Atom Superposition And Electron Delocalization Molecular Orbital Research Articles

Hydrogen embrittlement of a Fe–Cr–Ni alloy: Analysis of the physical and chemical processes in the early stage of stress corrosion cracking initiation

The interaction of two-hydrogen atoms in a zone of γ-Fe55Cr25Ni20 alloy having a vacancy (V) was studied by the atom superposition and electron delocalization molecular orbital (ASED-MO) method. The impurities are located aligned both along [1–10] direction and with the vacancy, in the (111) plane. This behavior can be related with a lineal hydrogen–vacancy clusterization, as a precursor to crack initiation. The electronic structure of the Fe, Cr and Ni atoms near the vacancy, changes after hydrogen's location. The interactions mainly involve Fe 4px and Cr 4py atomic orbitals. The 3dx2−y2,3dz2 and 3dxz metallic orbitals also have participation in the hydrogen–alloy interactions. An electron transfer to the H atoms from the Fe, Cr and Ni nearest neighbor atoms contributes to form the new H–metal bonds. The metal–metal bonds weakened as the new H–Fe, H–Cr, and H–Ni pairs were formed. The Cr atoms have an important role in the embrittlent process; the strengths of the Cr–Fe, Cr–Cr and Cr–Ni bonds are the most affected while the H–Cr interaction has the highest overlap population. Same H–H interaction is observed and could be associated with the precursor of hydrogen bubble but it is far away to a typical H2 chemical bond. All the cited physical and chemical processes play a key role in subsequent localized corrosion nucleation such as the initiation of stress corrosion cracking.

Interactive quantum chemistry: A divide‐and‐conquer ASED‐MO method

We present interactive quantum chemistry simulation at the atom superposition and electron delocalization molecular orbital (ASED-MO) level of theory. Our method is based on the divide-and-conquer (D&C) approach, which we show is accurate and efficient for this non-self-consistent semiempirical theory. The method has a linear complexity in the number of atoms, scales well with the number of cores, and has a small prefactor. The time cost is completely controllable, as all steps are performed with direct algorithms, i.e., no iterative schemes are used. We discuss the errors induced by the D&C approach, first empirically on a few examples, and then via a theoretical study of two toy models that can be analytically solved for any number of atoms. Thanks to the precision and speed of the D&C approach, we are able to demonstrate interactive quantum chemistry simulations for systems up to a few hundred atoms on a current multicore desktop computer. When drawing and editing molecular systems, interactive simulations provide immediate, intuitive feedback on chemical structures. As the number of cores on personal computers increases, and larger and larger systems can be dealt with, we believe such interactive simulations-even at lower levels of theory-should thus prove most useful to effectively understand, design and prototype molecules, devices and materials.

A model for atomic hydrogen–bimetal interactions

The adsorption of atomic H on the bimetallic FeNi(111) surface has been studied by ASED-MO tight binding calculations. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital (ASED-MO) method. Seven H locations on the alloy surface were selected and the hydrogen atom was positioned in their energy minima configurations.By ASED-MO calculations, the H atom presents its most stable position when it bonds on top Fe atom at 1.5Å and, on bridge Fe–Fe at 0.7Å, respectively. In these cases, the strength of the local Fe–Fe bond decreases 12% and 33% of its original bulk value, respectively. As a consequence of Fe–H interaction, a decohesion mechanism in the Fe–Fe bond could be evidenced. On the other hand, the Fe–Ni and Ni–Ni superficial bonds are slightly modified between 0.4 and 2%. A discussion based on electronic structure studies using the concept of density of states (DOS) and crystal orbital overlap population (COOP) curves is presented.

Multiple hydrogen location in a vacancy region of a FCC iron–nickel-based alloy

The interaction between four-hydrogen atoms and a FCC FeNi-based alloy ideal structure having a vacancy (V) was studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalisation molecular orbital (ASED-MO) method. The electronic structure was studied using the concept of density of states (DOS) and crystal orbital overlap population (COOP) curves. After a sequential absorption, the hydrogen atoms are finally positioned at their energy minima configurations, near to the vacancy. The energy difference for H agglomeration was also computed. The vacancy–H n complexes become less stable for n > 3. The changes in the electronic structure of Fe and Ni atoms near to the vacancy were also analysed. The interactions mainly involve Fe and Ni, 4s and 4p atomic orbitals. The contribution of 3d orbitals is much less important. The Fe–Fe, Fe–Ni and Ni–Ni bonds are weakened as new Fe–H, Ni–H and H–H pairs are formed. The effect of the H atoms is limited to its first neighbours. The detrimental effect of H atoms on the metallic bonds can be related to the decohesion mechanism for H embrittlement.

Simulation of hydrogen trapping at defects in Pd

Abstract The interaction of hydrogen with defects in palladium was studied using qualitative electronic structure calculations in the framework of the Atom Superposition and Electron Delocalization Molecular Orbital (ASED-MO) theory. Interatomic distances, energies and electronic structure for hydrogen at a dislocation and at a vacancy were determined and compared with that for an octahedral site in the Pd fcc lattice. We found that a repulsive H–H interaction is developed if these atoms occupy interstitial sites in a regular lattice. However, when the H atoms are close to a dislocation, the accumulation becomes possible. It was found that the dislocation allows hydrogen association at interatomic distances close to molecular hydrogen. The Density of States (DOS) and Crystal Orbital Overlap Population (COOP) curves were used to shed more light on the interstitial-Pd-defect interactions. Hydrogen produce changes in the local and global electronic structure of the host metal and induce changes in the cohesive forces between atoms in the host matrix. In all cases, it was found that strong bonds between Pd and H atoms are formed while metal–metal bonds are weakened. The metal–hydrogen bonds were characterized by charge transfer from metal atoms to hydrogen.

Solvatochromism, molecular and electronic structures of trans and cis isomers of a typical styryl pyridinium cyanine dye

This paper reports on solvatochromism and molecular and electronic structures of a typical styryl pyridinium cyanine dye, 1-methyl-4-( p- N, N-dimethyl-amino styryl) pyridinium iodide (Cy) in both ground and excited states. There is a hypsochromic shift of the longest absorption band, which is greater than that of the bathochromic shift of the fluorescence band, with a decrease of the Stokes shift as the solvent polarity decreases. The results confirm the importance of decreasing the dipole moment of Cy upon excitation and solvent polarity function F( D, n), which describes the orientation polarizability of the solvent, in the energetic stabilization of Cy dye in S o and S 1 states. Semiempirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) and PM3 methods were also performed. The change of the dipole moment upon excitation was explained on the basis of changes in the charge redistribution over the whole skeleton of the molecules, which agree well with the experimental results. Also, the nature and energy of the electronic transitions were elucidated and discussed in relation to the experimental data.

Mechanism of Water Attacking on Brooker’s Merocyanine Dye and Its Effect on the Molecular and Electronic Structures: Theoretical Study

Abstract Semiempirical molecular orbital calculations have been performed to study the mechanism by which water molecules attack the highly negative oxycyclic oxygen atom of Brooker’s merocyanine 1-methyl-4-(4′-oxidostyryl)pyridinium betaine, M, to form mono-, di-, and tri-hydrated merocyanine complexes. In the case of mono- and di-hydrated complexes, one or two water molecules attack the oxygen atom, respectively, but in the case of the tri-hydrated complex, only two water molecules attack the oxygen atom of M with sp2 hybridization, while the third one forms a H-bond with either one of the oxygen atoms of the coordinated water molecules. The effect of H-bond formation on the molecular and electronic structures of merocyanine is investigated using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. The calculations show that the hydrated complexes are shifted towards the benzenoid valence structures in the ground state, but shifted towards quinonoid ones upon excitation. On the basis of the calculated charge distribution over the whole skeleton of the complexes, it is found that the dipole moments of the complexes are slightly affected by H-bond formation in the ground state, but strongly decrease upon excitation. The formation of hydrated merocyanine complexes are highly exothermic, downhill reactions, which is explained with respect to the stabilization of the HOMO and oxygen lone-pair levels. We believe that the attacking by water with acidic character on the basic site oxygen atom of merocyanine is a kind of acid–base interaction.

Adsorbed Carbon Formation and Carbon Hydrogenation for CO2Methanation on the Ni(111) Surface: ASED-MO Study

Using the ASED-MO (Atom Superposition and Electron Delocalization-Molecular Orbital) theory, we investigated carbon formation and carbon hydrogenation for <TEX>$CO_2$</TEX> methanation on the Ni (111) surface. For carbon formation mechanism, we calculated the following activation energies, 1.27 eV for <TEX>$CO_2$</TEX> dissociation, 2.97 eV for the CO, 1.93 eV for 2CO dissociation, respectively. For carbon methanation mechanism, we also calculated the following activation energies, 0.72 eV for methylidyne, 0.52 eV for methylene and 0.50 eV for methane, respectively. We found that the calculated activation energy of CO dissociation is higher than that of 2CO dissociation on the clean surface and base on these results that the CO dissociation step are the ratedetermining of the process. The C-H bond lengths of <TEX>$CH_4$</TEX> the intermediate complex are 1.21 <TEX>$\AA$</TEX>, 1.31 <TEX>$\AA$</TEX> for the C<TEX>${\cdot}{\cdot}{\cdot}H_{(1)}$</TEX>, and 2.82 <TEX>$\AA$</TEX> for the height, with angles of 105<TEX>${^{\circ}}$</TEX> for ∠ <TEX>$H_{(1)}$</TEX>CH and 98<TEX>${^{\circ}}$</TEX> for <TEX>$H_{(1)} CH _{(1)}$</TEX>.

Absorption, fluorescence, and semiempirical ASED-MO studies on a typical Brooker's merocyanine dye

Solvatochromism and Solvatofluorchromism of Brooker's merocyanine 1-methyl-4- (4′-hydroxystyryl) pyridinium betaine, M, were studied in twelve polar protic and aprotic solvents. Moderate hypsochromic fluorescence energy shifts are 4.57 kcal mole −1 while strong hypsochromic absorption energy shifts are 16.63 kcal mole −1. Decreasing of the dipole moment of M upon excitation is the factor, which is responsible for the difference between the two energy shifts. The change of both energies rectilinearly with solvent acidity scale shows the importance of oxygen atom of M as a strong basic center. The application of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory reproduces geometrical and electronic structures for M, which agree well with the experimental observations. The calculations suggest strongly that the dye has a benzenoid valence structure in the ground state and shifts towards a quinonoid one upon excitation with an observed decreasing of the dipole moment. The changing of the dipole moment is explained clearly depending upon the calculated charge distribution over the whole skeleton of the molecule. The formation of a H-bond between the water molecule and the highly negative oxycyclic oxygen atom of M has slightly effect on its dipole moment in the ground state. This leads to suggest that this kind of interaction could be represented as attacking of water with acidic character on the basic site of M. Also, the calculations predict that the formation of monohydrated complex is an exothermic, down hill reaction, which is confirmed from the stabilization of the frontier molecular orbitals, oxygen lone-pair and the HOMO levels.

UV–vis, IR and 1H NMR spectroscopic studies of some mono- and bis-azo-compounds based on 2,7-dihydroxynaphthalene and aniline derivatives

The absorption spectra of mono- and bis-azo-derivatives obtained by coupling the diazonium salts of aromatic amines and 2,7-dihydroxynaphthalene have been studied in six organic solvents. The different absorption bands have been assigned and the effect of solvents on the charge transfer band is also discussed. The diagnostic IR spectral bands and 1H NMR signals are assigned and discussed in relation to molecular structure. Also, semi-empirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory have been performed to investigate the molecular and electronic structures of these compounds. According to these calculations, an intramolecular hydrogen bonding is essential for stabilization of such molecules.

BINDING ENERGY OF HYDROGEN TO MIXED AND SCREW DISLOCATION

We have studied the effect of hydrogen on the cohesion of two types of dislocation in bcc iron at an atomistic level, using the atom superposition and electron delocalization molecular orbital (ASED-MO) method. The most stable positions for one hydrogen at each dislocation core were determined. It was found that the total energy of the cluster decreases when the hydrogen is located at the core. This effect is higher in a mixed dislocation in accordance with the experimental data. The computed results show that hydrogen is a strong embrittler and that a decrease in the Fe–Fe overlap population plays a dominant role in the decohesion of the crystal structure.

Hydrogen adsorption on β-Ga 2O 3(1 0 0) surface containing oxygen vacancies

The understanding of hydrogen (H) adsorption on gallia is an important step in the design of molecular sensors and alkane dehydrogenation–aromatization catalysts. We have simulated the (1 0 0) surface of β-Ga 2O 3, which is the more frequent cleavage plane. Our study has considered oxygen vacancies in that plane. We have used the atom superposition and electron delocalization molecular orbital (ASED-MO), a semiempirical theoretical method, to understand both the electronic and bonding characteristic of H on β-Ga 2O 3 surface. As a surface model, we have considered both a cluster and a five-layer slab. We have found that H adsorption occurs on Ga sites close to oxygen vacancies. Two types of Ga have also been considered; namely, Ga(I) and Ga(II), with different coordination: Ga(I) is four-coordinated and Ga(II) six-coordinated. The Ga(I)–H bond is ∼24% stronger than Ga(II)–H while Ga(I)–O(I) bond is ∼44% stronger than Ga(II)–O(I). Also, Ga(I)–O(III) is ∼56% stronger that Ga(II)–O(III). The Ga–H and Ga–O interactions are always bonding. The Ga–Ga overlap population is null. We have assigned the 2003 and 1980 cm −1 infrared bands to the stretching frequencies of Ga(I)–H and Ga(II)–H bonds, respectively.

Dual emission of chalcone-analogue dyes emitting in the red region

Abstract The photophysical properties of new synthesized chalcones namely; 1-(4′-R-phenyl)-5-(4′-dimethylaminophenyl)-2,4- pentadien-1-one, [R=H (1), Cl (2) and OCH3 (3)] were studied in different solvents by using steady-state absorption and emission spectroscopy. The fluorescence spectra of these chalcones exhibit dual emission in medium and polar solvents. The dual emission was attributed to population of a polar locally excited (LE) state and a highly dipolar intramolecular charge transfer (ICT) state. The changes in dipole moments upon excitation were calculated from the solvatochromic plots. The total fluorescence quantum yields (φf) were also determined, and their values are strongly dependent on the nature of substitutent and the solvent polarity. Semiempirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) method were also performed to investigate the molecular and electronic structures of these chalcones in both the ground and excited state. The change of the dipole moment upon excitation was explained on the basis of changes in the charge redistribution over the whole skeleton of the molecules, which agree well with the experimental results. Also, the nature and energy of the electronic transitions were elucidated and discussed in relation to the experimental data.

Quantum chemical study of sulfur and hydrogen at the Σ=5 BCC Fe grain boundary

We investigate the effect of segregated atoms on metal–metal bonding at a grain boundary (GB). The structure and electronic properties of S and H impurities in Fe Σ=5 (013) [100] symmetrical tilt GB are studied using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. A large cluster containing 196 Fe atoms is used to simulate the local environment of the boundary. The results show that impurities induce large relaxation in the GB and that the GB gives rise to an energetically favorable zone for the H and S accumulation. No S–H association is found and the deleterious effect of H is of much less important as compared with S.

EFFECTS OF SULFUR AND CARBON ON THE ELECTRONIC STRUCTURE AND BONDING OF A GRAIN BOUNDARY IN α-IRON

The electronic structure of a grain boundary (GB) in α- Fe with segregated S and C impurity atoms was calculated by the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. An Fe 176 cluster model was used to simulate the local environment of the Σ5 (013), 36.9° [100] tilt boundary. The results show a site competition on the GB between S and C . The C drives the S away from the boundaries. The C segregation is 4.77 eV more favorable than the S segregation. Both interstitials weaken Fe – Fe bonds perpendicular to the GB plane and simultaneously form new interstitial- Fe bonds, although C presents the possibility of connecting the two halves of the crystal with a bridge Fe – C – Fe bond. These results should be important for understanding the complex mechanism for decohesion α- Fe .

The electronic effect of carbon and hydrogen in an ( [formula omitted]) edge dislocation core system in bcc iron

The Fe–C–H interaction in a dislocated bcc structure was studied using qualitative structure calculations in the framework of the atom superposition and electron delocalisation molecular orbital (ASED-MO) theory. Calculations were performed using Fe 85 cluster to simulate a dislocated bcc structure. The cluster geometry and atomic parameters were optimised to make a better approximation to the repulsive energy terms. The most stable position for C atom inside the cluster was determined. Therefore, and H atom was approximated to the minimum energy region where the C atom was previously located. The total energy of the cluster decreases with the C atom near the dislocation core. The a/2[ 1 1 ̄ 1 ] dislocation creates an energetically favourable zone for accumulation of C. The presence of C in the dislocation core make no favourable H accumulation. The C acts such as an expeller of H and could reduce the weakening of Fe–Fe bonds. In addition, a sort of Fe–C–Fe “bridge” could prevent dislocation displacement if a shear stress would be applied.

GRAIN BOUNDARY SEGREGATION OF HYDROGEN IN BCC IRON: ELECTRONIC STRUCTURE

The electronic properties of H impurity in an Fe Σ = 5, 53.1° [100] (012) symmetrical tilt grain boundary (GB) were studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. A large cluster containing 197 Fe atoms was used to simulate the local environment of the boundary. The most stable positions for one H atom and two H atoms at the GB core were determined. The total energy of the cluster decreases when the H atoms are at that location, making it a possible site for H accumulation. The binding energy found was less than that of a Σ = 5 [100] (013) GB. An analysis of the orbital interaction reveals that H–Fe bonding involves mainly the Fe 4s and H 1s orbitals. A higher contribution of d orbitals is present, which show a different behavior when compared with mixed dislocation and vacancy in the bulk Fe. The interatomic bonding along the Fe atom chains via H atoms is very inefficient, thus resulting in significant weakening of the interatomic bonding. H–H interaction was also analyzed.

The electronic structure and bonding of H pairs at Σ=5 BCC Fe grain boundary

The H–Fe interaction at a grain boundary (GB) in BCC Fe was studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Calculations were performed using an Fe 196 cluster to simulate the 36.9° [1 0 0] {0 1 3} symmetrical tilt GB structure. The most stable positions for one H atom and two H atoms at the GB core were determined. The total energy of the cluster decreases when the H atoms are at that location, making it a possible site for H accumulation. An analysis of the orbital interaction reveals that H–Fe bonding involves mainly the Fe 4s and H 1s orbitals. In general, H drain charge from the first neighbor Fe atoms. The crystal orbital overlap population (COOP) curves gives a measure of Fe–Fe bond weakening due to H segregate at GB. Some Fe–Fe bonds in the GB plane shows a 60% decrease in the overlap population when H is present. H–H interaction was also analyzed. Although some H–H association is reveled no bond is formed between the impurity atoms.

A comparative study of the electronic structure of H pairs near a/2 [formula omitted] and a[0 1 0] dislocations in bcc Fe

The H–Fe interaction in a dislocated bcc structure was studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalisation molecular orbital (ASED-MO) theory. The effect on the electronic structure after introducing an a/2 [1 1 ̄ 1] mixed dislocation was analyzed and compared with the effect of introducing an a[0 1 0] edge dislocation. Calculations were performed using Fe 85 clusters to simulate a dislocated bcc structure. The cluster geometry and atomic parameters were optimized to make a better approximation to the repulsive energy terms. The most stable positions for two H atoms inside the cluster were determined. The total energy of the cluster decreases when the H atoms are located near the dislocation void, making it a possible region for H-accumulation. The hydrogen atoms bond to their closest neighbors weakening the bond between iron atoms. The H–H interaction was also analyzed. In general, the behavior of H and its influence on the Fe atoms first neighbors result to have similarities for both dislocations analyzed.

Adsorption and dissociation reaction of carbon dioxide on Ni(1 1 1) surface: molecular orbital study

Calculations based on the atom-superposition and electron-delocalization molecular orbital (ASED-MO) method show that at low coverage, CO 2 binds on the di-σ (1), Δ, μ, and π/μ sites in the bent configuration of lying-down, and on the 1-fold, 2-fold, and 3-fold sites in the perpendicular configuration. The surface adsorbed binding site of carbon dioxide is strongest at the di-σ (1) site and weakest at π/μ site. Chemisorbed CO 2 bends because of metal mixing with 2π μ→6a 1 CO 2 orbital. The dissociation reaction occurred in two steps, with an intermediate complex composed of atomic oxygen and π-bonded CO. The OCO (1) angle of reaction intermediate complex structure for the dissociation reaction is 100° on the Ni(1 1 1) surface. The activation energy was 1.27 eV.