On the hyperfine structure in the yttrium ion

Experimental data on the hyperfine structure in the 4d5s, 4d ² and 4d5p configurations of the yttrium ion have been analysed by means of the effective operator formalism. The effective radial parameters of the magnetic dipole interaction are determined. A comparison with relativistic calculations gives an estimate of the effects due to configuration interaction.     

Formate incorporation in the structure of Ca-deficient apatite: Rietveld structure refinement

Two sets of non-stoichiometric apatites (Ca-deficient apatites) were prepared from calcium phosphate solutions by homogeneous precipitation through the hydrolysis of formamide at 95°C. One set of products contained monetite (CaHPO₄) and apatite, whilst the second, with more formamide, contained only apatite. Rietveld whole pattern fitting structure refinements were undertaken on all samples, and chemical analyses, IR and NMR spectroscopy, on the second set of samples. The Ca/P mol ratio was 1.596. Rietveld analysis gave lattice parameters a=9.4729(20) and c=6.8855(9) Å and showed that Ca²⁺ ions were lost exclusively from Ca2 sites, and that the PO₄ tetrahedron volume and P-O bonds were 4.4% and 1.4% smaller, respectively, than in hydroxyapatite (OHAp). Formate, HCO₂⁻, was clearly visible in the IR and NMR spectra, but the diffraction studies showed it was not present as a separate crystalline phase. Chemical analysis gave 5.8 wt % formate. We propose that the enlarged a-axis compared with OHAp (a=9.4243(55) Å) and reduced PO₄ dimensions and P occupancy are, respectively, caused by the partial replacement of OH⁻ and PO₄³⁻ ions in the structure by HCO₂⁻ ions. These substitutions would parallel the similar known substitutions of CO₃²⁻ ions in precipitated carbonate apatites.     

Crystal structure and magnetic structure of TbOOH

The monoclinic modification of terbium oxide hydroxide, TbOOH, was prepared using hydrothermal technique. The crystal structure was investigated by three-dimensional single-crystal X-ray analysis and was refined to a conventional R-value of 8.1%. The space group is $\text{P2}{}_1\text{m}$, No. 11, with a = 6.04 Å, b = 3.69 Å, c = 4.33 Å, and β = 109.0°. The terbium atom is seven coordinated with oxygen atoms, and the structure is not hydrogen bonded.The compound is antiferromagnetic with a Néel temperature of 10°K. Neutron diffraction powder patterns were measured at 300°K and 4.2°K. The magnetic super lattice reflections were indexed on the basis of a monoclinic unit cell with the dimensions a_M = 2a, b_M = b, c_M = c, and β_M = β, where a, b, c, and β are the dimensions of the chemical unit cell. The structure contains two independent magnetic atoms. A nonclinear antiferromagnetic arrangement of the spins describes the magnetic structure. The spin at one atom has an angle of 43° with the ac plane and the projection of the spin on the ac plane has an angle of 59° with the a axis. The spin on the other atom has an angle of ?43° with the ac plane, the projection having the same angle of 59° with the a axis.     

On the molecular structure of cycloheptadiene

Molecular mechanics calculations on cycloheptadiene indicate that the molecule has a structure which undergoes a wide pseudorotational motion between two C₁ forms, and a C₁ form, and this structure is in equilibrium with the C₂ form. It is shown that this equilibrium mixture is consistent with all of the available experimental data.     

Fine structure and hyperfine structure in the excited configuration 5p 26s of the antimony I spectrum

By optical interference and VUV spectroscopy the doublet system of SbI 5p ²6s was investigated, so that now the hyperfinestructure of all 8 levels of 5p ²6s (A- andB-splitting constants) are known. From the analysis we receive a spin-orbit parameter ζ_5p =3,538(57) cm^?1 and from the hyperfine-analysis single electron splitting constantsa _5p ⁰¹ =52.4(4.6),a _5p ¹⁰ =?1.6(7.3),a _5p ¹² =72.3(2.3),a _6s ¹⁰ =91.7(4.1),b _5p ⁰² =?49.6(1.1) andb _5p ¹¹ =30.4(3.2) (all values in mK). For all calculations in the fs- and hfs-analysis the 5p ²6s ² D _3/2 has to be excluded (see discussion). With the figures given above the quadrupole moment¹²¹ Q(5p ²6s)=?0.44(3) barn was obtained. It is in good agreement with the¹²¹ Q(5p ³). For the core-polarization by the 5p electron in the innerfieldns-shells (n=1, 2, 3, 4, 5) and the unpaired 6s electron a field of +500(300) kG was obtained.     

On the structure of n-sulphonylformamidines

The structure of N²-sulphonylformamidines has been investigated by means of IR, ¹H and ¹³CNMR spectroscopy. The compounds are shown to exist as two rotamers in solution. A previous report on the isolation of Z and E isomers (regarding the CN²-bond) for one N²-methane-sulphonylformamidine is shown to be dubious, the spectroscopical data indicating the two isomers to be rotamers at the C-N¹ -bond.     

On the structure of liquid methane

An analytical expression for the structure factor, S(k), obtained by treating the square-well potential as a perturbation on the hard-core in the mean spherical model approximation, has been used in computing the structure factor of liquid CH₄.     

Crystal structure refinements of the κ phases in the Hf-Mo-Se and Hf-Mo-Ge systems

The crystal structures of κ-(Hf-Mo-Se) and κ-(Hf-Mo-Ge) were determined using X-ray single-crystal diffractometry. Both structures crystallize in the space groupP6₃/mmc (No. 194) with the unit-cell dimensionsa = 8.6995(4)A?,c = 8.6234(7)A? for κ-(Hf-Mo-Se) anda = 8.6394(4)A?,c = 8.6827(5)A? for κ-(Hf-Mo-Ge). The structures have been refined onF² toR(F²) values of 0.0784 and 0.0661, respectively. κ-(Hf-Mo-Se) and κ-(Hf-Mo-Ge) are isostructural with κ-(Hf-Mo-P) with a variable degree of hafnium substitution on the molybdenum sites of the structure. In κ-(Hf-Mo-Ge) germanium vacancies were found to occur on the trigonal prismatic 2c site and the phase was also found to contain oxygen on the octahedral 6g site. According to the final refinements the compositions are Hf_9+xMo_4?xSe withx = 0.40(2) and Hf_9+xMo_4?xGe_yO₂ withx = 0.25(2),y = 0.88(1), andz = 1.47(12).     

The structure of the dithionite ion

The Raman spectra of aqueous and solid sodium dithionite have been recorded. Differences in the location, intensity, and number of observed bands are attributed to conformational changes in the dithionite ion. The structure of the aqueous ion is non-planar with a C_2h symmetry with an SS bond distance estimated to be 0.220–0.226 nm, as opposed to the dithionite structure in the Na₂S₂O₄·2H₂O salt which is known to have C_2ν structure with a bond distance of 0.2389 nm. The Raman spectra of aqueous dithionite are assigned to A_g (SO) = 997 cm^?1; B_g (SO) at 912 cm^?1, B_g SO₂ twist at 324 cm^?1. The remaining bands are a strong A_g, the SO₂ wag, the SO₂ scissor, and the SS stretch at 584, 461, and 232 cm^?1, respectively, but due to coupling all three motions are expected to exhibit substantial SS character. The variation of the spectra of the solid and aqueous sodium dithionite indicate strong environmental effect on the structure of the anion.     

Step structure in the atomic Kohn-Sham potential

In this work we analyze the exchange-correlation potentialv _xc within the Kohn-Sham approach to density functional theory for the case of atomic systems. The exchange-correlation potential is written as the sum of two potentials. One of these potentialsv _xc,scr is the long-range. Coulombic potential of the coupling constant integrated exchange-correlation hole which represents the screening of the two-particle interactions due to exchange-correlation effects. The other potentialv _xc,scr ^resp contains the functional derivative with respect to the electron density of the coupling constant integrated pair-correlation function representing the sensitivity of this exchange-correlation screening to density variations. As explicit expression of the exchange-part of this functional derivative is derived using an approximation for the Greens function of the Kohn-Sham system and is shown to display a distinct atomic shell structure. The corresponding potentialv _xc,scr ^resp has a clear step structure and is constant within the atomic shells and changes rapidly at the atomic shell boundaries. Numerical examples are presented for the Be and Kr atoms using the Optimized Potential Model (OPM).     

On the structure of the intermediate phases in the praseodymium oxide system

The defect crystal structures for two phases in the praseodymium oxide system (Pr₉O₁₆, ζ phase; and Pr₄₀O₇₂, ε phase) are proposed on the basis of the results of a high-resolution transmission electron microscope (HRTEM) study. Dynamical n-beam electron scattering calculations are used to provide a more accurate and reliable interpretation of the experimental micrographs. In addition, electron diffraction studies reveal the unit-cell dimensions for the n = 11 member in the homologous series of phases in this system. The structural relationships among the related praseodymium oxide phases are discussed.     

Phase equilibria and crystal structure of the complex oxides in the Sr-Fe-Co-O system

Phase relations in the Sr-Fe-Co-O system have been investigated at 1100 °C in air by X-ray powder diffraction on quenched samples. Solid solutions of the form SrFe_1−xCo_xO_3−δ (0?x?0.7), Sr₃Fe_2−yCo_yO_7−δ (0?y?0.4) and Sr₄Fe_6−zCo_zO_13±δ (0?z?1.6) were prepared by solid-state reaction and by the sol-gel method. The structural parameters of single-phase samples were refined by the Rietveld profile method. The variation of the lattice parameters with composition has been determined for each solid solution and a cross-section of the phase diagram at 1100 °C in air for the entire Sr-Fe-Co-O system has been constructed.     

Determination of the absolute structure of albaconol

The concise synthesis of (8aR)-(−)-albaconol (1) from (8aR)-albicanol (2) obtained from the lipase-assisted asymmetric acetylation of rac-2, was achieved in 45% overall yield (eight steps). By comparison of the sign of specific rotation of between synthetic (8aR)-(−)-albaconol (1) and natural (+)-albaconol (1), the absolute structure of natural (+)-1 was determined to be 1R,2R,4aS,8aS configuration.     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

Fast ionic conduction of fluorides with the fluorite-type structure

The authors have studied the Ca_1?xY_xF_2+x (0 < x ? 0.38), Pb_1?xBi_xF_2+x (0 < x ? 0.50), Pb_1?xTh_xF_2+2x (0 < x ? 0.25), MBiF₄ (M = K, Rb, Tl), and PbSnF₄ phases, which are outstanding conductors of the F^? ions. They give some quantitative information about the influence of various structural criteria on the electric properties of these materials.     

On the structure of quasi-crystals in the higher-dimensional space

Categories of the generalized crystallography, where structures are tiled into a large number of identical cells, include quasi-identity and quasi-equivalence. The hierarchy of organization levels including the n-dimensional space is considered. In the theory of proportions, irrational numbers, such as e, π, τ, etc., play an important role. The golden ratio τ is basic to the geometry of structures with five- or tenfold symmetry that are called quasi-crystals. The irrational nature of these numbers probably means that in the Euclidean space E3, we deal with projections of the fundamental polyhedra from a higher-dimensional space. Thus, the structures of quasi-crystals in the three-dimensional space represent slices of much more complex assemblies in the higher-dimensional space.     

Total synthesis of the proposed structure of `brahol' and the structural revision

Proposed structure of brahol, a natural product, has been disproved by total synthesis of the proposed molecule from myo-inositol. Readily available 1,2;4,5-di-O-isopropylidene-myo-inositol, 3 was converted to 2,5-di-O-acetyl-1,6;3,4-di-O-isopropylidene-allo-inositol by epimerization of the di-triflate of 3. The acetyl group at O-5-position was selectively deprotected by aminolysis or methanolysis enabling the total synthesis of 5-O-methyl-allo-inositol, the proposed structure of brahol in six steps from myo-inositol. A comparison of spectral data of synthetic 5-O-methyl-allo-inositol with that reported for natural brahol revealed that the proposed structure of brahol is incorrect. A detailed structural revision revealed that brahol is nothing but quebrachitol. This study contradicts the first and only report on the natural occurrence of allo-inositol derivative.     

Chromium ion incorporation in the crystal structure of BGO

Comprehensive investigations have been performed by EPR and optical spectroscopy for Bi₃GeO₄ crystals doped with chromium ions. It is demonstrated that the known optical absorption spectrum for chromium ions, specifically, the triplet in the region 600–900 nm has an analog in the EPR spectra — the center with electron spin S = 1. The spectrum is described by the spin-Hamiltonian with the parameters D = 550 G, E = 10 G, g _xx = g _yy = 1.915, g _zz = 1.932. The EPR spectrum is dictated by Cr⁴⁺ incorporation at the germanium sites. Luminescence observed in the region 1.2–1.7 μm is also caused by transitions of Cr⁴⁺ with tetrahedral surroundings to germanium sites. Original Russian Text Copyright ? 2005 N. V. Chernei, V. A. Nadolinnyi, N. V. Ivannikova, V. A. Gusev, I. N. Kupriyanov, V. N. Shlegel, and Ya. V. Vasiliev __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 46, No. 3, pp. 444–450, May–June, 2005.     

The influence of the supramolecular structure of water on the kinetics of vaporization

The kinetics of water vaporization was studied gravimetrically using a Q-1500 D derivatograph with an accuracy of ±5 × 10^?5 g under atmospheric conditions. Various supramolecular structures were created in liquid water using solutions of K, Na, Ba, and Zn chlorides with various concentrations. The kinetic dependences of weight P loss caused by the vaporization of solutions were compared with the data on pure water used to prepare the solutions. The dependence of the rate of vaporization V on the concentration of hydrated ions in solutions was used to show that the rate V is the sum of the rates of vaporization of particles of two types, (a) H₂O molecules and (b) supramolecular formations (H₂O clusters) with H-bonds. As a consequence, a nonlinear (piecewise linear) dependence of the kinetics of vaporization P = f(τ) of water and solutions is observed. The rate of vaporization (V ₁) along the initial P = f(τ) curve portions is substantially (by ～30%) higher than its stationary value (V).