Outer Valence Region Research Articles

A photoelectron study of the inner valence molecular orbitals of N 2O

Photoelectron spectra are reported for the inner and outer valence regions of N#2#O for several photon energies up to 120 eV. The outer valence region is characterized by four well resolved bands which can be assigned to ionisation from specific molecular orbitals. In contrast, the inner valence region displays a complex structure, composed of many overlapping lines, extending to a binding energy of approximately 45 eV. The experimental data are compared with theoretical calculations and with a spectrum recorded using X-ray excitation. Several of the features observed in the inner valence region can be interpreted with the aid of theoretical predictions.

The polychloromethanes - an experimental and theoretical investigation of their valence electronic structure

The CH 2Cl 2, CHCl 3 and CCl 4 molecules have been studied using electron momentum spectroscopy (EMS). The measurements were performed at incident electron energies of 1000 and 1800 eV, using symmetric noncoplanar kinematics. Binding energy spectra ranging up to 55 eV were measured at out-of-plane azimuthal angles from 0° to 30°, corresponding to target electron momenta from about 0.1–2.7 au. The separation energy spectra and electron momentum distributions obtained for the valence orbitals are compared with the results of Green function calculations for the ionization energies and their corresponding pole strengths and the spherically averaged momentum distributions obtained from the SCF molecular orbitals. In the outer valence region, where a one-particle picture hold for the description of the ionization processes, there is very good agreement between the theoretical energies and the present separation energies as well as photoelectron spectroscopic data, and between the present EMS results and theory of the pole strengths and the orbital momentum distributions. In the inner valence region a substantial splitting of the lines occurs. Because of the size of the molecules this splitting is only predicted in a qualitative way in the Green function calculations. In particular the splitting is found to occur over a larger energy range than predicted by the calculations. In the inner valence region the calculated SCF momentum distributions are not consistent with the measured ones. The theoretical distributions are much narrower than the experimental ones, suggesting that these orbitals are physically more localized in position space than predicted at the SCF level.

Gaussian basis sets for calculation of spin densities in first-row atoms

The suitability of Gaussian basis sets for ab initio calculation of Fermi contact spin densities is established by application to the prototype first-row atoms B-F having open shell p electrons. Small multiconfiguration self-consistent-field wave functions are used to describe relevant spin and orbital polarization effects. Basis sets are evaluated by comparing the results to highly precise numerical grid calculations previously carried out with the same wave function models. It is found that modest contracted Gaussian basis sets developed primarily for Hartree-Fock calculations can give semiquantitative results if augmented by diffuse functions and if further uncontracted in the outer core-inner valence region.

Orbital momentum distributions and binding energies for the complete valence shell of molecular iodine

The complete valence shell binding energy spectrum (8–43 eV) of I 2 has been measured by using electron momentum spectroscopy at 1000 eV. The complete inner valence region, corresponding to ionization from the 10σ u and 10σ g orbitals, has been measured for the first time and shows extensive splitting of the ionization strength due to electron correlation effects in the ion. Many-body calculations using the Green function method have been carried out and are compared with the data. Momentum distributions, measured in both the outer and inner valence regions, are compared with those given by SCF orbital wavefunctions calculated with a number of different basis sets. Computed orbital position and momentum density maps for oriented I 2 molecules are discussed in comparison with the measured and calculated spherically averaged momentum distributions.

Orbital momentum distribution and binding energies for the complete valence shell of molecular bromine

The binding energy spectrum of Br 2 has been recorded in both the outer and inner valence regions using electron momentum spectroscopy. The measurements are compared with the results of several Green function calculations using different approximations and based on both polarized and unpolarized wave functions. The inner valence region, observed for the first time, is found to exhibit complex structure that is shown to be due to many-body effects, thus indicating a breakdown of the simple MO picture for ionization in this region. Momentum distributions for the three outer valence orbitals are also measured and compared with spherically averaged calculations using the target Hartree-Fock and plane wave impulse approximations. The effect of polarization functions in the basis set is investigated. Orbital density maps in both momentum and position space have been calculated and compared with the experimental measurements.

Orbital momentum distributions and binding energies for the complete valence shell of molecular chlorine by electron momentum spectroscopy

The complete valence shall binding energy spectrum (10–50 eV) of Cl 2 has been determined using electron momentum (binary (e,2e)) spectroscopy. The inner valence region, corresponding to 4σ u and 4σ g ionization, has been measured for the first time and shows extensive splitting of the ionization strength due to electron correlation effects. These measurements are compared with the results of many-body calculations using Green function and CI methods employing unpolarised as well as polarised wavefunctions. Momentum distributions, measured in both the outer and inner valence regions, are compared with calculations using a range of unpolarised and polarised wavefunctions. Computed orbital density maps in momentum and position space for oriented Cl 2 molecules are discussed in comparison with the measured and calculated spherically averaged momentum distributions.

Interpretation of the Auger electron spectra of nitrous oxide

The nitrogen and oxygen Auger spectra associated with the nitrous oxide molecule have been calculated using a semiempirical model analogous to that used previously to explain atomic Auger spectra. The role of the central and terminal nitrogen atoms is elucidated. Good agreement with the experimental spectrum is obtained especially for the outer valence region.

The band structure of Ni(H 5C 3B 2). An example for energetic stabilization due to dimerization

The band structure of Ni(H 5C 3B 2) the first experimentally verified one-dimensional (1D) polydecker system, has been investigated by means of a semi-empirical crystal orbital (CO) formalism based on an improved INDO (intermediate neglect of differential overlap) hamiltonian. In order to allow for an analysis of the electronic structure of a 1D arrangement composed by unit cells with an ungerade number of electrons a grand canonical (GC) averaging scheme has been used for the definition of the crystal hamiltonian. The synthesized 1D material with 13 valence electrons per simplest stoichiometric unit is a semiconductor and shows nuclear distortions into the direction of the stacking axis (i.e. formation of non-equivalent metal-ligand contacts). The tight-binding calculations lead to a transparent theoretical explanation of this formal dimerization. The 1D arrangement built up by one Ni(H 5C 3B 2) half-sandwich per unit cell reproducing the full translational symmetry is unstable towards a dimerization that leads to a unit which is formed by two half-sandwiches. The energy of the 1D column with the doubled cell is stabilized due to a symmetry-violation of the spatial wavefunction (i.e. symmetry-breaking for a symmetry atomic arrangement). A nuclear distortion (formation of non-equivalent metal-ligand distances) causes an additional energy lowering of the 1D system. The band structure properties in the outer valence region are analyzed. The calculated band gap is in line with he experimentally observed semiconducting properties of the 1D chain. The microstates of the valence band contain both admixtures from the cyclic organic π ligand (leading contribution) and from the transition metal centers (3d xz or 3d yz orbitals).

Electronic structure of organometallic compounds. Part 28. Photoelectron spectra of transition-metal-1-methylborinato complexes in the outer valence region. Semiempirical model calculations based on Green's function formalism

The electronic structures of the 1-methylborinato complexes of V(CO)/sub 4/ (1), Mn(CO)/sub 3/ (2), Co(CO)/sub 2/ (3), and Co(C/sub 4/H/sub 4/) (4) have been studied by means of INDO calculations and their He I photoelectron (PE) spectra. The calculated rotational barriers and the orbital sequences of 1-3 have been compared with their isovalent (C/sub 5/H/sub 5/)M(CO)/sub n/ species. The measured ionization energies are discussed on the basis of those calculated by means of the Green's function formalism. It if found that Koopmans' theorem is only valid in the case of 1. Significant differences between the filling scheme of the canonical MO's in the ground state and the sequence of the ionic states are demonstrated for 2-4. It is shown that large relaxation energies are partially compensated by pair-relaxation contributions (i.e., many-body correction). 63 references, 10 figures, 5 tables.

Inelastic scattering and satellite fine structure in the high-resolution uv photoelectron spectrum of CS 2

The outer valence region in CS 2 has been studied by high-resolution UV photpelectron Spectroscopy. The spectra reveal detailed vibrational structure in the X $ ̃ 2Π g, A $ ̃ 2Π u, B $ ̃ 2Σ + u and C $ ̃ 2Σ + g bands. Some of the fine-structure peaks in the X , ̃ B $ ̃ and C $ ̃ bands are shown to be pressure-dependent. The reason for the pressure dependence is assumed to be inelastic scattering of electrons emitted in the adiabatic transitions. It is established that the two CI satellite bands present in the He(I)-excited spectrum contain vibrational structure.

Nature of the LiC bond in simple organolithium compounds: A transition operator-based MO electronegativity approach

The nature of the lithium—carbon bond (LiC) in some simple lithiated alkyl derivatives has been studied by using the direct theoretical approach of comparing the molecular orbital (MO) electronegativity corresponding to the two molecular fragments. The calculations of MO electronegativity have been carried out within the transition operator method by employing a recently proposed version of the INDO-MO model designed specifically to reproduce the vertical ionization potentials in the outer valence region. The LiC bonding is found to depend mainly on the geometry and the electron acceptor substituents at the carbon atom. Molecular fragments are presented where the LiC spans a range from nearly covalent to predominantly ionic bond.

Electron-propagator calculations with a transition-operator reference

The quest for a single election-propagator approximation, simple enough to permit applications to quite general molecular systems and sufficiently realistic to predict accurate electron binding energies from the core to the outer valence regions, has inspired a renewed study of some simple self-energy forms using transition orbitals and orbital energies. Applications to H 2O, C 2H 4, Ne, CN − and NH 2 − are reported.

Electronic structure of transition metal compounds: XXIII. Theoretical and spectroscopic investigations of heteroligand pentacarbonyl complexes of group via transition metals

The electronic structures of a series of 18 transition metal pentacarbonyl complexes of the general formula M(CO) 5L (M  Cr, Mo, W; L  Me 2XYMe, Me 2XXMe 2; X  P, As; Y  S, Se; Me  CH 3) have been investigated by semiempirical calculations of the CNDO and INDO type and by various spectroscopic studies. Information concerning the ground state of the pentacarbonyl derivatives has been derived from 1H NMR, 31P NMR and IR data; He(I) photoelectron spectra in the outer valence region have been measured in order to study the nature of the cationic hole-states. Theoretical and experimental data are combined to give a detailed description of the electronic structure of the M(CO) 5L complexes.

The electronic structure of transition metal tricarbonyl derivatives in the ground state and the cationic hole-states. a semiempirical INDO MO investigation based on the green's function formalism

The electronic structures of benzene chromium tricarbonyl (1), cyclopentadienyl manganese tricarbonyl (2), trimethylenemethane iron tricarbonyl (3), cyclobutadiene iron tricarbonyl (4) and butadiene iron tricarbonyl (5) have been studied in the ground state and in the cationic hole-states by means of semiempirical INDO calculations and many-body perturbation theory based on the Green's function formalism. The vertical ionization potentials of 1–5 in the outer valence region have been calculated with the inclusion of electron correlation and relaxation. In the cases of 1 and 2 it is found that the sequence of the ionization events corresponds to the ordering of the molecular orbitals (MO's) in the electronic ground state. The breakdown of Koopmans' theorem is demonstrated for the iron complexes 3–5 where the highest occupied MO'S are of ligand type while the first ionization potentials are due to MO's with predominant metal 3 d character. The different behaviour of the Cr, Mn and Fe tricarbonyl complexes is analyzed as a function of the localization properties of the orbital wavefunction and as a function of the corresponding two-electron integrals. The different bonding schemes in the series 1–5 are discussed.

A green's function approach to the photoelectron spectrum of irontetracarbonyl dibromide: Fe(CO) 4Br 2

The vertical ionization potentials of irontetracarbonyl dibromide, Fe(Co) 4Br 2, were dertermined by means of the Green's function approach in the computational framework of an improved CNDO Hamiltonian. The theoretically determined sequence of the ionization events was Br π <Fe 3d <FeBro, which leads to a straightforward assignment of the measured ionization energies in the outer valence region. The ground state properties of the title compound are studied by means of the CNDO approach; the degree of metal→ligand and ligand→Metal charge transfer for the various ligands (Br, CO ax, CO eq) of the dibromide is analyzed.

The electronic structure of transition metal tricarbonyl derivatives in the ground state and the cationic hole-states. A semiempirical indo mo investigation based on the Green's function formalism

The electronic structures of benzene chromium tricarbonyl (1), cyclopentadienyl tricarbonyl (2), trimthylenemethane iron tricarbonyl (3), cyclobutadiene iron tricarbonyl (4) and butadiene iron tricarbonyl (5) have been studied in the ground state and in the cationic hole-states by means of semiempirical INDO calculations and many-body perturbation theory based on the Green's function formalism. The vertical ionization potentials of 1–5 in the outer valence region have been calculated with the inclusion of electron correlation and relaxation. In the cases of 1 and 2 it is found that the sequence of the ionization events corresponds to the ordering of the molecular orbitals (MO's) in the electronic ground state. The breakdown of Koopman's theorem is demonstrated for the iron complexes 3–5 where the highest occupied MO's are of ligand type while the first ionization potentials are due the MO's with predominant metal 3 d character. The different behaviour of the Cr, Mn and Fe tricarbonyl complexes is analyzed as a function of the localization properties of the orbital wavefunction and as a function of the corresponding two-electron integrals. The different bonding schemes in the series 1–5 are discussed.

An INDO crystal orbital approach to the one-dimensional porphyrinatonickel(II) system

The band structure of the one-dimensional metallomacrocycle porphyrinatonickel(II), Ni(P), has been studied by means of the crystal orbital formalism based on a semi-empirical INDO Hamiltonian. The unoxidized Ni(P) polymer is an insulator, the highest occupied bands as well as the conduction band are localized in the ligand framework. Bands with significant Ni 3 d admixtures are predicted at lower energies. The dense manifold of ligand states in the vicinity of the Ni 3 d basis energies leads to a strong metal-ligand intermixing in Ni(P) bands in the outer valence region.

Electronic structure of dicarbonyl (cyclopentadienyl)—manganacumulene complexes

The electronic structures of CpMn(CO) 2C C(CH 3) 2 ( 1) and CpMn(CO) 2C C C( t − C 4H 9) 2 ( 2) have been investigated by means of He(I) photoelectron spectroscopy. The measured vertical ionization potentials of 1 and 2 are compared with Green's function results for 1 and for a methyl derivative ( 3) of 2 based on a semiempirical INDO Hamiltonian. An almost constant reorganization increment for all ionization potentials in the outer valence region is encountered in the perturbational Green's function technique. This behaviour can be traced back to very strong interfragment mixing of transition-metal and ligand orbitals leading to a set of complex orbitals that cannot be classified as typical metal 3 d or ligand functions.

On the quantum chemical origin for the nonvalidity of Koopmans' theorem in transitionmetal compounds

The quantum chemical origin for the nonvalidity of Koopmans' theorem in transitionmetal compounds of the 3d series is analyzed by means of the Green's function formalism applied in the framework of a semiempirical INDO Hamiltonian. In the case of ferrocene (1), cyclobutadiene iron tricarbonyl (2) and irontetracarbonyl dihydride (3) the self-energy part of a geometric approximation has been partitioned into relaxation and correlation (pair removal, pair relaxation) increments. The breakdown of Koopmans' theorem for strongly localized MOs with large Fe 3d amplitudes is predominantly the result of electronic relaxation lowering the calculated ionization potentials. On the other hand the variation of the pair correlation energy in the cationic hole-state is by no means negligible and acts into the opposite direction as the relaxation increment. These significant pair relaxation contributions explain the wellknown failtures of the ΔSCF approach in combination with large scaleab initio bases. The loss of ground state pair correlation in the outer valence region is small in comparison to relaxation and pair relaxation. The magnitude of the aforementioned reorganization increments has been studied as a function of the localization properties of the MOs and as a function of the one-electron energies of the available particle- and hole-states. The computational findings derived with the INDO model are compared with recentab initio studies.

The electronic structure of tricarbonyl(norbornadiene)iron. A semiempirical INDO MO study

The electronic structure of tricarbonyl(norbornadiene)iron ( 1) in the ground state and in the cationic hole-states has been investigated by means of semiempirical INDO calculations. The bonding characteristics of 1 can be rationalized in terms of Hoffmann's fragment approach for the Fe(CO) 3 moiety. The vertical ionization potentials of 1 in the outer valence region are calculated by a perturbational expansion based on the Green's function formalism. The strong correspondence between calculated reorganization energies and the localization properties of the orbital wavefunction leads to a dramatic breakdown of Koopmans' theorem.