Applications of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers

The core levels of a series of 83 homopolymers have been studied by electron spectroscopy for chemical analysis (ESCA). Comparisons of the experimentally determined core-level binding energies with theoretical calculations using the ground-state potential model in the complete neglect of differential overlap (CNDO/2) self-consistent field molecular orbital (SCF MO) formalism have been made on the C_1s and O_1s core levels for the oxygen-containing polymers in the series. A comparison of the ground-state potential model (GPM) and relaxation potential model (RPM) on a series of six model compounds representative on the series of polymers is given. Compilations are given of binding energies of C_1s, O_1s, N_1s, Cl_2p, S_2p, Si_2p, and Br_3d levels for typical structural features of common occurrence in polymer systems. These data, taken in conjunction with that previously published on fluoropolymers, provide a sound basis for the development of ESCA as a fingerprint tool in the elaboration of features of structure and bonding in polymers in general.     

Applications of ESCA to polymer chemistry. X. Core and valence energy levels of a series of polyacrylates

The core and valence levels of a series of poly(alkyl acrylates) have been studied by ESCA. From an analysis of the individual component peaks for the C_1s and O_1s core levels and from comparison of relative area ratios it is shown that ESCA may be applied to the study of surface compositions. The evidence presented strongly suggests that on the ESCA depth-profiling scale the technique statistically sample the repeat units with no evidence for preferential orientation of side chains at the surface. For some samples, ESCA provides evidence for a degree of surface oxidation and hydrocarbon contamination. The valence energy levels are shown to be characteristic of the polymer system. The measured absolute and relative binding energies of the core levels have been compared with model calculations using the charge-potential model in the CNDO/2 SCF MO formalism.     

Non empirical LCAO SCF MO investigations of electronic reorganizations accompanying core ionizations

Calculations have been carried out on an extensive series of molecules for both the neutral species and core ionized states. Substituent effects on C_1s , N_1s , O_1s , and F_1s levels have been investigated and where available comparison has been drawn with experiment. Comparison with Koopmans' theorem has allowed a relatively detailed study of change in relaxation energies as a function of substituent effect on a given core level. Whilst for C_1s levels the computed shifts in core binding energies are approximately linearly related to differences in relaxation energies for the N_1s , O_1s , and F_1s levels, the relative electronegativity of the substituent can invert this correlation. The empirical correction of Koopmans' theorem for differences in relaxation energies at different sites has been investigated for large molecules. The results compare well with direct hole state calculations.     

Applications of ESCA to polymer chemistry. XI. Core and valence energy levels of a series of polymethacrylates

The core and valence energy levels of a series of poly(alkyl methacrylates) have been studied by ESCA. Surface compositions have been determined both from a comparison of area ratios for the O_1s and C_1s core levels and from the relative areas for the individual component peaks for these levels. The evidence presented suggests that on the ESCA depth-profiling scale the technique statistically samples the repeat unit with little or no evidence for preferential orientation of the alkyl side chain at the surface. Little evidence was found for either surface oxidation or hydrocarbon contamination at the surface of the samples studied. The valence energy levels are shown to be characteristic of the polymer system.     

A theoretical investigation of molecular core binding and relaxation energies in a series of oxygen-containing organic molecules of interest in the study of surface oxidation of polymers

Nonempirical LCAO MO SCF computations (in the ΔSCF formalism) have been carried out of binding energies and relaxation energies for an extensive series of oxygen-containing organic systems encompassing most of the common functionalities. The molecules for which experimental data are available for comparison demonstrate the adequacy of the treatment for describing the relative binding energies for both the C_ls and O_ls core levels. A sound basis is therefore provided for the discussion of relative core binding energies (C_ls and O_ls) for functionalities for which experimental data are not available, that is, hydroperoxides, peroxides, peroxyacids, and peroxyesters, a knowledge of which is essential for investigations of the surface oxidation of polymers by means of ESCA. C_ls shifts are discussed in terms of primary and secondary substituent effects of oxygen, which greatly simplify the analysis of complex unresolved spectra, whereas for the O_ls levels a similar but less straightforward situation exists. The relaxation energies associated with both C_ls and O_ls core ionization follow a dependence on the binding energy for a given structural type.     

Surface aspects of the heat treatment of polyethylene terephthalate as revealed by ESCA

The changes in surface structure and bonding of PET films has been monitored as a function of temperature by means of ESCA. By monitoring both the C_1s and O_1s core levels it is shown that at remarkably low temperatures there is a reduction in oxygen functionality. This contrasts with the results in the literature relating to bulk techniques for monitoring thermal degradation.     

X-ray photoelectron spectroscopy of the YBa2Cu3O7−x superconductor system

X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) has been employed to investigate the chemical nature and the electronic structure of the YBa₂Cu₃O_7−x system at different stages of preparation. The binding energy measurements showed that O 1s, Ba 3d and Y 3d core levels undergo appreciable changes during the firing and subsequent heat treatment. The chemical shift in O 1s is believed to be due to a higher concentration of holes in the superconductor phase. No indication for the existence of a monovalent Cu(I) species was found at any stage of preparation.     

Sur un traitement unifie des deplacements chimiques en spectrometrie photoelectronique de rayons—X (ESCA), des moments dipolaires et des polarisabilites

The simplest and most general model for interpreting ESCA chemical shifts, the physical meaning of which is very clear, also allows the discussion of dipole moments and polarisabilities. Sixty-eight ESCA chemical shifts of C_1s, N_1s, O_1s and F_1s in 41 differently-substituted compounds (?F, ?Cl, ?Br, ?OH, ?NH₂), saturated or unsaturated (aldehydes, ketones, acids), have been calculated with this model from electric charge distribution; the mean quadratic deviations are respectively ±1.1 eV for C, ±1.6 eV for N, ±0.8 eV for 0, ±0.7 eV for F.With this model, electric charge repartitions can be deduced from ESCA data, even for aromatic molecules. The calculated electric dipole moments for 12 fluorinated aromatic molecules agree very well with experimental results. Other data have been calculated for molecules for which experimental data are not yet available, including examples where heavy atoms are present.     

X-ray spectra and core hole energy curves of some diatomic molecules

Soft X-ray emission spectra of the molecules CO, N₂, NO and O₂ are examined for the purpose of deriving information on their core hole energy curves. Molecular force constant and equilibrium bond lengths are determined for the core hole species C*O and N₂*, and a qualitative analysis is made for CO*, N*O, NO* and O₂*. The results show that differences of equilibrium geometries between the core hole states and the ground states are very well reproduced (better than 1 pm) by SCF calculations within the Hartree-Fock formalism. Inclusion of anharmonicity in the Franck-Condon analysis gives a small but significant effect on the best fitted value for the core hole state bond lengths (about 0.5 pm). Oxygen is binding energies determined from the X-ray spectra are shown to agree with ESCA data, in most cases within a few tenths of an eV. Calculated ΔSCF transition energies reproduce the experimental data within a few eV.     

The identification of peroxy-features at polymer surfaces by ESCA

A preliminary investigation of the identification of peroxy-features at polymer surfaces by ESCA is described. Oxidation of a hydrocarbon polymer in the effluent of a singlet molecular oxygen generator, based on a microwave discharge in oxygen, produces surfaces rich in peroxy-features. By comparison with experimental and theoretical studies of model systems these features can be identified in ESCA spectra by their characteristic O_1s binding energies. The oxygen atoms in the peroxy-linkage have essentially the same binding energy, which is within a narrow range of 534.0 eV for simple peroxides and hydroperoxides and a narrow range of 535.2 eV for peroxy-features in more highly oxygenated environments; for example, peracids and peresters. Peroxy-linkages decompose rather rapidly under the typical x-ray flux used in the ESCA experiment.     

Some theoretical aspects of vibrational fine structure accompanying core ionizations in N2 and CO

Ab initio calculations have been carried out on CO and N₂ and the relevant core hole states with different basis sets to investigate differences in geometries and force constants. From these calculations vibrational band profiles of the core level ESCA spectra for these molecules have been interpreted, obviating the need to rely on data pertaining to the equivalent core species. The agreement with experimental profiles is excellent. The O_1s level of CO which has not been subjected to detailed theoretical analysis previously, is predicted to show substantial vibrational structure in excellent agreement with recently acquired experimental data. The effect of temperature on the band profiles has also been considered. Theoretically derived core binding and relaxation energies of these systems have been investigated both as a function of basis set, and of internuclear distance. Density difference contours have been computed and give a straightforward pictorial representation of the substantial electron reorganizations accompanying core ionizations. Small basis sets with valence exponents appropriate to the equivalent core species when used in hole state calculations describe bond lengths, force constants, core binding energies and relaxation energies with an accuracy comparable to that appropriate to the corresponding extended basis set calculations.     

Theoretical studies of organometallic compounds. III. Structures and bond energies of FeCHn and FeCH (n = 1, 2, 3)

The geometries and dissociation energies for the Fe? C and C? H bonds of FeCH_n and FeCH (n = 1, 2, 3) have been calculated by ab initio quantum mechanical methods using different effective core potential models and Møller–Plesset perturbation theory. The HW3 ECP model, which has a configuration [core] (n?1)s², (n?1)p⁶, (n?1)d¹, (n)s^m for the transition metals, is clearly superior to the larger core LANL1DZ ECP model with the configuration [core] (n?1)d¹, (n)s^m. The Fe? C bond energies calculated at correlated levels using the HW3 ECP are in much better agreement with experiment than the LANL1DZ results. This effect is mainly due to the higher number of correlated electrons rather than the inclusion of the outermost core electrons in the Hartree–Fock calculation. At the PMP4/HW3TZ/6-31G(d)//MP2/HW3TZ/6-31G(d) level, the theoretically predicted Fe? C bond energies for FeCH are in the range of 80% of the experimental values and have nearly the same accuracy as all-electron calculations using large valence basis sets and the MCPF method for the correlation energy. © 1992 by John Wiley & Sons, Inc.     

Theoretical and experimental X-ray photoelectron spectroscopy investigation of ion-implanted nafion

The membrane properties of a Nafion surface can be modified by ion implantation with N⁺ or F⁺. The results are presented of an X-ray photoelectron spectroscopy (XPS) study of implanted surfaces. For the interpretation of the XPS spectra, calculations using a semiempirical quantum chemical formalism (AM1) have been applied, in conjunction with a charge-potential model, to predict the C_1s core electron binding energies. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 551–556, 2004     

Surface characterization of immunosensor conjugated with gold nanoparticles based on cyclic voltammetry and X-ray photoelectron spectroscopy

This investigation describes the surface characterization of rabbit immunoglobulin G (IgG) conjugated with gold nanoparticles. Goat anti-rabbit immunoglobulin G tagged with 5 nm gold nanoparticles was applied to detect the IgG. Then, the autocatalyzed deposition of Au³⁺ onto the surface of anti-IgGAu increased the surface area per gold nanoparticle. The immobilization chemistries and the atomic concentrations of Au_4f, P_2p, S_2p, C_1s, N_1s and O_1s of the resulting antibody-modified Au electrodes were determined by X-ray photoelectron spectroscopy (XPS). The sulfur that is involved in the cysteamine binding and the enlargement of the gold nanoparticles are identified using cyclic voltammetry. The results reveal that the surface area per gold particle, following the autocatalyzed deposition Au³⁺ on the surface of anti-IgGAu, was approximately seven times higher than that before deposition.     

Density functional calculation of core‐electron binding energies of isomers of C3H6O2 and C3H5NO

The core‐electron binding energies of six isomers of C₃H₆O₂ and four isomers of C₃H₅NO were calculated by a DFT/uGTS/scaled‐pVTZ approach. An average absolute deviation from experiment of 0.15 eV was found for 14 C, N, and O 1s energies. The results confirm the distinctive nature of the X‐ray photoelectron spectra (XPS) of isomers and support the use of electron spectroscopy complemented by accurate theoretical predictions as a tool for chemical analysis. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 44–50, 2000     

Gas sorption and transport in substituted polycarbonates

The effects of molecular structure manipulation of polycarbonates on sorption and transport of various gases were studied using tetramethyl, tetrachloro, and tetrabromo substitutions onto the aromatic rings of bisphenol A polycarbonate. Solubility and permeability measurements were made at 35°C over the pressure range of 1–20 atm for a variety of gases, namely CO₂, CH₄, O₂, N₂, and He. A threefold to fourfold increase in permeability was caused by the tetramethyl substitution, whereas the tetrachloro and tetrabromo substitutions reduced the permeability relative to the tetramethyl substitution. Lower activation energies for transport were found for the tetramethyl polycarbonate relative to the unsubstituted polycarbonate. Permeability coefficients were factored into solubility and diffusion coefficients. Sorption levels increased for all substitutions, but among the substituted polymers the levels remain practically the same. Solubility data were analyzed in terms of the dual sorption model. The Henry's law solubility coefficients obtained from this analysis were found to be consistent with a predictive equation developed for rubbery polymers. The usual correlation for predicting the Langmuir sorption capacity of the model overestimates the values for the substituted polycarbonates, and a proposal for the cause of this is offered. Thermal expansion of these polymers was measured using dilatometry, and the results are used in the interpretation of the sorption data. Diffusion phenomena are explained by segmental mobility and free volume considerations. The effects of CO₂ exposure history on sorption and transport were also investigated.     

Broken orbital-symmetry and the description of hole states in the tetrahedral [CrO4]− anion. I. Introductory considerations and calculations on oxygen 1s hole states

The localization of holes in systems containing spatially equivalent sites is discussed in terms of a simple one-particle model in which quantum mechanical delocalization effects compete with essentially classical polarization or dielectric relaxation effects. The predictions of the model for a tetrahedral system like CrO^?₄ compare favourably with the results of symmetry unrestricted SCF calculations on O_1s hole states. The connection with a Cl treatment using symmetry-restricted MOs is discussed. The calculated ionization energies are finally compared with XPS measurements on Na₂CrO₄. To this end the crystal surrounding of the CrO^?₄ anion has been represented by a point charge model and the ensuing Modelung field was included in the SCF calculations. In contrast to the Td restricted result of 551.4 eV, the completely localized C_3v results of 532.6 eV is in satisfactory agreement with the experimental data which are found around 530.0 eV.     

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2

The 300 K reactions of O₂ with C₂(X ¹Σ⁺_g), C₂(a ³ Π_u), C₃(X? ¹Σ⁺_g) and CN(X ²Σ⁺), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C₂H₃CN in the presence of O₂, CO molecules in the a ³Σ⁺, d ³Δ_i, and e ³Σ^? states were identified, as well as CH(A ²Δ) and CN(B ²Σ⁺) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C₂(X ¹Σ⁺_g, a ³Π_u) + O₂ → CO(X ¹Σ⁺ + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u) by O₂ were determined both from laser induced fluorescence of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10^?12 cm³ molec^?1 s^?1. The rate coefficient of the reaction of C₃ with O₂, which was determined using the IR MPD of allene as the source of C₃ molecules, is <2 × 10^?14 cm³ molec^?1 s^?1. In addition, we find that rate coefficients for C₃ reactions with N₂, NO, CH₄, and C₃H₆ are all < × 10^?14 cm³ molec^?1 s^?1. Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10^?11 cm³ molec^?1 s^?1. Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O₂ produces NCO in vibrationally excited states. Radiative lifetime of the ā ²Σ state of NCo and the ā ¹Π_u(000) state of C₃ are reported.     

Electron scattering differential cross sections for C2H2, C2H4, and C2H6 by gas phase electron diffraction - binding effects

Experimental differential cross sections for 40 keV electrons scattered by C₂H₂, C₂H₄ and C₂H₆ molecules were measured using the gas electron diffraction method in the range of the scattering variable s from s = 1 A^?1 to s = 30 A^?1. The differential cross sections for neon were also measured and compared with calculated differential cross sections to calibrate the diffractograph. Experimental differential cross sections show significant deviations with respect to theoretical differential cross sections calculated from the Debye-Ehrenfest model, mainly in the range of small scattering angles. The observed differences are connected to chemical binding effects. From the experimental data, an estimation of the binding energy was carried out. The deduced values: ?0.58 ± 0.20 au for C₂H₂, ?0.94 ± 0.30 au for C₂H₄ and ?1.23 ± 0.40 au for C₂H₆ are in agreement with those obtained by thermochemical methods.