On the photoemission spectra of CoO: Crystal-field analyses of the valence band structure

The valence band structure of CoO deduced from X-ray and ultraviolet photoemission spectroscopy is described in the framework of crystal-field theory. The energy of the transitions from the ground state of Co²⁺ to all states of Co³⁺ as well as the corresponding relative intensities are determined by taking, for the first time, strong-field configuration interaction into account.     

Electronic structure of UC and UN and its comparison with photoemission spectra

Molecular orbital calculations for UC and UN are carried out by the Xα discreet variational method, considering octahedral UC₆²⁰⁻ and UN₆¹⁵⁻ clusters. The main features of photoemission spectra are well described by the calculations. Whereas the highest occupied levels are composed mainly of 5f U orbitals, the levels, responsible for chemical bonding, contain important admixtures of 5f, 6d U orbitals with 2p (C(N) orbitals.     

SCF MO calculations of ultraviolet electronic spectra of azanaphtalenes with the variable Β approximation

The SCF MO method in the variable approximation has been applied to some azanaphtalenes. The transition energies and bond lengths of these molecules have been calculated. The results are in satisfactory agreement with experiment.

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Zusammenfassung Das SCF-Verfahren wurde in Form der -Variationsmethode nach nishimoto und Forter auf einige Azanaphthaline angewendet. Die berechneten übergangsenergien und BindungslÄngen stimmen in befriedigender Weise mit dem Experiment überein.

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Résumé Nous avons appliqué la méthode SCF MO dans l'approximation des variables à quelques azanaphtalènes et calculé les énergies de transition et les longueurs de liaison de ces molécules. Les résultats sont en accord satisfaisant avec l'expérience.

     

Atoms-in-molecules in momentum space: a Hirshfeld partitioning of electron momentum densities

A direct application of the Hirshfeld atomic partitioning (HAP) scheme is implemented for molecular electron momentum densities (EMDs). The momentum density contributions of individual atoms in diverse molecular systems are analyzed along with their topographical features and the kinetic energies of the atomic partitions. The proposed p-space HAP-based charge scheme does seem to possess the desirable attributes expected of any atoms in molecules partitioning. In addition to this, the main strength of the p-space HAP is the exact knowledge of the kinetic energy functional and the inherent ease in computing the kinetic energy. The charges derived from HAP in momentum space are found to match chemical intuition and the generally known chemical characteristics such as electronegativity, etc.     

Molecular engineering in symmetric end-substituted oligothiophene derivatives: analysis of condensed-phase photoemission spectra using semiempirical Hartree-Fock calculations

The electronic structures of two series of end-capped thiophene oligomers, one set containing the electron-deficient dimesitylboryl end-cap and one containing the electron-rich triaryl amine end-cap, have been modeled using semiempirical quantum chemical calculations and the results used to assign features in the photoemission spectra of the materials in the condensed phase. For the thiophene oligomers end-capped with the electron-deficient dimesitylboryl moieties, the energy of the occupied frontier orbitals is largely governed by pi-type orbitals of the thiophene repeat units in the oligothiophene main chain. Conversely, in oligomers end-capped with electron-rich triarylamine moieties, the occupied frontier orbital energies are largely governed by orbital states of heavily mixed character associated with thiophene pi-type systems and the low-lying nitrogen lone pairs of end capping groups.     

Outer- and inner-valence satellites of carbon dioxide: electron momentum spectroscopy compared with symmetry-adapted-cluster configuration interaction general-R calculations

The extensive study of outer- and inner-valence satellites of carbon dioxide by electron momentum spectroscopy is reported. The experiments have been performed using a high-sensitivity electron momentum spectrometer employing non-coplanar symmetric geometry at impact energy of about 1200 eV. Binding energy spectrum up to 50 eV, above the first double ionization threshold (~37.3 eV), is presented. Four main peaks and twelve satellites have been identified including four embedded in the double ionization continuum, among which the two beyond 42 eV are observed for the first time. High accuracy symmetry-adapted-cluster configuration interaction general-R calculation with aug-cc-pVTZ basis sets has also been performed and the result is in line with the experimental ionization spectrum except the relative intensities for some of the satellites in inner-valence region. The experimental momentum profiles for both the main ionization transitions and satellites have been obtained and compared with theoretical calculations by HF and B3LYP methods with 6-311++G? and aug-cc-pVTZ basis sets. Through comparison, the detailed assignments of the satellite bands have been achieved and the pole strengths for the relevant shake-up transitions are determined experimentally for the first time.     

Scaling the Coulomb interaction in calculations of electron spectra of transition metal complexes

A simple technique of scaling two-electron integrals in ab initio calculations of the electronically excited states of transition metal complexes is proposed. This technique uses the fact that one-center two-electron integrals depend linearly on the scaling factor when Slater type functions are subjected to scaling transformation. This leads to a linear dependence of the d—d transition energy on the “scale” of Coulomb interaction, which allows one to affect the calculation result by varying the Slater exponential. To test the technique, ab initio configuration interaction and full active space calculations of the low excited states of the CrF ₆ ^3- , MnF ₆ ^2- , and VF ₆ ^3- complexes are performed. For transition elements, a basis of Slater type effective functions chosen from the optical spectra of the atoms and ions of transition elements is used. It is shown that in the STO-6G basis with effective exponentials, experimental transitions are reproduced with an accuracy of about 2000 cm^-1 even with the use of small active space determined by the orbitals localized on the central atom of the complex.     

The fast atom bombardment mass spectra of furyl and thienyl silanes and germanes in comparison with their electron impact mass spectra

The interaction of the π electron system of furyl and thienyl substituents with the silicon (or germanium) atom is less significant in fragmentation under fast atom bombardment than under electron impact. Under fast atom bombardment the primary fragmentation of the title compounds is determined by steric influence of the heteroaromatic substituents.     

Study of the photoelectron and electron momentum spectra of cyclopentene using benchmark Dyson orbital theories

A complete study of the valence electronic structure and related electronic excitation properties of cyclopentene in its C(s) ground state geometry is presented. Ionization spectra obtained from this compound by means of photoelectron spectroscopy (He I and He II) and electron momentum spectroscopy have been analyzed in details up to electron binding energies of 30 eV using one-particle Green's function (1p-GF) theory along with the outer-valence (OVGF) and the third-order algebraic diagrammatic construction [ADC(3)] schemes. The employed geometries derive from DFT/B3LYP calculations in conjunction with the aug-cc-pVTZ basis set, and closely approach the structures inferred from experiments employing microwave spectroscopy or electron diffraction in the gas phase. The 1p-GF/ADC(3) calculations indicate that the orbital picture of ionization breaks down at electron binding energies larger than approximately 17 eV in the inner-valence region, and that the outer-valence 7a' orbital is also subject to a significant dispersion of the ionization intensity over shake-up states. This study confirms further the rule that OVGF pole strengths smaller than 0.85 foretell a breakdown of the orbital picture of ionization at the ADC(3) level. Spherically averaged (e, 2e) electron momentum distributions at an electron impact energy of 1200 eV that were experimentally inferred from an angular analysis of EMS intensities have been interpreted by comparison with accurate simulations employing ADC(3) Dyson orbitals. Very significant discrepancies were observed with momentum distributions obtained from several outer-valence ionization bands using standard Kohn-Sham orbitals.     

Surface Green's function calculations: a nonrecursive scheme with an infinite number of principal layers

A novel computational method for a surface Green's function matrix is introduced for the calculation of electrical current in molecular wires. The proposed nonrecursive approach includes an infinite number of principal layers and yields the second-order matrix equation for the transformed Green's function matrix. The solution is found by the direct diagonalization of the auxiliary matrix without any iteration process. As soon as complex roots of the auxiliary matrix (approximately GS) are calculated, the gaps and the bands in the surface electronic structure are found. It is shown that the solution of a second-order matrix equation determines the spectral density matrix, that is, the density of states for noninteracting electrons. Single and double principal layer models are studied both analytically and numerically. The energy interval for nonvanishing spectral matrices is determined. This method is applicable to matrices of any rank.     

Investigation into the valence electronic structure of norbornene using electron momentum spectroscopy, Green's function, and density functional theories

Results of a study of the valence electronic structure of norbornene (C(7)H(10)), up to binding energies of 30 eV, are reported. Experimental electron momentum spectroscopy (EMS) and theoretical Green's function and density functional theory approaches were utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all 19 valence orbitals of norbornene. This experimentally validated model was then used to extract other molecular properties of norbornene (geometry, infrared spectrum). When these calculated properties are compared to corresponding results from independent measurements, reasonable agreement is typically found. Due to the improved energy resolution, EMS is now at a stage to very finely image the effective topology of molecular orbitals at varying distances from the molecular center, and the way the individual atomic components interact with each other, often in excellent agreement with theory. This will be demonstrated here. Green's Function calculations employing the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than about 22 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet emission and newly presented (e,2e) ionization spectra. Finally, limitations inherent to calculations of momentum distributions based on Kohn-Sham orbitals and employing the vertical depiction of ionization processes are emphasized, in a formal discussion of EMS cross sections employing Dyson orbitals.     

Far-infrared band strengths in the water dimer: experiments and calculations

Most fundamentals modes of the water dimer have been experimentally determined, and the frequencies have been measured in either neon or parahydrogen matrices. The band strengths of all intramolecular and most intermolecular fundamentals of the water dimer have been measured. The results are further corroborated by comparison with the corresponding data for the fully deuterated water dimer. DFT calculations of the mode frequencies and band strength are in qualitative agreement with the experimental observations.     

Valence electronic structures of CH2BrCl and CF2BrCl: binding energy spectra and electron momentum distributions

The binding energy spectra (BES) of valence shells of CH2BrCl and CF2BrCl have been measured at a series of different azimuthal angles by an (e, 2e) electron momentum spectrometer employing noncoplanar symmetric geometry at an impact energy of 1200 eV plus binding energy. The experimental momentum profiles (XMPs) are extracted from the sequential BES and compared with the theoretical ones calculated by using Hartree-Fock (HF) and density functional theory (DFT-B3LYP) calculations with 6-311G, 6-311++G**, and aug-cc-pVTZ basis sets. In general, the DFT-B3LYP calculations using the larger basis sets 6-311++G** and aug-cc-pVTZ describe the XMPs well for both molecules. Moreover, the pole strengths of main ionizations from the inner valence orbitals 2a', 3a', and 1a' of CH2BrCl are determined, and the controversial ordering of two outer valence orbitals 3a' ' and 6a' of CF2BrCl has also been assigned unambiguously.     

EPR spectra of the trimethyl-1,4-benzoquinone anion-radical: Results of calculations by the density functional method and their comparison with experiment

Constants of the isotropic hyperfine interaction of an unpaired electron with the hydrogen nuclei of the trimethyl-1,4-benzoquinone anion-radical were calculated using the density functional method. The EPR spectrum of the radical was simulated on the basis of these calculations, which were in good agreement with the experimental EPR spectrum of trimethyl-1,4-benzosemiquinone dissolved in dimethylsulfoxide.     

Probing molecular conformations with electron momentum spectroscopy: the case of n-butane.

High-resolution (e,2e) measurements of the valence electronic structure and momentum-space electron density distributions of n-butane have been exhaustively reanalyzed in order to cope with the presence of two stable structures in the gas phase, namely the all-staggered and gauche conformers. The measurements are compared to a series of Boltzmann-weighted simulations based on the momentum-space form of Kohn-Sham (B3LYP) orbital densities, and to ionization spectra obtained from high-level [ADC(3)] one-particle Green's Function calculations. Indubitable improvements in the quality of the simulated (e,2e) ionization spectra and electron momentum profiles are seen when the contributions of the gauche form of n-butane are included. Both the one-electron binding energies and momentum distributions consistently image the distortions and topological changes that molecular orbitals undergo due to torsion of the carbon backbone, and thereby exhibit variations which can be traced experimentally. With regard to the intimate relation of (e,2e) cross sections with orbital densities, electron momentum spectroscopy can therefore be viewed as a very powerful, but up to now largely unexploited, conformational probe. The study also emphasizes the influence of thermal agitation in photoionization experiments of all kind.     

Qualitative comparison between the quantum calculations and electrospray mass spectra o complexes of polyammonium macrotricyclic ligands with dicarboxylic acids

The host-guest interactions play a very important role in chemical and biological processes. It is therefore important to be able to characterize these complexes. Electrospray mass spectrometry can be used to characterize the complex formation. It provides information on the mass and the charge of these ionic complexes. In this article, we show that the use of ab initio and semiempirical calculations, in addition to the results obtained by electrospray mass spectrometry, reveal to be a promising tool for the study of these noncovalent complexes. In this article, host-guest complexes formed by macropolycyclic polyammonium host molecules and dicarboxylic acids are studied.