Ab initio study of the molecular structure, isomerism, and vibrational spectra of MAF4. (M = Li, Na, K; A = Al, Ga)

The equilibrium geometrical parameters, force constants, vibration frequencies, IR intensities, and isomerization and dissociation energies of MAF₄ molecules are calculated by Hartree-Fock and second-order Möller — Plessett perturbation theory methods in the Li (9s3pld/4s3pld), Na, Al (12s8pld/6s4pld), K (14s11p3d/9s8p3d), F (9s5pld/4s2pld), and Ga (13s10p5d/6s5p2d) bases of grouped Gaussian functions. The calculations show that the structures corresponding to the bi-(b) and tridentate (t) coordinations of the M⁺ cation by the AF ₄ ^- anion are isomers. The b configuration is energetically most favorable for LiAF₄ molecules. The b and t structures of NaAF₄ have close energies, and the t configuration is basic for KAF₄ molecules. Simple empirical relations between the molecular constants in the series LiAF₄→NaAF₄→KAF₄ are found and the molecular parameters of RbAF₄ and CsAF₄ are estimated. A combined analysis of the ab initio and experimental data available in the literature for MAF₄ molecules is carried out. A number of bands observed in the IR spectrum of the matrix-isolated MAlF₄ molecules are assigned to their tridentate isomer.     

Near-resonant collisional energy transfer in Sr(5s6p 1p1-rare-gas systems

The total quenching cross sections for strontium (5s6p ¹p₁) collisions with the rare gases are determined by pulsed laser, time-resolved fluorescence measurements to be 80, 40, 73, 101, 140 Ų for He, Ne, Ar, Kr and Xe, respectively. For collisions with Kr, all of the near-resonant states, 4d5p ³F, 5s6p ³P and 4d5p ¹D₂, are observed to be excited. The cross sections are interpreted in terms of a curve crossing mechanism. The product branching can be explained in terms of strong coupling of the initial state with the near-resonant states, weighted by energy gap considerations.     

J-dependence of the isotope shift in Eu I 4f 75d6s(a 6 D)

The J-dependence of the isotope shift (IS) in the term a⁶D of the Eu I configuration 4f⁷5d6s was determined to be: z_5d(⁶D)=+34.5(1.5) MHz. This proves that in the phenomenological interpretation of the IS distinct parameters z_nl(^mL) for the J-dependence in one term are necessary.     

Hyperfine structure and Zeeman effect studies in the 6p7p-6p7s transitions in Bi II

The hfs and hfs Zeeman splitting of nine lines from the array 6p7p→6p7s as well as lines λ = 496.9 nm (6p8p ³D₂→6p7s ³P₂) and λ=581.8 nm (6p8s ³P₀→6p7p ³D₁) of Bi II have been analysed. The Zeeman effect studies were performed for transverse direction of observation and separated π; and σ components of lines. The electrodeless discharge tube containing metallic Bi was used as the light source. The spectral apparatus consisted of a sliver-coated Fabry-Perot etalon and a grating spectrograph combined with a diode array detector. In the analysis of the spectra we used the computer simulation technique. The magnetic-dipole (A) and the electric-quadruple (B) hfs constants as well as Lande-g_j factors for the level 6p8p ³D₂ and all levels of configurations 6p7s and 6p7p (with the exception of 6p7p ³S₁) were determined. Our results are compared with recent theory and other experiments.     

Configuration interaction satellites in the ESCA spectra of thorium and other actinide compounds

ESCA spectra of the Th 4d, 4f, 5s, 5p and 5d core levels have been recorded in ThF₄, Th(AcAc)₄ and other Th complexes. In addition to weak 4f and 5d satellites which are normally considered to be shake up satellites, a number of unusual satellites have been characterized in these compounds. In particular, the intense 5p_3/2 satellites, as well as previous similar 5p_{$\text{3}\text{2}$} satellites in other actinides, are shown to be due to configuration interaction of the 5p hole state with a 5d² hole state. Similarly, the 4d_{$\text{5}\text{2}$}, 5s and additional 5d satellites are probably also due to configuration interaction.     

Valence basis sets for lanthanide 4f-in-core pseudopotentials adapted for crystal orbital ab initio calculations

Crystal orbital adapted Gaussian (4s4p3d), (5s5p4d) and (6s6p5d) valence primitive basis sets have been derived for calculating periodic bulk materials containing trivalent lanthanide ions modeled with relativistic energy-consistent 4f-in-core lanthanide pseudopotentials of the Stuttgart-Koeln variety. The calibration calculations of crystalline A-type Pm₂O₃ using different segmented contraction schemes (4s4p3d)/[2s2p2d], (4s4p3d)/[3s3p2d], (5s5p4d)/[2s2p2d], (5s5p4d)/[3s3p3d], (5s5p4d)/[4s4p3d], (6s6p5d)/[2s2p2d], (6s6p5d)/[3s3p3d] and (6s6p5d)/[4s4p4d] are discussed at both Hartree–Fock (HF) and density functional theory (DFT) levels for the investigation of basis set size effects. Applications to the geometry optimization of A-type Ln₂O₃ (Ln = La-Pm) show a satisfactory agreement with experimental data using the lanthanide valence basis sets (6s6p5d)/[4s4p4d] and the standard set 6-311G* for oxygen. The corresponding augmented sets (8s7p6d)/[6s5p5d] with additional diffuse functions for describing neutral lanthanide atoms were applied to calculate atomic energies of free lanthanide atoms for the evaluation of cohesive energies for A-Ln₂O₃ within both conventional Kohn-Sham DFT and the a posteriori-HF correlation DFT schemes.     

Ab initio study of the molecular structures, force fields, and vibrational spectra of alkaline metal fluoride dimers MM′F2 (M, M′=Li, Na, K)

Ab initio calculations of the equilibrium geometrical parameters, force constants, and IR vibration frequencies and intensities of Li₂F₂, Na₂F₂, K₂F₂, LiNaF₂, LiKF₂, and NaKF₂ are reported. The calculations use the Hartree-Fock-Roothaan method and second-order Möller-Plesset perturbation theory along with configuration interaction theory including singly and doubly excited configurations and corrections for quartic excitations with basis sets of grouped Gaussian functions: Li — (9s3p1d/4s3p1d), Na — (12s8p1d/6s4p1d), K — (14s11p3d/9s8p3d), F — (9s5p1d/4s2p1d). According to the results of calculations, the equilibrium structures of the molecules are planar cyclic structures of D_2h (for M₂F₂) and C_2v (for MM′F₂) symmetries. The linear configurations M-F-M′-F (of C_∞v symmetry) are 70–190 kJ/mole less stable than the cyclic ones; for all molecules except M-F-K-F, these configurations correspond to local minima on the potential energy surface. The role of correlation effects in ab initio calculations of the geometry, force fields, and IR characteristics of molecules with highly polar chemical bonds is discussed. The theoretical force fields of the molecules are represented in canonical form in a system of redundant natural vibrational coordinates. The force fields of MM′F₂ molecules are studied. The results of the ab initio calculations are compared with the experimental structural data and vibrational spectra available in the literature.     

Simulation of isotopic selectivities and isotope ratio enhancement factors for gadolinium and lanthanum using narrow band laser excitation

Isotopic selectivities and isotope ratio enhancement factors have been calculated for 4f⁵5d6s^{2 9}D ₅ ⁰ -4f⁷5d6s6p ⁹D₆ (585.622 nm) transition of gadolinium and for 5d6s^{2 2}D_3/2-5d6s6p ⁴F _3/2 ⁰ (753.9 nm) transition of lanthanum by simulating the spectra assuming natural isotopic abundance of the samples. The atomic line shapes are assumed to be Lorentzian. Isotopic selectivities and isotope ratio enhancement factors for single step excitations have been computed. The isotope ratio enhancement factors estimated by us are half the value obtained by Young. et al. This is because the isotope ratio enhancement obtained by them is for a two colour RIMS where as our computations for selectivities are for a single step excitation. Considering the statistical error in the experimental values, our estimated isotopic selectivities are in good agreement with the available experimental data.     

584 Å pes of the Cs 5p levels in atomic Cs and in the CsCl molecule

Cs 5p photoelectron spectra of atomic Cs and the CsCl molecule are compared. The atomic spectrum shows seven peaks, four of which belong to the 5p⁵6s configuration and the remaining three belong to the 5p⁵5d configuration mixed through a configuration-interaction mechanism. The molecular spectrum shows five peaks, three of which can be correlated with the 5p ionization and the other two are proposed to result from a configuration interaction similar to the atomic case.     

Zeeman effect of superfluorescently populated 6snf 1F3 Ba I levels

Zeeman quantum beats of the Ba I 6snf ¹F₃ levels were measured between n=10 and 19 by time resolved recording of the exponential decay of the fluorescence on the 6snf ¹F₃ — 6s5d ¹D₂ transitions. The 6snf ¹F₃ levels were populated by superfluorescence pulses from the energetically higher lying 6s(n + 3)d ¹D₂ levels, the ¹D₂ levels were excited in a resonant two step process by two pulsed dye lasers. The experimental results of the g _J values in the 6snf ¹F₃ sequence are in fair agreement with the prediction of multi-channel quantum-defect theory.     

Temporal evolution of two-photon time-resolved optogalvanic signals of neon in the 600–630 nm region

Time-resolved optogalvanic (OG) signals of six two-photon transitions of neon were studied in the 600–630 nm region to 2p⁵4d[5/2]₂, 2p⁵4d[3/2]₂, 2p⁵4d[3/2]₁, 2p⁵4d[1/2]₁ and 2p⁵5s′[1/2]₁ states from the allowed states of the 2p⁵3s configuration (2p⁵3s[3/2]₁ and 2p⁵3s′[1/2]₀ states). The OG signals were recorded over a range of discharge currents from 2 to 10 mA. The decay rates of the upper and lower states were obtained by fitting the waveforms with the Han et al.’s mathematical rate equation model considering the three states contributing to the signal. Based on the values of decay rates of the upper states, it was proposed that, after excitation to 5s and 4d states, neon atoms radiatively decay to the lumped relevant electronic states of the 2p⁵3p and 2p⁵4p configurations which have the main contribution in producing the OG signals. It was found that, the decay rates of the upper states (the lumped relevant electronic states of 2p⁵3p and 2p⁵4p configurations) increase linearly and slowly with the discharge current for all the transitions considered in this work. The effective decay rates of the upper states and their electron collisional ionization rate parameters were also obtained. This study showed that the dominant relaxation process in the de-population of the upper states is the lengthened radiative decay in plasma medium after laser excitation.     

GRECP/5e‐MRD‐CI calculation of the electronic structure of PbH

The correlation calculation of the electronic structure of PbH is carried out with the generalized relativistic effective core potential (GRECP) and multireference single‐ and double‐excitation configuration interaction (MRD‐CI) methods. The 22‐electron GRECP for Pb is used and the outer core 5s, 5p, and 5d pseudospinors are frozen using the level‐shift technique, so only five external electrons of PbH are correlated. A new configuration selection scheme with respect to the relativistic multireference states is employed in the framework of the MRD‐CI method. The [6, 4, 3, 2] correlation spin–orbit basis set is optimized in the coupled cluster calculations on the Pb atom using a recently proposed procedure, in which functions in the spin–orbital basis set are generated from calculations of different ionic states of the Pb atom and those functions are considered optimal that provide the stationary point for some energy functional. Spectroscopic constants for the two lowest‐lying electronic states of PbH (²Π_1/2, ²Π_3/2) are found to be in good agreement with the experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Synthese und Kristallstruktur der molekularen Clusterverbindung W6Br14

Synthesis and Crystal Structure of the Molecular Cluster Compound W₆Br₁₄ Brownish-black crystals of W₆Br₁₄ are formed in the direct synthesis from W₆Br₁₂ and Br₂ (400 K). The compound crystallizes cubically with neutral cluster molecules ([W₆Br]Br): a = 13.458 Å; Pn3 (Nr. 201); d?(W? W) = 2.653 Å; d?(W? Brⁱ) = 2.616 Å; d?(W? Br^a) = 2.569 Å. The W atoms are 0.03 Å outside of the Br cube faces. The molecules are arranged according to a cF point configuration, but each is rotated ?23° about a threefold axis in order to avoid short inter cluster distances Br^a? Br^a. Nevertheless, via 12 short intermolecular distances per cluster of about d(Brⁱ …? Br^a) = 3.487 Å the clusters are interconnected by forming two independent and interpenetrating 3D nets (Cu₂O type). Although local distortion of the M₆X cluster does not occur, as is expected for this system with 22 electrons per M₆ octahedron, it is assumed that the Jahn-Teller theorem is fulfilled collectively via the low-symmetry nets of intermolecular interactions.     

Long-range molecular states dissociating to the three or four lowest asymptotes for the ten heteronuclear diatomic alkali molecules

Long-range energy matrix elements have been calculated in the multipolar expansion approximation for all the molecular states dissociating to the three or four lowest asymptotes for the molecules LiNa, LiK, LiRb, LiCs, NaK, NaRb, NaCs, KRb, KCs and RbCs using the semi-empirical perturbative model we proposed recently. Two different assumptions have been investigated: including or excluding the spin-orbit effects within each atom. Full numerical results are presented for NaK and LiCs which have been chosen as examples. For the ten molecules in the non-interacting assumption the long-range coefficients C₆ and C₈ have been found for each state when neglecting atomic spin-orbit effects while the fitted value C^*₆ and C^*₈ are presented for each state when including atomic spin-orbit effects. When considering the interacting states, those dissociating to ns + n′s and to ns + 5d(Cs) are seen to be slightly perturbed while the states dissociating to ns + n′p and to np + n′s are significantly perturbed. The wavefunctions for the interacting ³Σ⁺, ³Π, 0⁺, 0⁻, 1, 2 states for the molecules NaK and LiCs are presented for various internuclear distances.     

Characteristic and non-characteristic X-ray emission from SF6 and SO2 molecules by electron impact

Cross sections for K-shell ionization of sulphur in collisions of electrons with kinetic energies of 3.5–14.0 keV with SF₆ and SO₂ gases have been measured. In addition, the impact energy dependence of the bremsstrahlung radiation emitted at different photon energies was investigated. The experimental results are compared with theoretical calculations in plane wave Born approximation and with the available semi-empirical models.     

Quantum chemical topology at the spin–orbit configuration interaction level: Application to astatine compounds

We report a methodology that allows the investigation of the consequences of the spin–orbit coupling by means of the QTAIM and ELF topological analyses performed on top of relativistic and multiconfigurational wave functions. In practice, it relies on the “state-specific” natural orbitals (NOs; expressed in a Cartesian Gaussian-type orbital basis) and their occupation numbers (ONs) for the quantum state of interest, arising from a spin–orbit configuration interaction calculation. The ground states of astatine diatomic molecules (AtX with X = At F) and trihalide anions (IAtI⁻ , BrAtBr⁻ , and IAtBr⁻ ) are studied, at exact two-component relativistic coupled cluster geometries, revealing unusual topological properties as well as a significant role of the spin–orbit coupling on these. In essence, the presented methodology can also be applied to the ground and/or excited states of any compound, with controlled validity up to including elements with active 5d, 6p, and/or 5f shells, and potential limitations starting with active 6d, 7p, and/or 6f shells bearing strong spin–orbit couplings.     

Quasirelativistic energy-consistent 5f-in-core pseudopotentials for pentavalent and hexavalent actinide elements

Quasirelativistic energy-consistent 5f-in-core pseudopotentials modeling pentavalent (5f ^n?2 occupation with n = 2–6 for Pa–Am) and hexavalent (5f ^n?3 occupation with n = 3–6 for U–Am) actinides have been adjusted. Energy-optimized (6s5p4d) and (7s6p5d) valence basis sets contracted to polarized double- to quadruple-zeta quality as well as 2f1g correlation functions have been derived. Corresponding smaller basis sets (4s4p3d) and (5s5p4d) suitable for calculations on actinide(V) and actinide(VI) ions in crystalline solids form subsets of these basis sets designed for calculations on neutral molecules. Calculations using the Hartree–Fock and the coupled-cluster method with single and double excitation operators and a perturbative estimate of triple excitations for actinide pentafluorides show satisfactory agreement with calculations using 5f-in-valence pseudopotentials and experimental data, respectively. However, in the hexavalent case the 5f-in-core approximation seems to reach its limitations except for hexavalent uranium (5f⁰), where results for both uranium hexafluoride and the uranyl ion deviate only slightly from the 5f-in-valence reference data.     

UV and VUV spectrum of matrix-isolated In: an investigation by absorption,magnetic circular dichroism and emission yield spectroscopy

The electronic absorption spectra of In atoms isolated in neon, argon, krypton and xenon matrices have been measured in the energy range between 2.5 and 9.0 eV. This region includes the 5s²5p → 5s²6s and 5s²5p → 5s²5d resonance transitions, higher members of the corresponding s- and d-Rydberg series and the inner shell 5s²5p → 5s5p² transitions. A correlation of the absorption spectra with results obtained from magnetic circular dichroism and fluorescence measurements has made it possible to provide a detailed assignment of most of the features in the spectra in spite of the complexities associated with their behavior. For example, the transition to 5s²6s could not be detected in any of the matrices and the 5s²5d configuration was found to be strongly quenched in intensity as compared to the other transitions. In contrast, several Rydberg transitions could be observed for In in Ne. These were satisfactorily interpreted within the Frenkel formalism. Some of these observations have been rationalized by assuming that the average radius of the wavefunction for the excited state is the dominant parameter for the matrix interaction.     

1H NMR and spectrophotometric study of alkaline metal ion complexes with N-dansyl aza-18-crown-6

Formation of complexes of A18C6-Dns and metal cations (Ca²⁺, Sr²⁺, Ba²⁺ and Mg²⁺) in acetonitrile has been studied by NMR, absorption and fluorescence spectroscopy and PM5 semi-empirical methods. A18C6-Dns forms stable complexes with Ca²⁺, Sr²⁺ and Ba²⁺ cations. The stability constants of various complexes are determined by different methods and their structures are visualised by the PM5 semi-empirical calculations.