A theoretical investigation of the complete valence ionization spectra of cyanamide,isocyanamide, diazirine and diazomethane

The ionization potentials of the four isomeric molecules cyanamide, isocyanamide, diazirine and diazomethane are computed in the entire valence region by the Green function method. The Koopmans' ordering of the one-electron level is confirmed. The computations permit to unambiguously assign the photoelectron spectra of cyanamide and diazirine, and to reassign part of the spectrum of diazomethane. For isocyanamide, for which a PES spectrum is not available, the results of the computations are believed to represent a good approximation to the true ionization potentials. The usual breakdown phenomenon of the molecular-orbital model of ionization in the inner valence shell is observed and discussed. A strong hole-mixing effect is found in the isocyanamide molecule.     

Calculation of valence electron vertical ionization potentials using a limited Cl approach

A method is outlined for the calculation of valence electron vertical ionization potentials based on CNDO/2 and CNDO/S SCF molecular ground states improved by including a limited number of doubly excited configurations and ionic states obtained from a single Cl calculation considering a limited set of singly and doubly excited configurations based on the closed shell ground state MO's of the parent molecule. From numerical results on carbon dioxide, sulphur dioxide, ethylene, and trans-butadiene it is shown that the Cl procedure leads to the right sequence of ionic states and their reasonable relative location in contrast to respective Koopmans' ionic states predictions.     

The electronic structure of propiolic acid and propynal

A CNDO/2 calculation of ths total energy and electron distribution of propiolic acid and propynal shows that their dipole moment originates almost entirely from the charge distribution of the COOH and CHO groups and receives only a small contribution from the CCH group. These results provide a satisfactory explanation of very recent microwave dipole moment measurements on propiolic acid.     

Calculation of the HeI photoelectron spectrum of CS including satellite lines

The ionization potentials and the vibrational structure in the photoelectron spectrum of the CS molecule aer calculated using a Green's function approach. The inclusion of many-body corrections to Koopmans' theorem is necessary in order to obtain the correct ordering of the first two ionization potentials. In contrast to previous interpretations of the spectrum the third band has to be assigned to a shake-up line and the fourth to the 6σ orbital. The calculated vibrational structure is in good agreement with experiment and corroborates this assignment.     

Expanded electronic states of the fluorine molecule

Potential curves of electronically excited states of F₂ with an expanded outer orbital have been calculated using a modified frozen core technique: The ionic core has been described with a two-determinant wave function and for the excited states a mixing of configurations with different cores has been employed. An investigation of the valence shell states of F₂ is presented and potential curves for a singly excited as well as a doubly excited V-state of ¹Σ_u⁺ symmetry have been calculated. Further a low lying two-configuration state resulting from simultaneous excitation to a valence and a Rydberg orbital is predicted.     

Broken symmetry in valence molecular region within Hartree-Fock calculations

Symmetry restricted and unrestricted Hartree-Fock calculations at theab initio LCAO-MO-SCF level have been carried out on the ground, core and valence hole states of N₂ at various N-N distances. A one-particle criterion for symmetry breaking is discussed. Strong broken-symmetry effects in the inner valence molecular region of N₂ have been found at larger N-N distances. This breaking of symmetry accompanying the symmetry unrestricted Hartree-Fock calculations of the inner valence hole states at large internuclear separations can be considered to be a common phenomenon with all highly symmetric molecules. The outer valence broken-symmetry effects with N₂ have showed some deviations as compared with these effects in the inner valence and core molecular regions.     

Feshbach-resonances and dissociative electron attachment of H2O

In order to obtain a better understanding of the dissociative electron resonance capture processes of H₂O we have remeasured the ionization efficie The relative intensities of these curves are strongly dependent on the ion focusing conditions; the observed maxima however (7.0 eV, 9.1 eV, 11.8 eV) a We interpret the resonances as due to Feshbach states associated with the three lowest Koopmans' ions of H₂O; this interpretation is supported by a     

Molecular beam photoelectron spectroscopy of Ni(CO)4

The nickel carbonyl HeI (21.2175 eV) photoelectron spectrum has been reinvestigated with improved resolution and molecular beam sampling. The 9T₂ and 2E photoelectron bands are shown to be intrinsically diffuse, which is attributed to D_2d geometries. The ordering of the next seven outer valence electronic states is proposed from a linear least-squares fit of the spectrum.     

Rydberg-valence mixing in the carbon 1s near-edge X-ray absorption fine structure spectra of gaseous alkanes

We have acquired high-resolution carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of methane, ethane, propane, isobutane, and neopentane. These experimental measurements are complemented by high-quality ab initio calculations, performed with the improved virtual orbital approximation. The degree and character of Rydberg-valence mixing in the preedge of the NEXAFS spectra of these species is explored. Significant Rydberg-valence mixing only occurs when there are excited states of valence sigma(C-H) character that have the appropriate symmetry to interact with excited states of Rydberg character. Our results show that this mixing is only present when there are C-H bonds to the core excited carbon atom.     

Photoelectron He(I) Spectra of small silanes

The photoelectron He(I) spectra of methyl-, dimethyl-, trimethyl- and ethylsiliane are reported and assigned with the help of CNDO/2 calculations.For each silane the ordering of the valence MO's obtained from calculations is, very close to that of the corresponding alkane, and is in agreement with spectral evidence. Participation of silicon d orbitals in bonding is not substantial but is more important in the outermost SiC and SiH MO's than in the π MO's mainly localized on the alkyl groups.     

Fluorine core esca linewidths in CH3F and CF4

Ab initio calculations within the Hartree-Fock formation have been carried out on potential energy surfaces of the ground and the F_1s hole states of CH₃F and CF₄ in order to investigate linewidths of their ESCA spectra. The calculations show that potential energy surfaces of both hole states have dissociative character and can be approximated by straight lines in the region of interest. A simple formula for the ESCA fwhm linewidth is derived which yields results in good agreement with experiment. Theoretically derived relaxation and Koopmans' energies have been investigated as a function of geometry.     

Extended Koopmans' theorem and sudden ionization processes

The extended Koopmans' theorem eigenvalues of a correlated ground state are shown to represent the centroids of groups of ionization processes in the sudden limit. Lowest order relaxation effects are absent.     

SCF-Xα MO studies of the x-ray spectra of CuO and ZnO

SCF-Xα scattered wave cluster MO calculations for the oxyanions CuO^?6₄ (D_4h symmetry) and ZnO^?6₄ (Td symmetry) yield results in good agreement with the X-ray photoelectron and X-ray emission spectra of CuO and ZnO, respectively. Agreement of the calculations with optical data is fair. Calculations of the valence electron and core electron hole states of these oxyanions support the assignment of photoelectron shakeup satellites to valence band to conduction band transitions. Calculated shakeup energies for the Cu2p core spectrum in CuO are 7.4 and 9.9 eV (cf. experimental values of 7.5 and 10.0 eV) while shakeup peaks in the valence region spectrum are predicted at 6.1 and 8.0 eV. (Cf. a broad peak with maximum at 8.1 eV observed experimentally.) The absence of intense low energy satellites in the spectra of ZnO is explained by the small amount of electron reorganization in the outer valence levels attendant upon hole formation.     

Photoelectron spectra of hydrogen peroxide and hydrogen disulphide: ab initio calculations

Koopmans' theorem ionization potentials from ab initio gaussian basis set calculations on cis, trans and skew forms of HOOH and HSSH are compared with experimental values from photoelectron spectroscopy.     

Ab Initio calculation on BF3 and BCl3

Ab initio calculation are performed on the BF₃ and BCl₃ molecules, and the Koopmans' theorem ionization potentials are compared with the experimental ones obtained from photoelectron spectra.     

The calculation of heavy atom ionization potentials by the relativistic transition state approximation

The relativistic statistical exchange (Xα) model has been applied to the calculation of the ionization potentials of uranium and thorium. The results are comparable in accuracy to those of the relativistic Hartree-Fock method, if the transition state approximation is used, rather than interpreting the eigenvalues by Koopmans' theorem.     

Perturbation corrections to koopmans theorem. V. A study with large basis sets

The vertical ionization potentials of N₂, F₂ and H₂O were calculated by perturbation corrections to Koopmans' theorem using six different basis sets. The largest set used includes several sets of polarization functions. Comparison is made with measured values and with results of computations using Green's functions.     

Electronic (vibronic) origin of configurational symmetry breaking in condensed media

We have attempted to describe all symmetry breaking (structural phase transformations) in condensed matter from a unified point of view as electronically-controlled through vibronic interactions. In this case, the mechanism for symmetry breaking involves vibronic mixing of different electronic states with nuclear displacements lowering the symmetry of the system (the Jahn-Teller effect and the pseudo-Jahn-Teller effect). Along with previously studied structural phase transitions in crystals and liquid-crystal transitions, we also discuss the case of the gas-liquid transition.Institute of Chemistry, Academy of Sciences of the Moldavian SSR, Kishinev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 3, pp. 262–269, May–June, 1991. Original article submitted February 14, 1991.     

NdBaFe2O5+w and steric effect of Nd on valence mixing and ordering of Fe

NdBaFe₂O₅ above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Mössbauer spectroscopy and compared with GdBaFe₂O₅ that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Mössbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d_xz orbitals of Fe²⁺ is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe₂O₅. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe²⁺ nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe₂O₅ causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Mössbauer isomer shift. The charge ordering in NdBaFe₂O_5+w is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe^2.5+ is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms.     

Assignment of the four lowest ionized states of p-benzoquinone and the question of “lone pair splitting” in this system

The lowest ionized states of p-benzoquinone are assigned to be n_/²B_3g, n₄/²B_2u, π₁/²B_3u, and π₂/²B_1g in order of increasing energy. This sequence of states confirms Turner's original assignment and invalidates subsequent proposals. The origin of the difficulties with these assignments are traced back to the non-validity of Koopmans' theorem for the “oxygen lone pair” ionizations (i.e., ionization to the n_/²B_3g and n₊/²B_2u states).