Kinetic energy analysis of atomic multiplets

Summary A kinetic energy analysis of total energy differences in atomic multiplets arising fromf ^m (m=2–12) electronic configurations is performed within the nonrelativistic restricted Hartree-Fock framework. For these 1290 multiplets of 22 lanthanoid (Ce to Er) and actinoid (Th to Fm) atoms, a very good linear correlation between the total energy difference and the kinetic energy difference of the outermostf-electrons is found. The present results, together with our previous ones for the multiplets arising froms ^mpⁿ (m=1,2;n=2–4) ands ^mdⁿ (m=0–2;n=2–8) electronic configurations, demonstrate that the kinetic energy difference of electrons in open subshells is an excellent predictor of total energy differences among atomic multiplet states.     

Dependence of computer model structure of an amorphous solid on hardness of the repulsive potential

In models of an amorphous solid with a potential U = ar_–n(n = 3–20), the main structural elements were found to be tetrahedron and quartoctahedron. At lower n there are more tetrahedra and less quartoctahedra. In addition to the fom and size of separate Delaunay simplices, their mutual arrangement has been studied by percolafion analysis. For that we used the coloring of the Voronoi network: according to tetrahedricity of simplices to identify dense atomic configurations, and according to the radii of mouths of cavities (passages between simplices) to identify the cavities. We have found percolative cavities in liquids and percolating dense configurations in all amorphous solid models.Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 94–105, September–October, 1993.Translated by L. Smolina     

Computer simulation of disordered structures and nanosystems: An atomic-scale view

This paper reviews the motivations underlying the organization of the 2008 EMRS Symposium “Morphology and dynamics of nanostructures and disordered systems via atomic-scale modelling” and provides an overview of the main results contained in the nine papers composing the present special issue. We underline the use of molecular dynamics as a main tool to describe structural evolution for systems escaping a precise experimental determination of their atomic configurations (nanostructures and disordered networks).     

The Generalized Sturmian Method for Calculating Spectra of Atoms and Ions

The properties of generalized Sturmian basis sets are reviewed, and functions of this type are used to perform direct configuration interaction calculations on the spectra of atoms and ions. Singlet excited states calculated in this way show good agreement with experimentally measured spectra. When the generalized Sturmian method is applied to atoms, the configurations are constructed from hydrogenlike atomic orbitals with an effective charge which is characteristic of the configuration. Thus, orthonormality between the orbitals of different configurations cannot be assumed, and the generalized Slater–Condon rules must be used. This aspect of the problem is discussed in detail. Finally spectra are calculated in the presence of a strong external electric field. In addition to the expected Stark effect, the calculated spectra exhibit anomalous states. These are shown to be states where one of the electrons is primarily outside the atom or ion, with only a small amplitude inside.     

From amorphous solid to defective crystal. A study of structural peculiarities in close packings of hard spheres

The structure of models of close packings of hard spheres is examined, with density of the packings exceeding the maximum value for uniform disordered packings (η = 0.64). High densities are achieved due to regions of the closest packing (η = 0.74) emerging in the models. Spatial geometry of good tetrahedral atomic configurations and of simplest elements of the crystal structure identified by Delaunay simplices was studied using the Voronoi— Delaunay method. Models with a packing coefficient η varying from 0.639 to 0.706 were considered. At smaller densities, a well-known disordered close “Bernal packing” is realized. At η = 0.706 (the greatest density achieved), a unified crystal structure with numerous defects is formed. At intermediate densities, stochastically oriented crystalline nuclei are observed. Specific atomic aggregates — stacks of five-membered rings in good tetrahedral configurations of spheres — are revealed in models having a substantial fraction of crystalline phase (η = 0.664). Such non-trivial structures can occur only in packings that are intermediate between amorphous and crystalline phases.     

Synthese und Konfigurationszuordnung eines potentiell antimikrobiellen 5,6-Dihydroxyisobenzofuranons

Summary Isobenzofuranone3 was prepared by KMnO₄-oxidation of2. 3 was reacted to give acetals4–10, which adopt the boat-conformation.3 and its acylated derivatives11–13 on the other hand prefer the chair-conformation. The relative configurations of3–13 at the chiral centres 3a, 5, 6, and 7a were determined by¹H-NMR-spectroscopy.

     

Preparation of 6-chloro-6-desoxy- and 6-iodo-6-desoxycellulose

Conclusions The reaction of 2,3-di-O-acetylcellulose with N-chloro- or N-iodosuccinimide and triphenylphosphine gave the 6-chloro-6-desoxy- and 6-iodo-6-desoxycellulose acetates with a respective degree of substitution (DS) in halogen of 0.8–1.0 and 0.7. The deacetylation of these compounds gave 6-chloro-6-desoxycellulose (DS 0.8) and 6-iodo-6-desoxycellulose (DS 0.6).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2158–2160, September, 1977.     

Disordered Crystal Structures of Three Halogenated Propionamides CF3CFXCONH2 (X = Cl, Br, I)

The disordered crystal structures of three halogenated propionamides of the general formula CF₃CFXCONH₂, X = Cl (I), Br (II), and I (III), were studied by X-ray diffraction (CAD-4 automatic diffractometer, MoK ). All these structures are monoclinic with space group P2 _1/c . The structures were solved by direct methods and refined by the full-matrix least-squares procedure anisotropically for nonhydrogen atoms. It was established that structures II and III is isomorphic to each other, whereas structure I is not isomorphic to them, although it has a similar molecular packing. The same crystallographic position is statistically (with different probabilities) occupied by two identical molecules (I, II, or III) with different configurations, which may be regarded as one disordered molecule where two substituents at the C₂ atom (halogen X atom and CF₃ group) are mutually statistically interchangeable in the space of the unit cell. The amide fragment (–CONH₂) of these disordered molecules is practically ordered. The crystal structures of I, II, and III have a developed system of intermolecular H-bonds of N–H O type and unusual intramolecular H-bonds of N–H F–C type.     

Temperature Distribution in a Pilot Plasma Tundish: Comparison Between Plasma Torch and Graphite Electrode Systems

Arc, bath, and refractory wall temperatures are measured in a pilot transferred-arc plasma furnace by atomic emission spectroscopy (AES) and multiwavelength pyrometry. Argon plasma torch and graphite electrode with nitrogen as plasma gas are both examined and compared using the steel bath as anode. With argon, a two-slope characteristic curve is measured for arc temperature, which ranges from 9000 to 25,000 K. Another trend is observed with nitrogen for temperatures in the range 8000–12,000 K. In this latter case, the bath temperature is very sensitive to arc length: more than 100 K increase results in arc length rise from 50 to 150 mm. Experimental data shows the variation of heat transfer efficiency between the two configurations, which is supported by results about surface emissivity in the spectral domain 1–15 m.     

Substitutional and orientational disorder in organic crystals: a symmetry-adapted ensemble model

Modelling of disorder in organic crystals is highly desirable since it would allow thermodynamic stabilities and other disorder-sensitive properties to be estimated for such systems. Two disordered organic molecular systems are modeled using a symmetry-adapted ensemble approach, in which the disordered system is treated as an ensemble of the configurations of a supercell with respect to substitution of one disorder component for another. Computation time is kept manageable by performing calculations only on the symmetrically inequivalent configurations. Calculations are presented on a substitutionally disordered system, the dichloro/dibromobenzene solid solution, and on an orientationally disordered system, eniluracil, and the resultant free energies, disorder patterns, and system properties are discussed. The results are found to be in agreement with experiment, when some physically implausible configurations are removed from the ensemble average for eniluracil, highlighting the dangers of a completely automated approach to organic crystal thermodynamics which ignores the barriers to equilibration once the crystal has been formed.     

Sizes and Conformation of the Molecules of DNA–Surfactant Complexes in Dilute Solutions and on the Atomic Smooth Substrates

The conformations of the molecules of DNA–surfactant complexes in dilute solutions and on the atomic smooth surfaces of mica and highly oriented pyrolytic graphite were comparatively studied by the methods of isothermal diffusion, electric birefringence, and atomic force microscopy. The DNA–surfactant complexes were deposited onto the substrates from a chloroform solution. The number of particles of the DNA–surfactant complex on the substrate was changed by varying the concentration of the initial solution within three orders of magnitude. The particles of a shape close to ellipsoidal, 25–70 nm in diameter and 2–4 nm high, were observed at the lowest concentration of DNA–surfactant solution on the mica substrate. The shape and size of these particles correspond to those of a single DNA–surfactant complex, calculated from its translational diffusion coefficient and the time of orientational relaxation in dilute solutions. An increase in the number of molecules deposited onto the substrate leads to an increase in the characteristic sizes of DNA–surfactant complex particles observed by the atomic force microscopy. This may be associated with the aggregation of DNA–surfactant complexes.     

Some N-alkyl-naphthylazo pyridinium salts as new reagents for the spectrophotometric determination of anionic surfactants

Summary Eleven pyridinium azo dyes with straight-chain alkyl groups C_nH_2n+1–(n=6–18) and bromoalkyl groups BrC_mH_2m–(m=6–12) were synthesized with the intention of developing reagents for the determination of low levels of anionic surfactants in an aqueous medium. The effect of the alkyl chain length of these reagents on the reactivity with anionic surfactants such as sodium dodecylsulphate (SDS), sodium linear-dodecylbenzenesulphonate (DBS), sodium dodecylsulphonate (DS) and sodium laurate (SL) was studied. It was found that the alkyl chain length played an important role in the formation of ion associates and the composition of the ion associates formed. These reagents were classified into four groups with respect to the reactivity with anionic surfactants. The first group (n,m=6) reacts only with DBS. The second group (n,m=8) reacts with SDS, DBS and DS. The third group (n,m=10, 12) reacts with SDS, DBS and DS; however, the colour intensity of the DBS-ion associate was unstable. The fourth group (n=14, 16, 18) reacts with all anionic surfactants examined, and the composition of the ion associates with SDS and DS was 2:1 ([reagent]/[surfactant]) though that of the ion associates of the three reagent groups mentioned above was 1:1. The optimal conditions for the determination of anionic surfactants in river water with 1-octyl-4-(4-aminonaphthylazo)-pyridinium bromide was examined. The calibration graph was linear up to 3×10^–6 mol/l, and the apparent molar absorptivity of the ion associate was 3.8×10⁴ l mol^–1 cm^–1 (at 427 nm). The relative standard deviation for 2.4×10^–6 mol/l SDS was 4.9%. Recoveries of 88–107% were found for 8.0×10^–7 mol/l SDS in river water samples.     

Linear Approximation of Basis Set Orbital Exponents for Isoelectronic Series of Atoms

The orbital exponents of Slater type atomic orbitals (AOs) in isoelectronic series of atoms may be approximated by the linear dependence on the nuclear charge using a technique developed for optimization of AO basis sets in Hartree–Fock–Roothaan calculations. This approach yields the analytical Hartree–Fock wave functions for any ion in the isoelectronic atomic series without optimization of orbital exponents. The approximated linear equations for atomic orbital basis sets of B, C, O, and F in the ground state are presented as an example.     

Two-Stage Atomizer for Electrothermal Atomic Absorption Spectrometry: Dynamics of the Spatial Temperature Distribution

A two-stage atomizer for atomic absorption spectrometry is proposed. Its distinctive feature is the introduction of an extra stage of the fractional condensation of analyte atoms and the carrying out of the analytical cycle by a vaporization–condensation–atomization scheme. A special computer-driven power supply unit allows the heating of the upper and lower parts of the graphite furnace to be controlled independently. The dynamics of the temperature of the inner surface of the furnace for various temperature programs is studied. Using aqueous solutions of lead as an example, it is shown that one can control the processes of condensation–revaporization of elements to be determined proceeding in the atomizer volume.     

Determination of Antimony in Environmental Samples by ETAAS Using Different Permanent Modifiers

Single noble metal permanent modifiers such as, Rh, Ir, and Ru, as well as mixed tungsten plus noble metal (W–Rh, W–Ru, W–Ir) permanent modifiers thermally deposited on the integrated platform of a transversally heated graphite atomizer (TGA) were employed for the determination of antimony in sludge, soil, sediment, coal, ash and water samples by electrothermal atomic absorption spectrometry (ETAAS).Microwave digests of solid samples and water samples were directly introduced into different pre-treated platforms of graphite tubes. The performance of the modifiers for accurate antimony determination in real samples depended strongly on the type of permanent modifier chosen. The single noble metal (Rh, Ir and Ru) permanent modifiers were suitable for analyte determinations in simpler matrices, such as waters (recoveries of certified values 95–105%), but the analyte recoveries of certified values in sludge, soil, sediment, coal, and ash samples were always lower than 90%. On the other hand, for the determination of antimony, using W–Rh, W–Ru, and W–Ir permanent modifiers presented recoveries of certified values within 95–105% for all the samples.Long-term stability curves obtained for the determination of antimony in environmental samples with different permanent modifiers (Rh, Ir, Ru, W–Rh, W–Ir, W–Ru) showed that the improvement in tube lifetime depends on the tungsten deposit onto the platform. The tungsten plus noble metal permanent modifier presents a tube lifetime of at least 40% longer compared to a single permanent modifier.     

Nature of the Chemical Bond and the Mutual Influence of Ligands in Co(III) Dioximinates

The results of MWG calculations of the electronic structures of real Co(III) complexes [Co(HD)₂L¹L²] ⁿ were used to analyze the electron density distribution and to determine the charges on atoms and configurations, where nis the charge of the complex and HD is the acid residue of dimethylglyoxime (H₂D); L¹= NH₃at L²= NH₃, Cl^–, Br^–, or I^–and L¹= L²= Cl^–; and L¹= H₂O or NO^– ₂at L²= NO^– ₂, with self-consistency over all atoms of the system and over d, s, and pconfigurations of cobalt. The mutual influence of the ligands (trans- and cis-) was shown to be determined by the atomic charges and bond orders on the axial coordinate and in the equatorial plane of the complex. The following order of the trans-effect was proposed: I^–> Br^–> Cl^–> NO^– ₂> NH₃> H₂O. The effects of the electronic factors on distorsion and conformational processes in the complexes were discussed.     

Ab initio Study of the Geometrical and Electronic Structures of Sc(MDA)2 and Sc(MDA)3 Molecules

The geometrical parameters, force fields, and vibrational spectra of various geometrical configurations of Sc(MDA)₂ and Sc(MDA)₃ molecules were calculated using the restricted Hartree—Fock method taking into account electron correlation by second–order Möller–Plesset perturbation theory. The calculations were performed using effective pseudopotentials for describing atomic cores and valence double zeta basis sets supplemented by polarization functions. It was shown that the configurations of D ₃ [Sc(MDA)₃] and D _2h [Sc(MDA)₂] symmetry correspond to the minimum on the potential energy surface of the ground electronic state. The configurations of D _3h symmetry for Sc(MDA)₃ and D _2d for Sc(MDA)₂ correspond to first–order saddle points. The ground electronic states of Sc(MDA)₂ and Sc(MDA)₃ molecules are the states of ² A _g and ¹ A ₁ symmetries, respectively. For both configurations (D _2h and D _2d of the Sc(MDA)₂ molecule, the energies of vertical electron transitions were calculated using the CASSCF method. The formation of chemical bonding in the compounds considered was analyzed. It was shown that the Sc–O bond is predominantly ionic and the conjugation is characteristic only of the oxygen–carbon skeleton of the chelate fragment. The results obtained are compared with literature experimental and theoretical data for related compounds.     

Calorimetric study of phase transitions in the thiourea 1,1,2,2-tetrachloroethane clathrate compound

Heat capacities of the thiourea clathrate compound of 1,1,2,2-tetrachloroethane, {(NH₂)₂CS}₃(CHCl₂)₂, were measured at temperatures between 13 and 330 K. Two phase transitions were found. The enthalpy and entropy changes of the transition are 5940 J·mol^–1 and 28.1 JK^–1· mol^–1 for the one occurring at 224 K and 2756 J·mol^–1 and 11.3 JK^–1·mol^–1 for the other at 248 K. It is concluded from the transition entropy values that the guest molecules are orientionally disordered nearly to the same extent as in the neat liquid.Contribution No. 56 from the Microcalorimetry Research Center     

A kinetic study of methane conversion by a dinitrogen microwave plasma

Conversion of CH₄ with a N₂ microwave plasma (2.45 GHz) is studied. The experiments cover the absorbed microwave power range 300–700 W with 17–62% of methane in the gas mixture, with pressures of 10–40 mbar and flow rates of 140–650 ml· min^–1. The yields of C₂ hydrocarbons and dihydrogen are analyzed by gas chromatography. The distance of methane addition downstream of the plasma plays an important role on the composition and the concentration of the products obtained. This distance mainly determines the energy concentrated in the active species of the plasma when they react with methane. Different behaviors for acetylene formation, on the one hand, and for ethane and ethene formation, on the other hand, have been observed, and this finding allows us to propose a kinetic mechanism for the decay of methane and for the formation of C₂ hydrocarbons.     

Study of Aluminum–Oxygen Binary Systems: Determination of Partial Interatomic Distances

This paper reports on an Auger-electron study of the oxidation of the perfect surface of polycrystalline Al in residual gas atmosphere at a partial pressure of oxygen of 10 ^-10 torr. Under supervacuum conditions, layer-by-layer oxidation of the aluminum surface takes place along with stabilization of the chemical composition of the oxide film. Oxygen can penetrate into the oxidation region not only by deposition from the residual atmosphere but also by other mechanisms. After the chemical composition of the oxide film was stabilized, its local atomic structure was studied by Al and O K-edge extended electron energy loss fine structure (EELFS) spectroscopy. The estimated partial interatomic distances for Al–Al, Al–O, O–Al, and O–O atomic pairs permitted us to conclude that an oxide-like phase is formed in the subsurface layers of aluminum oxidized under supervacuum conditions and to construct a model of the local atomic structure of this oxide-like phase.