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Martin Jansen

Publications and source records attributed to Martin Jansen.

At least 55 records · Page 3Linked to original sources
PubMed2003

Crystal and molecular structure of rubidium peroxodicarbonate Rb2[C2O6].

We report the crystal structure of rubidium peroxodicarbonate, which was synthesized by electrocrystallization at T=257 K, from laboratory X-ray powder diffraction data. The compound crystallizes in the monoclinic space group P2(1)/c with four formula units per unit cell and cell parameters of a=7.9129(1), b=10.5117(1), c=7.5559(1) A, beta=102.001(1) degrees, and V=614.75(1) A(3). The packing can be considered as a strongly distorted CsCl type of structure. The conformation of the peroxodicarbonate anion was found to be planar (C(2h) symmetry), in contrast to the staggered conformation of the peroxodicarbonate anion in the respective potassium peroxodicarbonate. The different conformation is attributed to packing effects.

Journal Article↗
PubMed2003

High lithium ionic conductivity in the lithium halide hydrates Li3-n(OHn)Cl (0.83 < or = n < or = 2) and Li3-n(OHn)Br (1 < or = n < or = 2) at ambient temperatures.

Lithium ionic conductivity and phase transitions in a series of lithium halides hydrates and hydroxides with general formula Li3-n(OHn)X (0.83 < or = n < or = 2; X = Cl,Br) were studied using impedance measurements and 1H and 7Li NMR spectroscopy. All compounds studied in this work crystallize in the antiperovskite structure or are closely related to this structure type. With the exception of LiCl. H2O, all compounds with integer lithium content exhibit good lithium ionic conductivity in their high temperature cubic phases above T = 33 degrees C. Lithium doping of samples LiX.H2O and Li2(OH)X leads to a suppression of the phase transition into the noncubic phases and the good ionic conductivity is extended down to lower temperatures (T < 0 degree C). Thus, lithium doping of the lithium halide hydrates provides a promising tool for tailoring the ionic conductivity at ambient temperatures to its optimum value.

Journal Article↗
PubMed2003

Crystal and molecular structures of alkali oxalates: first proof of a staggered oxalate anion in the solid state.

The molecular and crystal structures of solvent-free potassium, rubidium, and cesium oxalates have been determined ab initio from high-resolution synchrotron and X-ray laboratory powder patterns. In the case of potassium oxalate K(2)C(2)O(4) (a = 10.91176(7) A, b = 6.11592(4) A, c = 3.44003(2) A, orthorhombic, Pbam, Z = 2), the oxalate anion is planar, whereas in cesium oxalate Cs(2)C(2)O(4) (a = 6.62146(5) A, b = 11.00379(9) A, c = 8.61253(7) A, beta = 97.1388(4) degrees, monoclinic, P2(1)/c, Z = 4) it exhibits a staggered conformation. For rubidium oxalate at room temperature, two polymorphs exist, one (beta-Rb(2)C(2)O(4)) isotypic to potassium oxalate (a = 11.28797(7) A, b = 6.29475(4) A, c = 3.62210(2) A, orthorhombic, Pbam, Z = 2) and the other (alpha-Rb(2)C(2)O(4)) isotypic to cesium oxalate (a = 6.3276(1) A, b = 10.4548(2) A, c = 8.2174(2) A, beta = 98.016(1) degrees, monoclinic, P2(1)/c, Z = 4). The potassium oxalate structure can be deduced from the AlB(2) type, and the cesium oxalate structure from the Hg(99)As type, respectively. The relation between the two types of crystal structures and the reason for the different conformations of the oxalate anion are discussed.

Journal Article↗
PubMed2003

X-ray diffraction studies on mesophases of cetyl- and dodecyltrimethylammoniumbromide in liquid ammonia.

We have studied solutions of the surfactants cetyltrimethylammoniumbromide (CTAB) and dodecyltrimethylammoniumbromide (DTAB) in liquid ammonia with respect to the formation of lyotropic phases. For this purpose, a set-up for performing X-ray scattering experiments at temperatures up to 120 degrees C on samples containing liquid ammonia has been developed. Both systems form hexagonal and monoclinic lyotropic phases above the dissolving temperature of the surfactant, thus representing the first examples for lyotropic phases in liquid ammonia, and for monoclinic phases in nonaqueous solvents. The phase diagrams of CTAB/liquid NH(3) and DTAB/liquid NH(3) show similarities to their respective aqueous systems. However, the regions of existence of monoclinic phases are much larger in the ammonia system, while the cubic phases, as observed in the water based systems, do not seem to exist. The liquid-crystalline phases found provide potentiality for preparing mesoporous, nitride-based solids.

Journal Article↗
PubMed2002

A concept for synthesis planning in solid-state chemistry.

There is a widely-held belief that the preparation of new solid-state compounds based on rational design is not possible. Herein, we present a concept that points the way towards a rational design of syntheses in solid-state chemistry. The foundation of our approach is the representation of the whole material world, that is, the known and not-yet-known compounds, on an energy landscape, which gives information about the free energies of these compounds. From this it follows that all chemical compounds capable of existence are present on this landscape. Thus the chemical synthesis always corresponds to the discovery of compounds, not their creation. Consequently, the first step in planning a synthesis can and must be to identify a synthesizable compound. Up to now, materials capable of existence are discovered in the course of an experimental exploration of the energy landscape; however, an a priori identification of a synthesis goal requires an exploration using theoretical methods. In contrast to those computational approaches currently employed for structure determination for fixed composition and already known unit cells, our aims clash with such restrictions and full global optimizations have to be performed on the landscape. Although for reasons of computational feasibility the accuracy of the energy calculations is not yet as high as one would wish, our approach proves to be surprisingly robust. One always finds the already known compounds of a given chemical system, and, in addition, further plausible structure candidates are discovered. The second step of a rational planning of syntheses is the design of feasible synthesis routes. Modeling such routes requires highly accurate computations for realistic thermodynamic conditions, however this is usually beyond our current capabilities. Thus, we have not seriously pursued such a deductive approach; instead we have attempted, to reproduce the "computational annealing" employed during our structure predictions in the experiment. Educts, generated by vapor deposition methods, that are disperse on an atomic level are found to react with surprisingly low activation energies to give highly crystallized products. However, even this technique does not yet provide the possibility to selectively synthesize a specific solid compound. For this final step, modeling and experimental control of nucleation processes will be the key ingredient. Only when viewed superficially, our goal of a "rational design" of solid-state syntheses and the "high-throughput" syntheses are in contradiction. But an exhaustive exploration of the unimaginably large combinatorial diversity of chemistry remains beyond our capabilities, even with an exceedingly high throughput. The future of solid-state synthesis will be found in a union of these two conceptual approaches.

Journal Article↗
PubMed2002

PX4+, P2X5+, and P5X2+ (X=Br, I) salts of the superweak Al(OR)4- anion [R=C(CF3)3].

PX(4) (+)[Al(OR)(4)](-) (X=I: 1 a, X=Br: 1 b) was prepared from X(2), PX(3), and Ag[Al(OR)(4)] [R=C(CF(3))(3)] in CH(2)Cl(2) at -30 degrees C in 69-86 % yield. P(2)X(5) (+) salts were prepared from 2 PX(3) and Ag[Al(OR)(4)] in CH(2)Cl(2) at -30 degrees C yielding almost quantitatively P(2)X(5) (+)[Al(OR)(4)](-) (X=I: 3 a, X=Br: 3 b). The phosphorus-rich P(5)X(2) (+) salts arose from the reaction of cold (-78 degrees C) mixtures of PX(3), P(4), and Ag[Al(OR)(4)] giving P(5)X(2) (+)[Al(OR)(4)](-) (X=I: 4 a, X=Br: 4 b) with a C(2v)-symmetric P(5) cage. Silver salt metathesis presumably generated unstable PX(2) (+) cations from PX(3) and Ag[Al(OR)(4)] (X=Br, I) that acted as electrophilic carbene analogues and inserted into the Xbond;X (Pbond;X/Pbond;P) bond of X(2) (PX(3)/P(4)) leading to the highly electrophilic and CH(2)Cl(2)-soluble PX(4) (+) (P(2)X(5) (+)/P(5)X(2) (+)) salts. Reactions that aimed to synthesize P(2)I(3) (+) from P(2)I(4) and Ag[Al(OR)(4)] instead led to anion decomposition and the formation of P(2)I(5)(CS(2))(+)[(RO)(3)Al-F-Al(OR)(3)](-) (5). All salts were characterized by variable-temperature solution NMR studies (3 b also by (31)P MAS NMR), Raman and/or IR spectroscopy as well as X-ray crystallography (with the exception of 4 a). The thermochemical volumes of the Pbond;X cations are 121 (PBr(4) (+)), 161 (PI(4) (+)), 194 (P(2)Br(5) (+)), 271 (P(2)I(5) (+)), and 180 A(3) (P(5)Br(2) (+)). The observed reactions were fully accounted for by thermochemical calculations based on (RI-)MP2/TZVPP ab initio results and COSMO solvation enthalpy calculations (CH(2)Cl(2) solution). The enthalpies of formation of the gaseous Pbond;X cations were derived as +764 (PI(4) (+)), +617 (PBr(4) (+)), +749 (P(2)I(5) (+)), +579 (P(2)Br(5) (+)), +762 (P(5)I(2) (+)), and +705 kJ mol(-1) (P(5)Br(2) (+)). The insertion of the intermediately prepared carbene analogue PX(2) (+) cations into the respective bonds were calculated, at the (RI-)MP2/TZVPP level, to be exergonic at 298 K in CH(2)Cl(2) by Delta(r)G(CH(2)Cl(2))=-133.5 (PI(4) (+)), -183.9 (PBr(4) (+)), -106.5 (P(2)I(5) (+)), -81.5 (P(2)Br(5) (+)), -113.2 (P(5)I(2) (+)), and -114.5 kJ mol(-1) (P(5)Br(2) (+)).

Journal Article↗
PubMed2002

Computational design and prediction of interesting not-yet-synthesized structures of inorganic materials by using building unit concepts.

The computational design of new and interesting inorganic materials is still an ongoing challenge. The motivation of these efforts is to aid the often difficult task of crystal structure determination, to rationalize different but related structure types, or to help limit the domain of structures that are possible in a given system. Over the past decade, simulation methods have continuously evolved towards the prediction of new structures using minimal input information in terms of symmetry, cell parameters, or chemical composition. So far, this task of identifying candidate structures through an analysis of the energy landscape of chemical systems has been particularly successful for predominantly ionic systems with relatively small numbers of atoms or ions in the simulation cell. After an introductory section, the second section of this work presents the historical developments of such simulation methods in this area. The following sections of the work are dedicated to the introduction of the building unit concept in simulation methods: we present simulation approaches to structure prediction employing both primary (aggregate of atoms) and secondary (aggregate of coordination polyhedra) building units. While structure prediction with primary units is a straightforward extension of established approaches, the AASBU method (automated asssembly of secondary building units) focusses on the topology of network-based structures. This method explores the possible ways to assemble predefined inorganic building units in three-dimensional space, opening the way to the manipulation of very large building units (up to 84 atoms in this work). As illustrative examples we present the prediction of candidate structures for Li(4)CO(4), the identification of topological relationships within a family of metalphosphates, ULM-n and MIL-n, and finally the generation of new topologies by using predefined large building units such as a sodalite or a double-four-ring cage, for the prediction of new and interesting zeolite-type structures.

Journal Article↗
PubMed2002

Synthesis, characterization, and bonding properties of polymeric fullerides AC70.nNH3 (A = Ca, Sr, Ba, Eu, Yb).

Reduction of C70 with alkaline earth and rare earth metals dissolved in liquid ammonia results in metal fulleride solvates AC70.nNH3 (A = Ca, Sr, Ba, Eu, Yb) containing linear polymeric, anionic chains infinity 1 [C70(2-)]. The compounds were characterised by means of Raman spectroscopy and single-crystal structure determination. The accurate crystal structure of [Sr(NH3)8]C70.3NH3, determined with atomic resolution, allowed for a comparison with results of quantum chemical calculations. The nature of the C-C bonds in the fulleride is analysed in detail leading to a model explaining the unexpected polymerisation of C70(2-).

Journal Article↗
PubMed2002

Reversible dimerization of C60 molecules in the crystal structure of the bis(arene)chromium fulleride [Cr(C7H8)]2C60.

Bis(toluene)chromium fulleride, [Cr(C(7)H(8))(2)]C(60), has been synthesized as a black microcrystalline powder from C(60) and [Cr(C(7)H(8))(2)] in toluene. [Cr(C(7)H(8))(2)]C(60) is an ionic compound in which the fullerene is negatively charged and the bis(toluene)chromium molecule positively charged. At T = 250 K a reversible first-order phase transition from a primitive cubic high-temperature phase to a triclinic low-temperature phase occurs. The high-temperature phase [Pm3 macro m, a = 9.9840 (1) A, T = 295 K] is composed of dynamically disordered fulleride anions and bis(toluene)chromium(I) cations in a CsCl-type arrangement. The triclinic low-temperature modification [P1 macro, a = 13.6414 (8), b = 13.8338 (7), c = 13.8548 (7) A, alpha = 91.830 (3), beta = 116.776 (2), gamma = 119.333 (2) degrees, T = 235 K] consists of ordered C(60) dimers and two crystallographically distinct bis(toluene)chromium entities.

Journal Article↗
PubMed2002

Structure of haloform intercalated C60 and its influence on superconductive properties.

CHCl3 and CHBr3 intercalated C60 have attracted particular interest after a superconductivity transition temperature (Tc) of up to 117 K was discovered. We have determined the structure using synchrotron x-ray powder-diffraction and found that the expansion of the lattice mainly takes place in one dimension (triclinic b axis), leaving planes of C60 molecules on an approximately hexagonal, slightly expanded lattice. We have performed tight-binding band structure calculations for the surface layer. In spite of the slight expansion of the layers, for the range of dopings where a large Tc has been observed, the density of states at the Fermi energy is smaller for C60.2CHCl3 and C60.2CHBr3 than for C60. This suggests that the lattice expansion alone cannot explain the increase of Tc.

Journal Article↗
PubMed2002

Tetrakis(chloromethyl)phosphonium chloride monohydrate.

Tetrakis(chloromethyl)phosphonium chloride monohydrate, C(4)H(8)Cl(4)P(+) x Cl(-) x H(2)O or P(CH(2)Cl)(4)(+) x Cl(-) x H(2)O, is the first crystal structure determination of a tetrakis(halogenomethyl)phosphonium compound to date. The only comparable structures known so far are of phosphonium ions containing just one halogenomethyl group. The solvent water molecule interacts with the Cl(-) anion via hydrogen bonds, with O...Cl distances of 3.230 (2) and 3.309 (2) A. The structure also contains several C-H...Cl(-) and C-H...O contacts, though with longer D...A distances [D.A 3.286 (3)-3.662 (2) A] or bent D-H...A angles. For these reasons, the C-H...Cl(-) and C-H...O interactions should not be considered as strong hydrogen bonds.

Journal Article↗
PubMed2002

Treatment of systemic lupus erythematosus by immunoadsorption in a patient suffering from tuberculosis.

We report on a 40-year-old man, admitted with fever and weight loss, in whom systemic lupus erythematosus (lupus nephritis World Health Organization type IV) and concomitant acute lung tuberculosis were diagnosed. Conventional treatment of diffuse proliferative nephritis with cytotoxic drugs was thought to be too dangerous in the presence of active tuberculosis. A combination of immunoadsorption and steroids was instituted for the treatment of systemic lupus erythematosus. Antibodies against double-stranded DNA decreased, and proteinuria decreased from 10 g/24 hours to less than 1 g/24 hours. Tuberculosis was treated initially with quadruple-drug therapy, then a triple-drug protocol. Primarily enlarged lymph nodes decreased to normal size after 3 months. The combined treatment modality of steroids and immunoadsorption was effective and safe, even in this patient with active tuberculosis.

Administration, Oral↗
PubMed2002

Atorvastatin in low-density lipoprotein apheresis-treated patients with homozygous and heterozygous familial hypercholesterolemia.

To further reduce low-density lipoprotein-cholesterol (LDL-C), atorvastatin treatment was investigated in patients with homozygous (n = 4) and heterozygous (n = 10) familial hypercholesterolemia (FH) undergoing LDL-apheresis. After a wash-out period of 4 weeks, atorvastatin therapy was administered in escalating doses (10 up to 80 mg/d). LDL-apheresis was performed at weekly intervals during the entire study period. The LDL-C concentration decreased from 240 +/- 35 mg/dL after the wash-out period to 206 +/- 63 mg/dL during treatment with 10 mg atorvastatin. Four weeks of treatment with 80 mg atorvastatin resulted in an additional 24% (P <.05) reduction in LDL-C. LDL-C increased from 28.8 +/- 14.2 mg/dL immediately after apheresis to 156.6 +/- 25.5 mg/dL at day 7. LDL-C values remained below the recommended target range for an extended duration of 48 hours in atorvastatin-treated patients, but not in those without concomitant lipid-lowering drug therapy. The levels of high-density lipoprotein-cholesterol (HDL-C) and plasma fibrinogen were unchanged during the entire study period. No adverse events were observed with atorvastatin treatment. Finally, high-dose atorvastatin therapy resulted in a 40% reduction in LDL-apheresis sessions in these patients. Our results show that LDL-C reduction by atorvastatin is a safe and effective therapy in LDL-apheresis patients with severe heterozygous or homozygous FH.

Adolescent↗