Transient absorption studies on 3,6-dibromo polyvinylcarbazole and its model compounds

3,6-Dibromo-N-ethylcarbazole (DBNEC) and its polymeric analogue poly-3,6-dibromovinylcarbazole (PDBVCz) were studied by transient absorption spectroscopy. The transient absorption spectrum of the 3,6-dibromo-N-ethylcarbazole radical cation and decay rate constants of radical cations of 3,6-dibromo-N-ethylcarbazole and its polymeric analogue are presented. In the case of unsubstituted carbazole, the ratio of the yield of radical cation of monomer to polymer is 2.0, whereas in the case of PDBVCz, under the same experimental conditions, the yield of the radical cation is an order of magnitude less in comparison with the monomer model compound DBNEC. This drastic difference in yield has been correlated to the difference in the conformational structure of the polymer as evidenced by nuclear magnetic resonance spectroscopy. (C) 1997 Elsevier Science S.A.

Studies of ternary Li2SO4-Li2O-P2O5 glasses

Glasses in a wide range of compositions in the ternary system xLi(2)SO(4-y)Li(2)O-zP(2)O(5) where x ranges from 0 to 30 mol%, y ranges from 35 to 55 mol% and z ranges from 25 to 50 mol% have been prepared and their properties measured using infra-red, Raman, and P-31 magic angle spinning nuclear magnetic resonance spectroscopic techniques. We conclude that a random close packing of phosphate and sulphate ions which also leads to formation of connected voids in the structure is consistent with our data. There is also evidence for formation of condensed sulphate-phosphate species in the liquid which may be retained in the glass structure. (C) 1999 Elsevier Science B.V. All rights reserved.

Studies on the Effect of Li2SO4 on the Structure of Lithium Borate Glasses

Thermal and spectroscopic investigations have been carried out on a number of glasses with a wide range of compositions in the pseudoternary glass system, Li2SO4-Li2O-B2O3, to understand the role of sulfate ions in modifying the borate glass structure. Both nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic results indicate that four-coordinate boron atoms are retained in the glass structure to a greater extent in sulfate-containing glasses than in pure lithium borate glasses. There seems to be some evidence for the existence of sulfoborate-type units in Raman spectra in the region of 800-960 cm(-1). These conclusions are supported by the observed behavior of glass transition temperatures and molar volumes. The possibility of formation of sulfoborate-type units is discussed from bonding and thermodynamic points of view.

The steady state nuclear Overhauser effect revisited

The steady state of a two spin system coupled to an isotropic environment and to each other through a dipolar interaction and under irradiation by a monochromatic, circularly polarized radio frequency field is determined ab initio using thermodynamic arguments. This steady state is used to describe the well known nuclear Overhauser effect in liquids. The steady state is also derived from the Solomon-Bloch set of equations used to describe the driven spin. It is shown that in the limit of weak driving, the two solutions coincide. (C) 1999 American Institute of Physics. [S0021-9606(99)71210-9].

Chemistry of dihydrogen complexes containing only phosphorus co-ligands

A series of new dicationic dihydrogen complexes of ruthenium of the type cis-[(dppm)(2)Ru(eta(2)-H-2)(L)][BF4](2) (dppm = Ph2PCH2PPh2; L = phosphite) have been prepared by protonating the precursor hydride complexes cis-[(dPPM)(2)Ru(H)(L)][BF4] using HBF4.Et2O. The precursor hydride complexes have been obtained from trans-[(dppm)(2)Ru(H)(L)][BF4][(L = phospfiite) via a rare acid-catalysed isomerization reaction in six coordinate species. The trans-[(dppm)(2)Ru(H)(L)][BF4] complexes (L = phosphine) upon protonation gave the isomerized derivatives, however, further addition of acid resulted in a five-coordinate species, [(dppm)(2)RuCl](+) presumably via an intermediate phosphine dihydrogen complex. The electronic as well as the steric properties of the co-ligands seem to strongly influence the structure-reactivity behaviour of this series of complexes.

Chemistry of tetrathiomolybdate: Applications in organic synthesis

The utility of tetrathiomolybdate in a variety of organic transformations is presented in this account. The sulfur transfer ability of tetrathiomolybdate is exploited in the synthesis of organic disulfides under mild reaction conditions. The induced internal redox reactions associated with tetrathiomolybdate have been thoroughly exploited in developing various methodologies, which include the reduction of organic azides, synthesis of diselenides, cyclic imines, thioamides, and thiolactams. In addition, novel deprotection strategies using tetrathiomolybdate have been developed to cleave the propargyl and propargyloxy carbonyl (POC) protecting groups. Tetrathiomolybdate mediated tandem sulfur transfer-reduction-Michael reactions have been applied to the synthesis of sulfur containing bicyclic systems. Furthermore, the reactions in the solid state and the reactions in water medium assisted by tetrathiomolybdate have greatly simplified the synthesis of organic disulfides.

Developments in quantum information processing by nuclear magnetic resonance: Use of quadrupolar and dipolar couplings

Use of dipolar and quadrupolar couplings for quantum information processing (QIP) by nuclear magnetic resonance (NMR) is described. In these cases, instead of the individual spins being qubits, the 2(n) energy levels of the spin-system can be treated as an n-qubit system. It is demonstrated that QIP in such systems can be carried out using transition-selective pulses, in (CHCN)-C-3, (CH3CN)-C-13, Li-7 (I = 3/2) and Cs-133 (I = 7/2), oriented in liquid crystals yielding 2 and 3 qubit systems. Creation of pseudopure states, implementation of logic gates and arithmetic operations (half-adder and subtractor) have been carried out in these systems using transition-selective pulses.

High temperature phase chemistry of system Eu-Pd-O

An isothermal section of the phase diagram for the system Eu - Pd - O at 1223 K has been established by equilibration of samples representing 20 different compositions, and phase identification after quenching by optical and scanning electron microscopy, X-ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides, Eu4PdO7, Eu2PdO4, and Eu2Pd2O5, were identified. Liquid alloys and the intermetallic compounds EuPd2 and EuPd3 were found to be in equilibrium with EuO. The compound EuPd3 was also found to coexist separately with Eu3O4 and Eu2O3. The oxide phase in equilibrium with EuPd5 and Pd rich solid solution was Eu2O3. Based on the phase relations, four solid state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 925 to 1350 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to crosscheck the data. An advanced version of the solid state cell incorporating a buffer electrode with yttria stabilised zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high temperature thermodynamic measurements. Equations for the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides Eu2O3 with C type structure and PdO have been established. Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Eu - Pd - O have been developed.

Organometallic chemistry of chiral diphosphazane ligands: Synthesis and structural characterisation

The diphosphazane ligands of the type, (C20H12O2)PN(R)P(E)Y2 (R = CHMe2 or (S)-*CHMePh; E = lone pair or S; Y2 = O2C20H12 or Y = OC6H5 or OC6H4Me-4 or OC6H4OMe-4 or OC6H4But-4 or C6H5) bearing axially chiral 1,1'-binaphthyl-2,2′-dioxy moiety have been synthesised. The structure and absolute configuration of a diastereomeric palladium complex, [PdCl2{ηsu2}-((O2C20H12)PN((S)-*CHMePh)PPh2] has been determined by X-ray crystallography. The reactions of [CpRu(PPh3)2Cl] with various symmetrical and unsymmetrical diphosphazanes of the type, X2PN(R)PYY′ (R = CHMe2 or (S)-*CHMePh; X = C6H5 or X2 = O2C20H12; Y=Y′= C6H5 or Y = C6H5, Y′ = OC6H4Me-4 or OC6H3Me2-3,5 or N2C3HMe2-3,5) yield several diastereomeric neutral or cationic half-sandwich ruthenium complexes which contain a stereogenic metal center. In one case, the absolute configuration of a trichiral ruthenium complex, viz. [Cp*Ruη2-Ph2PN((S)-*CHMePh)*PPh (N2C3HMe2-3,5)Cl] is established by X-ray diffraction. The reactions of Ru3(CO)12 with the diphosphazanes (C20H12O2)PN(R)PY2 (R = CHMe2orMe; Y2=O2C20H12or Y= OC6H5 or OC6H4Me-4 or OC6H4OMe-4 or OC6H4But-4 or C6H5) yield the triruthenium clusters [Ru3(CO)10{η-(O2C20H12)PN(R)PY2}], in which the diphosphazane ligand bridges two metal centres. Palladium allyl chemistry of some of these chiral ligands has been investigated. The structures of isomeric η3-allyl palladium complexes, [Pd(η3-l,3-R′2-C3H3){η2-(rac)-(02C20H12)PN(CHMe2)PY2}](PF6) (R′ = Me or Ph; Y = C6H5 or OC6H5) have been elucidated by high field two-dimensional NMR spectroscopic and X-ray crystallographic studies.

Organometallic chemistry of diphosphazanes. Part 15. Half-sandwich cyclopentadienyl ruthenium complexes of achiral and chiral diphosphazanes

Reaction of [CpRu(PPh3)(2)Cl] (1) {Cp = eta(5)-(C5H5)} with X2PN(CHMe2) PYY' {X = Y = Y' = Ph (L-1); X = Y = Ph, Y' = OC6H4Me-4 (L-4); X = Y = Ph, Y' = OC6H3Me2- 3,5 (L-5); X = Y = Ph, Y' = N2C3HMe2 (L-6)} yields the cationic chelate complexes, [CpRu(eta(2)-(X2PN(CHMe2) PYY')) PPh3] Cl. On the other hand, the reaction of 1 with X2PN(CHMe2)PYY' {X = Ph, YY' = O2C6H4(L-3)} gives the complex, [CpRu(eta(1)-L-2)(2)PPh3] Cl. Both types of complexes are formed with X2PN(CHMe2) PYY' {X = Ph, YY' = O2C6H4 (L-3)}. The reaction of 1 with (R),(S)-(H12C20O2) PN(CHMe2) PPh2 (L-7) yields both cationic and neutral complexes, [CpRu{eta(2)-(L-7)} PPh3] Cl and [CpRu{eta(1)-(L-7)}(2)PPh3] Cl and [CpRu{eta(2)-(L-7)}Cl]. The reactions of optically pure diphosphazane, Ph2PN(*CHMePh) PPhY (Y = Ph (L-8); Y = N2C3HMe2-3,5 (L-9)) with 1 give the neutral and cationic ruthenium complexes, [CpRu{eta(2)-(Ph2PN(R) PPhY)} Cl] and [CpRu{eta(2)-(Ph2PN(R)PPhY)} PPh3] Cl. "Chiral-at-metal" ruthenium complexes of diphosphazanes have been synthesized with high diastereoselectivity. The absolute configuration of a novel ruthenium complex, (SCSPRRu)-[(eta(5)-C5H5) Ru*{eta(2)-(Ph2PN(*CHMePh)P*Ph( N2C3HMe2-3,5))} Cl] possessing three chiral centers, is established by X-ray crystallography. The reactions of [CpRu{eta(2)-(L-8)} Cl] with mono or diphosphanes in the presence of NH4PF6 yield the cationic complexes, [CpRu{eta(2)-(L-8)}{eta(1)-(P)}] PF6 {P = P(OMe)(3), PPh3, Ph2P(CH2)(n)PPh2 (n = 1 or 2)}.

Transformation in heterokaryons of Neurospora crassa is nuclear rather than cellular phenomenon

In Neurospora crassa, multinucleate macroconidia are used for genetic transformation. The barrier for such a transformation can be either at the cell membrane level or at the nuclear membrane level. For assessment of these possibilities, a forced heterokaryon (containing two genetically marked nuclei and auxotrophic for histidine) of Neurospora crassa was transformed with a plasmid containing his-3(+) gene. The transformants, which could grow without histidine supplementation, were then resolved into component homokaryons to determine into which nucleus or nuclei the plasmid had entered. Our results suggest that the barrier for transformation in Neurospora crassa is at the nuclear level, not at the cell membrane level. In a heterokaryon containing two genetically distinct nuclei, plasmid DNA integrated into only one of the nuclear types at any instance, but never into both nuclear types. Thus, in Neurospora crassa, the competent nucleus is essential for the transformation event to take place, and at a given time only one type of nucleus is competent to take up the exogenous DNA. Genomic Southern analysis showed that the transformants harbor both ectopic and homologous integrations of the plasmid DNA. The type and number of integrations were reflected at the post-translational level, since the specific activity of histidinol dehydrogenase (the translation product of his-3+ gene) was variable among several transformants and always less than the level of the wild type.

Carbothermal reduction and nitridation reaction of SiOx and preoxidized SiOx: Formation of α-Si3N4 fibers

The chemical composition of amorphous SiOx has been analyzed by oxidation studies and is found to be SiO1.7. SiO1.7 appears to be a monophasic amorphous material on the basis of 29Si nuclear magnetic resonance, high resolution electron microscopy, and comparative behavior of a physical mixture of Si and SiO2. Carbothermal reduction and nitridation reactions have been carried out on amorphous SiO1.7 and on amorphous SiO2 obtained from oxidation of SiO1.7. At 1623 K reactions of SiO1.7 lead exclusively to the formation of Si2N2O, while those of SiO2 lead exclusively to the formation of Si3N4. Formation of copious fibers of α-Si3N4 was observed in the latter reaction. It is suggested that the partial pressure of SiO in equilibrium with reduced SiO1.7 and SiO2 during the reaction is the crucial factor that determines the chemistry of the products. The differences in the structures of SiO2 and SiO1.7 have been considered to be the origin of the differences in the SiO partial pressures of the reduction products formed prior to nitridation. The effect of the ratios, C:SiO1.7 and C:SiO2, in the reaction mixture as well as the effect of the temperature on the course of the reactions have also been investigated.