatic interaction Our joint experimental and theoretical results demonstrate that an electrostatic model based on only charge induction from the surrounding medium fails to properly describe the internal charge redistributions upon hydrogen bonding