Spektroskopische und strukturelle Untersuchungen an hexagonalen Elpasolithen

Inhaltsübersicht. Es werden die Ergebnisse der röntgenographischen, ligandenfeldspektro-skopischen sowie IR-spektroskopischen Untersuchungen des Mischkristallsystems Sr_2–xBa_xNi TeO₆ mitgeteilt und mit denen entsprechender Untersuchungen der Mischkristalle Ba₂NiTe_xW_1–xO₆ und Ba₂Ni_xMg_1–xTeO₆ verglichen. In diesen Systemen ist mit wachsendem x ein Übergang von der Elpasolith- über die hexagonale BaTiO₃-(6-Schichten-) zur hexagonalen Ba₂NiTeO₆-(12-Schichten-) Struktur zu beobachten. Im Falle der Verbindungen Sr_2–xBa_xNiTeO₆ konnten zusätzlich bei kleinen x-Werten Übergänge von der monoklinen Struktur (Sr₂NiTeO₆) über eine tetragonale Variante der Elpasolithstruktur zum kubischen Gitter gefunden werden. Mittels bei 4 K und 293 K aufgenommener Ligandenfeldspektren wird die Auswirkung der strukturellen Veränderungen beim Ubergang von der geordneten Elpasolithstruktur zu den hexagonalen Varianten auf die Symmetrie und die Bindungsverhältnisse der NiO₆-Oktaeder verfolgt, während die IR-Spektren zusätzlich die Symmetrieverände-rungen der TeO₆-Oktaeder erkennen lassen. Schließlich konnte mit Hilfe beider spektroskopischen Methoden Aussagen über die Kationenverteilung in den hexagonalen Phasen gemacht werden. Spectroscopic and Structural Investigations on Hexagonal Elpasolites The results of X-ray as well as ligand field and i.r. spectroscopic investigations of the mixed crystal series Sr_2–xBa_xNiTeO₆ are reported and compared with those of Ba₂NiTe_xW_1–xO₆ and Ba₂Ni_xMg_1–xTeO₆. In these compounds transitions from the elpasolite to the hexagonal BaTiO₆ (6 layers) and finally to the hexagonal Ba₂NiTeO₆ (12 layers) structure are observed with increasing x. In addition a change from the monoclinic structure (Sr₂NiTeO₆ type) to a tetragonal variant of the elpasolite lattice and finally to the cubic structure is found in the mixed crystal series Sr_2–xBa_xNiTeO₆ at low x values. The influence of these structural changes on the symmetry and the bonding within the NiO₆ polyhedra is studied by the parameters taken from the 4 K and 293 K ligand field spectra. In addition the i.r. spectra give information with respect to the symmetry of the TeO₆ polyhedra. Finally the cation distribution in the hexagonal phases could be deduced from the spectroscopic data.     

The Oxidation of Hexahydrotriazines

Summary. N-Alkyl-2,4-dioxohexahydro-1,3,5-triazines oxidize readily with oxygen to the corresponding cyanuric acid derivates. The oxidation of 1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazine resulted in a stable form of hydroperoxide. During the oxidation of 1,3,5-trimethyl- 6-phenyl-2,4-dioxohexahydro-1,3,5-triazine this hydroperoxide could not be identified, however, the result was the stable reaction product bis-[6-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazin]peroxide.     

13C-NMR. Spectra of Pteridines

¹³C-NMR. spectra of pterin, xanthopterin, isoxanthopterin, leucopterin, lumazine and of the model compounds isocytosine and desamino-isocytosine have been measured as anions and cations in 1 M NaOD, CF₃COOH, H₂SO₄ and FSO₃H solutions. The spectra were analysed by means of heteronuclear double resonance, with the aid of non-decoupled spectra, and by spectral comparison. The results are interpreted in terms of the ionisation state of the pteridines in the four solvents and are compared with those obtained from ¹H-NMR. spectroscopy.     

Anion and cation defect structure in magnesium fluorogermanate

Single crystals of magnesium fluorogermanate have been grown from a melt composition of 3.5MgO·0.5MgF₂·GeO₂ in a lead fluoride flux. The crystals are isomorphous with magnesium germanate, Mg₂₈Ge₁₀O₄₈; the space group is Pbam and a = 14.343(1), b = 10.196(1), and c = 5.9075(4) Å. A final unit cell composition of Mg₂₈Ge_7.5O₃₈F₁₀ was deduced from a detailed analysis of high quality X-ray diffraction data. A residual, R = 0.045, was achieved with a data-parameter ratio of 17 in the final anisotropic refinement.     

A Route to Amino Functionalized Hypericin Derivatives and their Chemical and Photochemical Properties Pertaining to Photodynamic Therapy

Syntheses of amino functionalized hypericin derivatives could be achieved starting from the recently prepared emodin derived 1,3,8-trimethoxy-6-amino-9,10-anthraquinone. Our strategies for the preparation of 10,11-didemethyl-10,11-diaminohypericin, 10,11-didemethyl-10,11-di(acetylamino) hypericin, and its hypericinoidic diazepine derivative include synthetical modifications on the levels of the anthraquinone, anthron, and the phenanthroperylenequinone system itself. The chemical as well as photochemical properties of these unique hypericin derivatives, which might constitute new photodynamic therapy agents, are reported.     

Die Struktur des Spiro-bis(ethylendithia)silans

Contributions to the Chemistry of Silicon Sulfur Compounds. XXXII. The Structure of Spiro-bis (ethylendithia) silane The reaction between sodium methylene dimercaptide with SiCl₄ yielded spiro-bis (ethylendithia)silane. The colorless title compound crystallizes monoclinically with Z = 4 molecules per elementary cell (space group C2/c; a = 1058.4, b = 950.7, c = 1011.9 pm, β = 120.07°; 902 observed unique reflections, R = 0.026). The 4, 4-spiro system nearly is of 222-D₂ symmetry. The twisted five-membered rings as well as the deformation of the spiro angle (ε = 74.4°) result in the deviation from mm2-C₂ symmetry. The mean bond length [d(Si? S) = 211.6pm] was found to be very short. Comparative MNDO/1-calculations of the molecule and for some ortho-acids give no energy minimum for the observed configuration (ε = 74.4°) but show a reduction of the torsional potential to energies comparable to van der Waals interactions, which are caused at the spiro silicon atom by the twisted 5-membered rings.     

Trans-N2F2 and cis-N2F2: A green's function calculation on their photoelectron spectra

The vertical valence ionization potentials of trans-N₂F₂ and cis-N₂F₂ have been computed by a many-body Green function method. For trans-N₂F₂ the agreement with experiment is very satisfactory in general and the calculations permit an analysis and assignment of the experimental photoelectron spectrum. The ionization potentials of cis-N₂F₂ are predictions. The ordering of the ionization potential is for trans-N₂F₂ 5ag(n₊), 2a_u(π), 4b_u(n_?), 4ag, 1b_g(π), 1a_u(π), 3b_u, 3a_g, 2b_u and for cis-N₂F₂ 4b₂ (n_?), 2b₁ (π) + 5a₁(n+), 3b₂, 1a₂ (π), 1b₁(π), 4a₁, 3a₁, 2b2, n₊ and n_? denote lone pairs on the N atoms except for the 4b_u(n_?) orbital which has the largest contribution from the F atoms.     

Monitoring accuracy and precision — Improvements by introducing robust and resistant statistics

A method is presented to monitor for accuracy and precision of chemical analyses based on the use of a control reference sample (CRS) and blind duplicates of project samples. The major advantage of the method is that it works with real samples instead of international (or laboratory) reference standards. Thus, it will take into account changes in absolute or relative errors over the whole observed concentration range. For each project and each sample type, respectively, and for all elements analysed, it provides realistic estimates of precision and — if there are any determinations at low concentrations — of the practical instead of the rather meaningless theoretical detection limit. By introducing robust and resistant statistics it is possible to drastically reduce the number of samples necessary for the monitoring procedure. As an additional advantage, these statistics are independent of any distribution model and solely reflect the data structure. A program for the whole monitoring procedure, the Laboratory Control Package (LCP), has been written in FORTRAN. It can be implemented on any personal computer with graphic capabilities.     

Microfibril diameter in celery collenchyma cellulose: X-ray scattering and NMR evidence

Cellulose isolated from celery collenchyma is typical of the low-crystallinity celluloses that can be isolated from primary cell-walls of higher plants, except that it is oriented with high uniformity. The diameter of the microfibrils of celery collenchyma cellulose was estimated by three separate approaches: ¹³C NMR measurement of the ratio of surface to interior chains; estimation of the dimensions of the crystalline lattice from wide angle X-ray scattering (WAXS) measurements using the Scherrer equation; and the observation that microfibrils of this form of cellulose have the unusual property of packing into an irregular array from which small angle X-ray scattering (SAXS) shows features of both form and interference functions. The interference function contributing to the SAXS pattern implied a mean microfibril centre-to-centre distance of 3.6 nm, providing an upper limit for the diameter. However modelling of the scattering pattern from an irregular array of microfibrils showed that the observed scattering curve could be matched at a range of diameters down to 2.4 nm, with the intervening space more or less sparsely occupied by hemicellulose chains. The lateral extent of the crystalline lattice normal to the 200 plane was estimated as a minimum of 2.4 nm by WAXS through the Scherrer equation, and a diameter of 2.6 nm was implied by the surface: volume ratio determined by ¹³C NMR. The WAXS and NMR measurements both depended on the assumption that the surface chains were positioned within an extension of the crystalline lattice. The reliability of this assumption is uncertain. If the surface chains deviated from the lattice, both the WAXS and the NMR data would imply larger microfibril diameters within the range consistent with the SAXS pattern. The evidence presented is therefore all consistent with microfibril diameters from about 2.4 to 3.6 nm, larger than has previously been suggested for primary-wall cellulose. Some degree of aggregation may have occurred during the isolation of the cellulose, but the larger microfibril diameters within the range proposed are a consequence of the novel interpretation of the experimental data from WAXS and NMR and are consistent with previously published data if these are similarly interpreted.     

Zur Elektronenstruktur hochsymmetrischer Verbindungen der f‐Elemente. 41 Synthese,Kristall‐, Molekül‐ und Elektronenstruktur eines Bis(cyclohexylisonitril)‐Addukts des Grundkörpers Tris(bis(trimethylsilyl)amido)erbium(III) sowie Elektronenstrukturen ausgewählter Monoaddukte

Electronic Structures of Highly Symmetrical Compounds of f Elements. 41 Synthesis, Crystal, Molecular and Electronic Structure of a Bis(cyclohexylisonitrile) Adduct Derived from the Tris(bis(trimethylsilyl)amido)erbium(III) Moiety and Electronic Structures of Selected Mono Adducts The reaction of tris(bis(trimethylsilyl)amido)erbium(III) (Er(btmsa)₃) with two equivalents of cyclohexylisonitrile yields the corresponding bis adduct [Er(btmsa)₃(CNC₆H₁₁)₂] ( 1 ). Complex 1 crystallizes in the monoclinic space group C2/c with a = 2542.9(11) pm, b = 1208.4(4) pm, c = 1783.0(2) pm, β = 122.39(3)°, V = 4.638(5)·10⁹ pm³, Z = 4 and R = 0.0380. The structure of compound 1 features the five coordinate Er³⁺ central ion in a nearly exact trigonal bipyramidal environment, with three btmsa ligands in the equatorial and the two cyclohexylisonitrile molecules in the axial positions. On the basis of the absorption spectra of bis adduct 1 and the mono(tetrahydrofuran) as well as the mono(triphenylphosphine oxide) adducts [Er(btmsa)₃(THF)] ( 2 ) and [Er(btmsa)₃(OPPh₃)] ( 3 ), respectively, the underlying truncated crystal field (CF) splitting patterns of these compounds could be derived, and simulated by fitting the free parameters of a phenomenological Hamiltonian. Reduced r.m.s. deviations of 13.0 cm^?1 (42 assignments), 16.0 cm^?1 (63 assignments) and 17.5 cm^?1 (55 assignments) could be achieved for compounds 1 , 2 and 3 , respectively. Making use of the phenomenological CF parameters of the fits, the experimentally based non‐relativistic molecular orbital schemes of complexes 1 , 2 and 3 were set up, and compared with that of base‐free Er(btmsa)₃.