Core Electron Binding Energy Shifts Research Articles

Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and 1H NMR Spectra Using DFT Calculations.

Intramolecular hydrogen bonding (HB) is a complex phenomenon that extends beyond a simple valence event, affecting the core electrons of a molecule. Salicylic acid (SA) and its conformers provide an excellent model compound for studying intramolecular HB as the proton donor (H) and acceptor (O) can be toggled by rotating the C-O and C-C bonds to form up to seven potential conformers through various HB. In this study, we computationally investigated intramolecular interactions in SA conformers with and without such HB, by examining their calculated O 1s core electron-binding energy (CEBE) and 1H NMR chemical shifts validated using recent measurements. The quantum mechanically stable SA conformers are fully defined by three rotatable bonds in the compound, which are abstracted as SA(η1η2η3) digital structures, where ηi = 0 if the ηi angles match the most stable SA conformer (000) and ηi = 1 otherwise. Our findings suggest that the stability is dominated by the appearance of the intergroup intramolecular HB of Hp···O (where O is in the carboxylic acid functional group and Hp is the phenolic proton in -OHp), and η3 serves as a switch of such HB. As a result, the (η1η20) SA conformers containing such Hp···O HB are more stable than other SA conformers (η1η21) without such the Hp···O HB. The present density functional theory calculations reveal that this Hp···O HB results in splitting of the O 1s CEBEs of two hydroxyl groups (-OH) by up to 1 eV and deshielding the Hp proton 1H NMR (δHp) up to 11.68 ppm for the (η1η20) conformers. Without such Hp···O HB, the O 1s XPS binding energies of two -OH groups will be closely located in the same band, and the 1H NMR chemical shift of the Hp atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The present study indicates that the O 1s CEBE splitting between two -OH groups serves as an indicator of the presence of the Hp···O HB in SA conformers, which is also supported by the 1H NMR results.

Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in [formula omitted] thin films

Ab initio calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti1−δOxN1−x). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by ∼0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti1−δOxN1−x thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti1−δOxN1−x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.

Surface, size and thermal effects in alkali metal with core-electron binding-energy shifts

Consistency between density functional theory calculations and X-ray photoelectron spectroscopy measurements confirms our predications on the undercoordination-induced local bond relaxation and core level shift of alkali metal, which determine the surface, size and thermal properties of materials. Zone-resolved photoelectron spectroscopy analysis method and bond order-length-strength theory can be utilized to quantify the physical parameters regarding bonding identities and electronic property of metal surfaces, which allows for the study of the core-electron binding-energy shifts in alkali metals. By employing these methods and first principle calculation in this work, we can obtain the information of bond and atomic cohesive energy of under-coordinated atoms at the alkali metal surface. In addition, the effect of size and temperature towards the binding-energy in the surface region can be seen from the view point of Hamiltonian perturbation by atomic relaxation with atomic bonding.

Positive XPS binding energy shift of supported CuN-clusters governed by initial state effects

An initial state effect is established as origin for the positive 2p core electron binding energy shift found for CuN-clusters supported by a thin silica layer of a p-doped Si(100) wafer. Using the concept of the Auger parameter and taking into account the usually neglected Coulomb correlation shift in the Auger final state (M4,5M4,5) it is shown that the initial state shift is comparable to the measured XPS shift while the final state relaxation shift contributes only marginally to the binding energy shift. The cluster results differ from the negative surface core-level shift of crystalline copper which has been explained in terms of a final state relaxation effect.

Coverage-Dependent Adsorption at a Low Symmetry Surface: DFT and Statistical Analysis of Oxygen Chemistry on Kinked Pt(321)

We explore the influence of adsorbate interactions on the thermodynamic and spectroscopic properties of oxygen on the stepped, kinked Pt(321) surface. The ground state arrangements of atomic oxygen are identified with the aid of a cluster expansion and analyzed for coverages up to one oxygen per surface Pt (1 ML). We find that oxygen prefers to bind in bridge sites at the step edge at coverages up to 0.2 ML, but at higher coverages oxygen atoms actually experience mild, localized attractions such that both bridge and threefold hollow sites are occupied to form square planar, fourfold-coordinated PtO4-like structures. These structures progressively dominate the surface with increasing coverage up to 0.8 ML, at which point every kink Pt is saturated with four oxygens. We compute stability regions for these ground states with respect to gas-phase O2 and to NO/NO2 mixtures. The ground state structures at 0.2, 0.6, and 0.8 ML dominate over a wide range of conditions, with the 0.6 ML structure being most prominent. We also explore site preferences for molecular O2 adsorbed on key O ground state structures. Calculations of vibrational modes and core electron binding energy shifts allow us to relate both ground state and non-equilibrium structures to experimental HREELS and XPS results. We show that adsorption sites are primarily characterized by their surface coordination, such that O in atop, bridge, and threefold hollow sites possess distinct and identifiable vibrational modes and core level shifts. However, within these broad categories, we find variability due to interactions with proximal adsorbates. Adsorption energies and vibrational modes of O2 are found to be particularly sensitive to the local adsorption environment. Lastly, we develop a one-dimensional adsorption model to understand and rationalize experimentally observed non-equilibrium behavior at low coverages.

Substituent effects in chain and ring π-systems studied by core-electron binding energies calculated by density functional theory

The substituent effects of X- in 1-X-hexatrienes, which are chain π-systems with conjugated double bonds, was compared with those in Ph X, which are ring π-systems, using core electron binding energy shifts (ΔCEBE) of the carbon atoms in the molecules. The ΔCEBE of C 1 C 4 in 1-X-hexatrienes are generally close to ΔCEBE of C 1 C 4 in Ph X. The ΔCEBE of carbon atoms in the 1-X-hexatrienes are not only highly correlated to the Hammett σ substituent constants, but their numerical values are also close to each other. The core electron binding energies (CEBE) of the six carbons in 1-X-hexatriynes, which are the chain π-systems with conjugated triple bonds, were calculated by density functional theory (DFT) and the substituent effect was investigated. Average ΔCEBE of the five carbon atoms between C 2 and C 6 in 1-X-hexatriyne were calculated. The magnitude and sign of the average ΔCEBE provides quantitative strength and direction, electron withdrawing or donating, of a substituent X.

Accurate calculation of N1s and C1s core electron binding energies of substituted pyridines. Correlation with basicity and with Hammett substituent constants

Substituent shifts of the energetics of four related ionization processes of pyridines and benzoic acids (Fig. 1) were investigated. The first process is core-electron ionization of gas-phase pyridines (Fig. 1A), while the second concerns gas-phase acid-base reaction between a substituted pyridine and a conjugated acid (Fig. 1B), and the third and fourth processes are the acid dissociation of substituted benzoic acids in aqueous solution (Fig. 1C) and in vacuum (Fig. 1D), respectively. Core-electron binding energies for the first process were calculated using density-functional theory with the scheme Δ E KS (PW86x-PW91c/TZP+ C rel)//HF/6-31G∗. Average absolute deviation of calculated core electron binding energy shifts at N atom in substituted pyridines from experiment was 0.08 eV. The shift at N coincides highly with that at a ring carbon atom. The four shifts corresponding to the four processes shown in Figs. 1A–D correlate strongly with one another, with numerical values fairly close to each other when expressed in unit of electron volts.

Geometry, solvent, and polar effects on the relationship between calculated core-electron binding energy shifts (ΔCEBE) and Hammett substituent (σ) constants

According to Lindberg et al. there exists an equation ΔCEBE= κσ for substituted benzene derivatives. Core-electron binding energy shift (ΔCEBE) is the difference between the CEBE of a specific carbon in monosubstituted benzene derivatives (C 6H 5–Z) and in benzene (C 6H 5–H); κ is related to a reaction constant and σ is the experimental Hammett substituent constant. The object of the present work is to investigate geometry, solvent, and polar effects on Lindberg's equation using theoretically calculated ΔCEBE. The CEBEs were calculated using DFT within the scheme ΔE KS (PW86x-PW91c/TZP+C rel). The geometry has only little effect on the CEBE values. A regression relation between ΔCEBE and σ takes the form ΔCEBE= κσ− C with κ≅1.17 and C≅0.17. We estimated 69 σ constants in water that have not been presented in the literature. Theoretical resonance ( σ R) and inductive ( σ I) effects were calculated using Taft equations. ΔCEBE (R) and ΔCEBE (I) effects on ΔCEBE were also calculated using Taft-like equations. The quality of the correlation to the resonance is better than that to the inductive effect, in water. The regression quality in aqueous organic solvent is poorer than in water in both Lindberg and Taft equations. The solvent effect is greater on the resonance than on the inductive effect.

Core electron binding energy (CEBE) shifts as descriptors in structure activity relationship (SAR) analysis of cytotoxicities of a series of simple phenols

Core electron binding energy shifts (ΔCEBE's) of carbon atoms calculated with the semi empirical HAM/3 method were shown to serve as a useful descriptor for SAR analysis of cytotoxicities of a series of simple phenols. Using three selected ΔCEBE's of carbon atoms in the phenyl ring of the compounds, the compounds were well separated by a pattern recognition method such as principal component analysis.

Core electron binding energy (CEBE) shifts as descriptors in structure activity relationship (SAR) analysis of neolignans tested against Leishmania donovani

Core electron binding energy shifts (ΔCEBEs) of carbon atoms calculated with the semi-empirical Hydrogenic Atoms in Molecules, version 3, method were shown to serve as a useful descriptor for structure activity relationship analysis of 12 neolignans studied. Using five selected ΔCEBEs of carbon atoms in the two phenyl rings of the compounds, the compounds were well separated by pattern recognition methods such as principal component analysis and SIMCA.

Core electron binding energy (CEBE) shifts applied to structure activity relationship (SAR) analysis of neolignans

Core electron binding energy shifts (DCEBE's) and CEBE of carbon atoms calculated with the semi empirical HAM/3 method were shown to serve as a useful descriptor for SAR analysis of six neolignans studied. Using five selected DCEBE's of carbon atoms in the two phenyl rings of the compounds, the compounds were well separated by HCA, PCA, KNN and SIMCA methods.

Surface Resonance X-Ray Scattering Observation of Core-Electron Binding-Energy Shifts of Pt(111)-Surface Atoms during Electrochemical Oxidation

Surface resonance x-ray scattering, sensitive to the monolayer-level change of an oxidation state of a buried interface, is used for the investigation of electrochemical oxidation of Pt(111) single crystal surface. The strongly $\mathbf{Q}$-dependent energy scans through the ${L}_{\mathrm{III}}$ resonance energy of Pt atoms were accounted for by a large increase of the electron binding energy of the surface atoms as a result of surface anodic oxidation.

Thermally induced core-electron binding-energy shifts in transition metals: An experimental investigation of Ta(100)

High-resolution photoemission spectra from the 4 f 7/2 levels of Ta~100! have been obtained between 77 K and room temperature. The data show an increase in both the surface and bulk core-level binding energies ~BE’s ! as the temperature is raised: between 77 and 293 K the bulk and surface BE’s increase by 31 63 and 1362 meV, respectively. A model calculation of the bulk binding-energy increase, which is based upon the lattice expansion of the solid, is in good agreement with the experimental results and indicates that the shifts arise from both initial- and final-state effects that are of comparable magnitude. The model is further used to estimate thermally induced shifts for the whole 5 d transition-metal series. @S0163-1829~96!02348-X#

Core electron chemical shifts in conjugated molecules and polymers

Modern experiments and quantum calculations have revealed interesting trends in core electron spectra of oligomers and polymers. In particular, the core electron chemical shifts show salient features that are structurally dependent, but which have not been completely understood. From π-electron theory we derive a model for core electron binding energy shifts in conjugated molecules and polymers. The alternant behavior of the site dependent shifts in the linear and polycyclic hydrocarbons are predicted by the model. The alternations show end-to-bulk amplification, respectively, damping for the cyclic and the linear polyenes. The polycyclic hydrocarbons show the strongest alternation effects for the bulk atoms, while for the polyenes the alternation is strongest for the terminal atoms. The analysis is based on renormalized perturbation theory and on the analytic solutions of the Hückel equations. The theory and the proposed model can be applied also for investigations of the influence of impurities on physical properties of polymers.

Surface properties of the semiconductor CsAu

Photoemission from the CsCl structure, n-type semiconductor CsAu confirms the presence of an adsorbed Cs layer on films produced by vapor phase reaction between bulk Au and Cs vapor. Cs 4d and 5p spectra yield a well-defined Cs core-electron binding-energy shift of −0.58 ± 0.06 eV for this layer relative to the Cs in bulk CsAu. The Au 4f and 5d spectra do not exhibit a measurable surface-atom binding-energy shift. Annealing of reacted CsAu films produced by the reaction of Au(100) with Cs vapor show ordered diffraction patterns that are coverage dependent.

XPS and XAES study of the interaction of palladium and copper overlayers with fullerene films

X-ray photoelectron and Auger spectroscopies were used to examine surface bonding and overlayer growth during palladium and copper deposition onto films of fullerene, C60 The results were consistent with metal cluster formation on C60. The observed positive core electron binding energy shifts in small metallic clusters supported on C60 were shown to originate in metal-fullerene interaction accompanied by charge transfer. Palladium-fullerene intermixing at temperatures as low as ss 50°C was observed for small Pd coverages.

PdTi surface alloy formation at room temperature: its indication by electronic relaxation processes as evidenced in Pd (CVV) Auger spectra

Experimental data obtained by X-ray excited photoelectron and Auger spectroscopy of Pd deposited on quasi-amorphous Ti are presented. The interaction between Pd and the Ti substrate is discussed in terms of Pd M 4,5VV Auger spectrum variation with Pd coverage. The surface PdTi phase is determined via core-electron binding energy shifts, Auger Pd (M 4,5VV) electron kinetic energy shifts and corresponding linewidths. At Pd coverages between one and two monolayers interfacial PdTi-alloy formation at room temperature is found.

Thermal and surface core-electron binding-energy shifts in metals.

High-resolution photoemission spectra from the shallow core levels of alkali metals and of In have been obtained between 78 K and room temperature. The data yield values for the alkali-metal surface-atom core-level shift and show thermal shifts of comparable size for bulk and surface. The positive surface shifts are due to the spill-out of conduction-electron charge, which is responsible for the surface dipole layer. The surface shifts are in good agreement with values obtained from a Born-Haber cycle expressed in terms of surface energies. The thermal shifts are proportional to the lattice expansion, and arise from both initial-state and final-state effects. As the lattice expands, the Fermi level decreases, decreasing the core-electron binding energy. At the same time, the expansion of the conduction-electron charge increases ${\mathit{r}}_{\mathit{s}}$, thereby decreasing the potential at the core level and increasing the binding energy. The expansion also decreases the relaxation energy, further increasing the core-electron binding energy. In the alkali metals, the combined potential- and relaxation-energy terms dominate the Fermi-level term, making the shifts positive. In divalent metals the three terms tend to cancel, while in trivalent metals it is the Fermi-level term that dominates, making the shifts negative.

A theoretical study of x-ray photoelectron spectra of model molecules for polymethylmethacrylate

We explore the usefulness of the delta self-consistent-field (ΔSCF) approximation in connection with high-resolution x-ray photoelectron spectra for component and structural analysis of organic compounds. Results for core electron binding energy shifts for model molecules of the polymethylmethacrylate polymer are presented. A previously devised method for proper self-consistent-field solutions for core hole states in molecules is evaluated. The results indicate that chemical shifts can be obtained within a few tenths of an eV. A discussion is presented on the inherent errors in the ΔSCF approximation, the proper corrections for zero-point vibrational energies, and the role of relaxation of core orbitals.

Photoemission and electron microscopy of small supported palladium clusters

Small palladium clusters, produced by vapour deposition onto amorphous carbon substrates, were analysed in situ by photoelectron spectroscopy (XPS, UPS) and characterized ex situ by transmission electron microscopy. These combined measurements provide the actual cluster size dependence of the two well-known effects in the photoelectron spectra of small supported clusters — the valence band narrowing and the core-electron binding energy shift — suggesting the following explanations: the valence band narrowing is due to an initial state effect, while the core-level shift is dominated by the effect of the changed final state, as it is predicted by Wertheim et al.