ESCA Shifts Research Articles

On the determination of atomic charge via ESCA for organometallics II

Following a method originally proposed by Folkesson and Larsson, linear relationships between atomic charges and experimentally determined core-electron ESCA shifts obtained from model compounds were established. The relations revealed for metal ions allow subsequent characterisation of changes in the charge on metal ion sites, thereby enabling the study of the relation between metal ion charge and, e.g., catalytic activity. We have extended previous work establishing charge – core binding energy relations for a series of metals, now completing the series Ti – Cu.

Transient fs-time-resolved ESCA photoemission of iodophenylphenol molecules adsorbed as a monolayer on a silicon (100) surface

A new type of time-resolved photoelectron spectroscopy experiment is presented: the ESCA shifts of the 4d fine structure split core-level of iodine atoms in the iodophenylphenol molecules adsorbed perpendicular to the (100) surface of silicon has been studied and observed transiently in fs-time resolved photoelectron spectroscopy as a function of the delay between the exciting ultrashort UV pulse and the ionizing ultrashort XUV pulse.

On the determination of atomic charge via ESCA including application to organometallics

A consistent set of relative atomic charges for several light elements has been obtained employing the semiempirical AM1 method. Following a method previously proposed by Folkesson and Larsson, linear relationships between charges and experimentally determined core-electron ESCA shifts obtained from model compounds were established. The relations revealed for metal ions allow subsequent characterisation of changes in the charge on metal ion sites, thereby enabling the study of the relation between metal ion charge and catalytic activity.

X-ray photoelectron spectroscopy studies of a nitroxyl free radical

X-ray photoelectron spectra of 2,2,6,6-tetramethyl oxypiperidine 1-oxyl, a stable free radical, were obtained. The surface composition determined from the spectral data compared favourably with the molecular formula C 9H 16NO 2. The experimentally measured ESCA shifts of C 1s, N 1s and O 1s correlated with Pauling's charge on the atoms.

Semiempirical calculations of the ESCA chemical shifts of nitrogen atoms in a chemical environment: failure of the PM3 and AM1 methods

The performance of the AMI and PM3 semiempirical methods in calculating ESCA shifts in the inner-shell binding energies of the nitrogen atom is tested within the electrostatic potential model expressed in the atomic monopole approximation. Relaxation energies accompanying the formation of a positive highly localized inner hole by photoionization are estimated by the equivalent core approach. Both ESCA shifts and relaxation energies are in poor agreement with the experimental data and the available ab initio results. A possible source of errors is detected in the screening ζ constants of the nitrogen atom AOs and its EC counterpart the oxygen atom, which are particularly unsatisfactory in the PM3 approach. A reparameterization of the PM3 and AMI methods for nitrogen is therefore desirable. In contrast to the AMI and PM3 methods, the self-consisted charge method, which is based on the model hamiltonian and retains all overlap integrals, reproduces ESCA shifts and reorganization energies in a satisfactory fashion.

Nuclear-charge scaling in molecular systems

Several general properties of molecular systems that can be demonstrated using nuclear charge scalings are reviewed. In particular, the property according to which the energy of the ground state of a molecule can be written in terms of the derivative of screening functions, evaluated at each nucleus of the molecule, has been reviewed. The validity of some results of this type already known in the literature is extended to the case in which the scaling of the charge of only some of the nuclei of the molecule is considered. This is done both for the fixed-nuclei model and for the nuclear-motion quantum-mechanical model. Some of these results are discussed in the context of the problem of relaxation in ESCA shifts in electron spectra, taking into account the nuclear motion.

Critical appraisal of some current semiempirical methods in calculating ESCA chemical shifts

Semiempirical MINDO/3 and MNDO methods are tested by calculating N(1 s), O(1 s), F(1 s) and Si(2 p) ESCA chemical shifts. Inner-shell binding energies (ISBE) are estimated by electrostatic potentials exerted at the host nuclei, computed in atomic monopole approximation (AMEP). Reorganization energy upon ionization is taken into account by the equivalent core model whenever possible. The calculated ESCA shifts are in moderately good agreement with experimental data. Comparison with earlier EHT, SCC-MO and CNDO/2 studies shows that the SCC-MO approach is the method of choice if ESCA shifts are to be calculated at the semi-empirical level, as this approach gives a better description of the electron charge distribution in molecules, and in particular gives better estimates of atomic monopoles. It is suggested that ESCA shifts should be used in the empirical adjustment of parameters in semiempirical methods.

ESCA studies of carbanionic compounds: Butyllithium in heptane solution and dibutylmercury in the gas phase

AbstractTwo organometallic reagents, butyllithium in heptane solution and dibutylmercury in the gas phase, have been studied by means of core electron spectroscopy. The property of particular interest was the charge polarization as reflected by the ESCA shifts. In the butyllithium compound the C1s binding energy is shifted to a lower value by 1·8 eV for C‐1 (situated closest to the Li+ ion) relative to the heptane solvent carbon. This is due to the anionic character of the C‐1. The interpretation of the experiments was supported by comparing the results with ab initio calculations made on geometry‐optimized butyllithium, butyl anion and butane.

Semiempirical vs. Ab initio calculations of molecular properties: Part 4. Nuclear quadrupole coupling constants of 14N in some medium sized heterocyclic molecules

The performance of CNDO/2D and SCC-MO methods in estimating NQCCs of 14N atom in planar compounds is tested against the ab initio DZ results and experimental data. A salient feature of the procedure is a rigorous calculation of electric field gradient matrix elements. The semiempirical values are in good qualitative accordance with results of more sophisticated methods. Quantitative discrepancies may serve as a useful guide in determining satisfactory basis sets for semiempirical schemes. The electric field gradients (EFG) provide useful information on screening constants of 2p orbitals and their relative populations. It is suggested that EFGs should be synergetically used with other observables such as ESCA shifts in finding optimal parametrization for semiempirical methods.

Theoretical and experimental ionization potentials of (CH 3) 2S and (CH 3) 2SO. The effect of substituents R on the sulphur ESCA shifts in the series R 2S, R 2SO, R 2SO 2

Theoretical and experimental inner shell ionization potentials (IP) have been determined for (CH 3) 2S and (CH 3) 2SO. This information is combined with other data to draw conclusions on the effect of methyl and phenyl substituents in the series R 2S, R 2SO and R 2SO 2, on the chemical shifts of the sulphur 2P IP. It was found that while methyl substitution has no significant effect on the chemical shifts of the sulphur 2P IP, phenyl substition modifies them significantly implying a donation of electron density from the phenyl groups to sulphur.

Application of the potential model for ESCA shifts to core-hole state geometric relaxation

The different characteristics of the equilibrium geometries of the C 1s and 0 1s ionized species of CO are derived from the potential model for ESCA binding energy shifts.

Semiempirical studies of inner-core energy levels: IX. Esca shifts of silicon atoms in various chemical environments by the scc-mo method

The semiempirical self-consistent charge molecular orbital (SCC-MO) method has been applied to a number of representative molecules involving Si atoms in various chemical environments. The calculated point charges were correlated with Si(2 p) inner-core energy level shifts by using ground state potential, relaxation potential and transition potential models (GPM, RPM and TPM). The results are in good agreement with experimental data. Various contributions to the relaxation energy are briefly discussed.

Semiempirical Studies of Inner-Core Energy Levels. IV. ESCA Shifts of Nitrogen Atoms in Different Chemical Environments by the SCC-MO Method

Abstract The binding energy shifts of the Is levels of nitrogen atoms in 15 different bonding situations are calculated by the self-consistent charge molecular orbital (SCC-MO) method employing an electrostatic potential model in the point charge approximation. The standard deviation of the obtained correlation is ~ 0.5 eV, which is substantially better than the earlier CNDO/2 GPM and RPM results. The scatter of the correlated values is diminished to roughly 1/3 of the previous CNDO/2 results. The merits and shortcommings of the electrostatic potential model are briefly discussed. It is concluded that the present approach provides a useful tool for studying ESCA shifts in large molecules of biochemical interest.

ESCA shifts and effective charges

A simple semi-experimental method of Gianturco and Coulson (1968) has been employed to estimate the effective charges on B, C, N, O, F, P and S ions in various compounds for which the measured ESCA shifts are available. The present values are compared with those obtained from other theoretical methods.

The phenol-pyridine hydrogen bonded complex: a cndo/2 study

The CNDO/2 method has been applied to the phenol-pyridine hydrogen bonded complex. The dissociation energy, hydrogen bond force constants, dipole moment and ESCA shifts of phenol-pyridine have been calculated and compared to the available experimental data. Two model structures have been investigated in detail. In one of these, the two aromatic rings are mutually perpendicular and the (O-H-N) hydrogen bond is predicted to be linear. In the other, the two rings are co-planar and the hydrogen bond deviates from linearity by about 20°. The latter effect may be attributed to an inter-ring repulsion arising from the close proximity of two of the ring hydrogens.

Equivalent cores and transition potentials for ESCA shifts and auger electron spectra

A simple derivation of transition potentials, based on equivalent cores and the Hellman-Feynman theorem, is given. It relates to previously given transition potential models with the potential at the ionisation site, which has a modified nuclear charge, thus including relaxation.

The electronic structure of NbC and NbN

A self-consistent band-structure calculation has been performed for niobium carbide using the non-relativistic APW method and the X alpha exchange approximation. An analogous calculation already exists for NbN. Both band structures are fitted with a Slater-Koster LCAO scheme including overlap contributions. From a comparison of the two compounds a pronounced change in the non-metal p and niobium d interaction is found, which is discussed in connection with the binding mechanism, ESCA shifts and superconductivity. The X-ray emission spectra of NbC as computed from the present APW results agree well with experiments.

Correlation of ESCA shifts and Hammett substituent constants in substituted benzene derivatives

ESCA shifts of substituted benzene derivatives are shown to correlate with Hammett σ constants. An explanation of this correlation is given in terms of the thermodynamic model for core electron binding energy shifts, and it is shown that ESCA shifts in benzene derivatives can be treated as a linear free energy relationship.

Electron Spectroscopy and Molecular Structure

Abstract Electron spectroscopy can now be applied to solids, liquids and gases. Some fields of research require ultrahigh vacuum conditions, in particular those directly concerned with surface phenomena on the monolayer level. Liquids have recently been subject to studies and several improvements and extensions of this technique can be done. Much advance has lately been achieved in the case of gases, where the pressure range presently is 10 –5 − 1 torr. Signal-to-background ratios for core lines can be ˜ 1000:1 and the resolution has been increased to the extent that vibrational fine structures of 1s levels in some small molecules have been observed. These improvements are based on the monochromatization of the exciting AlK α radiation. Under such conditions the background is so much reduced that shake-up structures are more generally accessible for closer studies. ESCA shifts are also much easier to resolve and to measure with higher precision around 0.02 eV. It is interesting to compare the results from electron spectroscopy with those recently obtained from X-ray emission spectra of free molecules. By means of grating at grazing incidence a resolution has been obtained at a sufficient intensity to record not only transitions from resolved molecular orbitals to the 1s levels but also in some cases vibrational fine-structures inside such separate lines. The photoionization dynamics including atomic and molecular relaxations have been investigated, both experimentally and theoretically. In the valence electron region improvements in energy resolution and in the application of the intensity model based on the MO-LCAO approximation greatly facilitate the assignments of the valence orbitals. Accumulation of empirical evidences gathered from series of similar chemical species and also better methods of calculation, both ab initio and semi-empirical, have gradually resulted in a much better understanding of the molecular orbital description. The experience of the latest ESCA instrument with monochromatization has motivated an attempt to design an optimized apparatus according to the general principles of this prototype. A considerable gain in intensity can be made at an improved resolution set by the inherent diffraction pattern of the focussing spherical quartz crystals.

Hyperfine Interactions and ESCA Data

A comparison is made of those data obtained from ESCA and from hyperfine interactions that bear on the same problems. A formal Hamiltonian can be written in which ESCA shifts and structure are discussed as E0, M1, E2,... terms. Under the E0 term core-level shifts are compared with isomer shifts and diamagnetic shifts. The M1 term refers to exchange splitting (core polarization). Multiplet splitting can identify local moments with fluctuation times down to 10-15 s. Rare earths and the 3d group are considered. The E2 term yields a crystal-field energy transforming as Y20 and correlated with e2qQ in metals. Finally the valence-band density of states in noble metals can be related to a Y4 term if nearest neighbors in metals are effectively negative; i.e., conduction electrons, rather than ion cores, are dominant.