Excited and ionized states of RuO4 and OsO4 studied by SAC and SAC-CI theories

The excitation and ionization spectra of RuO₄ and OsO₄ are studied theoretically by the symmetry-adapted cluster (SAC ) and SAC-CI theories. This is the attempt to assign whole of the spectra by ab initio calculations including electron correlations. In the ground state, electron correlations work to reduce the polarity of the M–O bond overestimated in the Hartree–Fock calculation. The Os–O bond is stronger than is the Ru–O bond, which is reflected in the differences of the excitation and ionization spectra of RuO₄ and OsO₄. The excitation energies of the experimental spectra are well reproduced by the SAC-CI theory, though the calculated intensities of some peaks are very small in comparison with the experiments. The outer-valence ionization spectra calculated by the SAC-CI theory agree well with the experimental photoelectron spectra. Some shake-up peaks that are accompanied with an electron-transfers from oxygen to metal are also calculated.     

Theoretical study of the satellite bands in the valence shell XPS spectra of N2, CO,O2 and NO molecules

The energies for the ionized states of N₂, CO, O₂ and NO are obtained by using an ab initio full valence configuration interaction. (FVCI) method with a minimal basis set. The intensities of the corresponding valence shell MgKα X-ray photoelectron spectroscopy (MgKα-XPS) spectra are estimated with plane wave approximation and the satellite bands in the observed spectra are analyzed. The present FVCI results give a quite reasonable interpretation of the bands, both with regard to the positions and the relative intensities. New assignments are proposed for some bands Of O₂ and NO.     

Theoretical study of valance photoelectron spectra of hypoxanthine, xanthine, and caffeine using direct symmetry-adapted cluster/configuration interaction methodology

UV photoelectron spectra of hypoxanthine, xanthine, and caffeine, up to 20 eV, were calculated and compared with the experimental spectra reported in literature. The calculations were performed using a novel version of the quantum mechanical symmetry-adapted cluster/configuration interaction (SAC-CI) method termed, direct SAC-CI. The Duning/Huzinaga valance double-zeta D95+(d,p) Gaussian basis set was also employed with this method. The ionization energies and intensities were calculated, and the corresponding spectral bands were assigned. Natural bonding orbital (NBO) calculations were employed for better spectral band assignment. The calculated ionization energies and intensities reasonably produced the experimental photoelectron spectra.     

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.     

Negative shake-up energy in core ionization

In recent years unusual multipeak structures have been detected in the core electron ionization spectra of certain highly polar aromatic molecules. In this work the ionization spectrum of para-nitroaniline, NO₂C₆H₄NH₂, in the Nls energy region is calculated by ab initio methods taking configuration interaction between the single hole-hole configurations and manifold of two-hole-one-particle excited configurations into account. The splitting of the Nls(NO₂) line observed in the ESCA spectrum of para-nitroaniline is reproduced by the calculation. The calculation shows that in the presence of the core hole the lowest π → π* excitation energy is negative. This negative shake-up energy is responsible for the multi-peak structure of the Nls(NO₂) line. It is shown that the computational results can be simply rationalized by considering the charge distribution of the highest occupied and lowest unoccupied valence orbitals. The close analogy between the multi-peak phenomena in the K-shell ESCA spectra of nitroaniline (and related highly polar aromatic compounds) and molecules chemisorbed on metal surfaces is emphasized.     

2s and 2p X-ray photoelectron spectra of Ti4+ ion in TiO2

A detailed experimental study of the 2s- and 2p XPS spectra of TiO₂ revealed new (charge-transfer) shake-up satellite structures (at 4.0–5.0 eV from the main peaks) which can be explained using molecular orbital representation. The satellite peaks, situated at about 26 eV from the main peaks, the origin of which was in doubt in the previously reported works are shown conclusively in the present investigation to be energy loss peaks. The relative energies and intensities of the observed satellites are presented and the theoretical implications of the work are discussed.     

SAC-CI general-R study of the ionization spectrum of HCl

The valence ionization spectrum of HCl is studied by symmetry-adapted-cluster configuration-interaction general-R and SD-R methods. The general-R method describes well the peak positions and intensities of seven satellite lines observed below the double ionization threshold. The twinning shake-up states due to the (4σ)⁻¹ state and the Rydberg states of HCl⁺ are correctly reproduced. Received: 26 June 1998 / Accepted: 9 September 1998 / Published online: 8 February 1999     

Theoretical fine spectroscopy with symmetry adapted cluster-configuration interaction general-R method: first-row K-shell ionizations and their satellites

Molecular core ionization spectra and their satellites were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) general-R method. The core-electron binding energies of C, N, O, and F atoms of 22 molecules were calculated with an average deviation of 0.11 eV from the experimental values. The energy splittings between K-shell gerade and ungerade states were calculated and discussed in relation to the bond length. The satellite spectra of the C 1s and N 1s core ionizations of methane and ammonia were investigated. The SAC-CI general-R method gave many shake-up states with moderate intensities, reproducing the general feature of the experimental spectra, and thus enabling the detailed understanding and assignments of the core-electron ionization spectra.     

Effects of relativity in the He(I) photoelectron spectroscopy of the transient species TeCI2 and TeBr2

Valence ionization energies of the transient species TeCl₂ and TeBr₂, obtained with He(I) photoelectron (PE) spectroscopy, are presented. The interpretation is based on the results of Hartree-Fock-Slater calculations, using STF basis sets of double zeta quality. Implementation of relativistic corrections to the ionization energies of TeBr₂ show that off-diagonal matrix-elements of the spin-orbit operator give rise to a splitting in the non-relativistically almost degenerate bromine “lone pair” orbitals. This splitting is observed experimentally. The assignments find additional support in a comparison with PE results of related oxygen-, sulphur- and seleniumdihalogenides, a simple LCBO model, sum rule considerations and a modified Walsh empirical diagram for AB₂ molecules with 20 valence electrons.     

Shake-up satellites in x-ray photoelectron (ESCA) spectra of 5f0 Th(IV) compounds

X-ray photoelectron (ESCA) spectra of the Th 4f levels have been recorded in 12 Th compounds. Weak satellites are observed in all the 4f spectra at binding energies 4–6 eV from the primary photolines. On the basis of a multiple scattering calculation in ThO₂, these satellites are assigned to ligand (e.g. O 2p) to Th 5f shake-up transitions. As with La 3d satellites, the satellite intensities correlate well with the nephelauxetic order of ligands.     

Crossover and valence band Kβ X-rays of chromium oxides

Kβ X-ray spectra of chromium metal and selected chromium oxides were measured twice using medium resolution flat crystal spectrometer and high resolution spectrometer employing Johansson geometry after excitation with 2 MeV proton beams. The positions and intensities of crossover (Kβ″) and valence (Kβ_2,5) band X-rays relative to the primary Kβ X-ray components were extracted in a consistent way. The results were compared with the existing data obtained by proton and photon induced ionization mechanisms and theoretical predictions. The obtained results in peak relative positions and intensities were analyzed in order to study dependence on the chromium oxidation states and chromium-oxygen bond lengths in selected chromium oxides. Our results obtained by both spectrometers confirm that the linear trend observed for the valence peak relative energy shift as a function of chromium oxidation number does not depend on the experimental resolution. Experimental results for normalized intensities (i.e. relative intensities divided with the number of chromium-oxygen pairs) of crossover and valence band X-rays obtained by both spectrometers are in very good agreement, and follow exponential relationship with the average Cr―O bond lengths in corresponding chromium oxides. The observed trends in crossover and valence X-rays normalized intensities could be used to measure the average chromium-oxygen bond length in various chromium oxides, with the sum of both crossover and valence X-ray normalized intensities being the most sensitive measure.     

On the calculation of the satellite peaks observed in the high energy photoelectron spectra of small molecules

The shake-up peaks which are observed to high binding energy of the main photoionization peak in the high energy photoeletron spectra of CO (carbon and oxygen core holes), H₂O and NH₃ have been interpreted by an extension of the singly excited configuration interaction method used previously in the study of the low-lying excited states of these molecules. The results obtained provide a good interpretation of the experimental spectra of these molecules in terms of many shake-up states. It is concluded that relaxed orbitals and an extensive basis set, including diffuse functions, should be used in the calculations in order to obtain good results.     

Theoretical study of shake-up in the core photoelectron spectra of the alkali atoms

Relativistic and non-relativistic Hartree-Fock calculations have been performed for the shake-up lines relative to core ionization of the alkali atoms. Good general agreement with the experimental data is achieved both for the energy and the intensity. Relativistic effects are found to be small, amounting TO = 0.2 eV in Cs. As concerns the shake-up energies, a correlation effect is detected, the magnitude of which increases along the series.     

痂囊腔菌素A分子构型和分子内氢键的理论研究

用半经验量子化学AM1方法对天然苝醌化合物痂囊腔菌素A(EA)的分子构型和分子内氢键进行了研究;从EA可能的64种构型中选择16种进行了计算.结果表明,EA的X射线晶体结构对应的构型是II型左旋(A)a,a型(II-L-A-a,a);小的生成热差值可以使得异构体间的转换容易进行,有利于发生分子内质子传递.尽管采用AM1方法计算得到的EA各种构象的平面性有所差别,但都很接近晶体的平面性.此外,EA分子内氢键键能的平均值为22.9kJ/mol;II型的氢键键能比I型的大,(9,10)位的氢键键能比(3,4)位的大;EA的平面性是由苝醌环上的侧链取代所决定,而与分子内是否存在氢键无关.     

An ab initio CI study of the ground and excited states of p-quinodimethane

Configuration interaction (CI) studies of the ground, electronically excited singlet and triplet states and of the ionized states (cations) are reported for p- quinodimethane (p-xylylene). The calculated ionization potentials are compared with the experimental photoelectron spectrum for the low-energy ionization region. The two high-energy low-intensity flanks of the second and third band observed in the photoelectron spectrum are assigned to be due to the two non-Koopmans' cation states, ascribing to shake-up ionizations.The calculated singlet-singlet and singlet-triplet excitation energies are compared with previous semiempirical MO results and experimental data.     

Accurate ab initio binding energies of the benzene dimer

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.     

Valence and core ionization energies for SiH4, H3SiCl and H3CCl from SCF and PNO-CEPA calculations

Valence and core ionization energies for the molecules SiH₄, H₃SiCl and H₃CCl have been obtained from RHF-SCF and PNO-CEPA wavefunctions. The calculated vertical ionization energies IE_n^v(n = 1, 2, 3) agree within about 0.2 eV with the PE spectroscopically determined values for SiH₄, H₃SiCl and H₃CCl. The increase in IE₁^v for H₃SiCl relative to H₃CCl, which has been previously ascribed to a different d-orbital participation in the two molecules, is already reproduced by calculations with (s, p) basis sets. The ΔSCF method has been used to predict several core ionization energies. In addition, relative intensities calculated for the ionizations into the ²A₁ states of SiH₄⁶ are compared with available experimental data, and some spectroscopic constants for SiH₄ and its ionic states are predicted.     

Multiple excitations in the core photoelectron spectrum of acetylene

A high-resolution core photoelectron spectrum of free acetylene is presented. Configuration interaction and complete active space SCF calculations were carried out for assignment and prediction of energies and intensities in the spectrum. The shake-up structure is found to be strongly dominated by intervalence π_g to π_g transitions for which multiple 2-, 3- and 4-fold excitations are important.     

A theoretical study of electronic spectra of radical cations of some dihydroxynaphthalenes

Electronic transition energies of radical cations of 1.2-, 1.3-, 1.6-, and 1.7?dihydroxynaphthalenes are calculated using an open-shell SCF method with configuration interaction. The results are critically analyzed and a correlation diagram is given that shows the energy-shift and intensity variation in the electronic transitions when moving from one system to another, thus revealing the characteristic behavior of the transitions depending on the positions of the hydroxyl substituents. An interesting relation connecting the electronic spectroscopy with the UV photoelectron spectroscopy is suggested on the basis of which the first ionization potentials (IPS ) of the substituted aromatic systems can be inferred from the calculated energy of the A-type (HOMO → LUMO ) transitions for their radical cations. Furthermore, the predictability of the IP s is found to be considerably increased with the incorporation of “molecular size” in the regression.