High-Pressure Low-Temperature Locknut Cell For Both Electron-Paramagnetic-Res And Nmr-Studies To 10 Kilobars And 77-K

A locked high-pressure cell with working pressure range up to 10 kbars suitable for low-temperature studies to 77 K has been described. It can be used for both EPR and NMR studies of single crystals (and other solid samples). The high-pressure seal and all other aspects of the cell remain the same for either application. Only a change of the bottom plug is required for a switch from a nuclear-magnetic-resonance (NMR) to an electron-paramagnetic-resonance (EPR) experiment. Details of the procedure for the calibration of pressure inside the cell at various temperatures are discussed. The performance of the cell in EPR (Cr3+ion) and NMR (27Al nucleus) studies is reported.

Magnetic-properties of quasi-2-dimensional la1-xsr1+xmno4 and the evolution of itinerant electron ferromagnetism in the sro.(la1-xsrxmno3)n system

Quasi-two-dimensional oxides of the La,+,Sr,+,Mn04 system, possessing the KZNiF4 structure, show no evidence for ferromagnetic ordering in contrast to the corresponding three-dimensional La,+.Sr,MnO~ perovskites. Instead, there is an increasing tendency toward antiferromagnetic ordering with mcreasmg x m La,+,Sr,,, MnOp. Furthermore, these oxides are relatively high-resistivity materials over the entire compositional range. Substitution of Ba for Sr in La&r,.5Mn04 decreases the ferromagnetic interaction. Increasing the number of perovskite layers in SrO (La,-,Sr,MnO& causes an increase in electrical conductivity as well as ferromagnetic interaction. The oxide becomes a highly conducting ferromagnet when n 2 2.

Cresols - An investigation into their electronic structure and spectra

The three isomeric cresols were subjected to the all-valence-electron CNDO/2 andPPP-CI calculations. Results from this study were used: (i) to compare the electronic structures of these isomers vis-Ã-vis parent compounds-phenol and toluene, (ii) to obtain a quantitative picture of their chemical reactivities and electronic absorption spectra. Using the sgr-core charges derived from CNDO/2 calculations and subsequently revising the valence-state ionisation potential and one-center-two-electron repulsion integrals, thePPP-CI calculations were performed on the title compounds according toNishimoto andForster scheme. In these calculations the pseudo-unsaturated nature of the methyl group has been given due consideration. In spectral assignment, compared to the conventionalPPP approach, the CNDO/2-basedPPP-CI method gave a better agreement with the experimental data.

Noble Metal Ionic Catalysts

Because of growing environmental concerns and increasingly stringent regulations governing auto emissions, new more efficient exhaust catalysts are needed to reduce the amount of pollutants released from internal combustion engines. To accomplish this goal, the major pollutants in exhaust-CO, NOx, and unburned hydrocarbons-need to be fully converted to CO2, N-2, and H2O. Most exhaust catalysts contain nanocrystalline noble metals (Pt, Pd, Rh) dispersed on oxide supports such as Al2O3 or SiO2 promoted by CeO2. However, in conventional catalysts, only the surface atoms of the noble metal particles serve as adsorption sites, and even in 4-6 nm metal particles, only 1/4 to 1/5 of the total noble metal atoms are utilized for catalytic conversion. The complete dispersion of noble metals can be achieved only as ions within an oxide support. In this Account, we describe a novel solution to this dispersion problem: a new solution combustion method for synthesizing dispersed noble metal ionic catalysts. We have synthesized nanocrystalline, single-phase Ce1-xMxO2-delta and Ce1-x-yTiyMxO2-delta (M = Pt, Pd, Rh; x = 0,01-0.02, delta approximate to x, y = 0.15-0.25) oxides in fluorite structure, In these oxide catalysts, pt(2+), Pd2+, or Rh3+ ions are substituted only to the extent of 1-2% of Ce4+ ion. Lower-valent noble metal ion substitution in CeO2 creates oxygen vacancies. Reducing molecules (CO, H-2, NH3) are adsorbed onto electron-deficient noble metal ions, while oxidizing (02, NO) molecules are absorbed onto electron-rich oxide ion vacancy sites. The rates of CO and hydrocarbon oxidation and NOx reduction (with >80% N-2 selectivity) are 15-30 times higher in the presence of these ionic catalysts than when the same amount of noble metal loaded on an oxide support is used. Catalysts with palladium ion dispersed in CeO2 or Ce1-xTixO2 were far superior to Pt or Rh ionic catalysts. Therefore, we have demonstrated that the more expensive Pt and Rh metals are not necessary in exhaust catalysts. We have also grown these nanocrystalline ionic catalysts on ceramic cordierite and have reproduced the results we observed in powder material on the honeycomb catalytic converter. Oxygen in a CeO2 lattice is activated by the substitution of Ti ion, as well as noble metal ions. Because this substitution creates longer Ti-O and M-O bonds relative to the average Ce-O bond within the lattice, the materials facilitate high oxygen storage and release. The interaction among M-0/Mn+, Ce4+/Ce3+, and Ti4+/Ti3+ redox couples leads to the promoting action of CeO2, activation of lattice oxygen and high oxygen storage capacity, metal support interaction, and high rates of catalytic activity in exhaust catalysis.

Influence of Solvent on Photoinduced Electron-Transfer Reaction: Time-Resolved Resonance Raman Study

Time-resolved resonance Raman spectroscopy (TR3) has been used to study the effect of solvent polarity on the mechanism and nature of intermediates formed in photoinduced electron-transfer reaction between triplet flouranil ((FL)-F-3) and tetramethylbenzene (TMB). Comparison of the TR3 spectra in polar, nonpolar, and medium polar media suggests that formation of radical anion due to electron-transfer reaction between (FL)-F-3 and TMB is favored in more polar solvents, whereas ketyl radical formation is more favored in less polar media. Compared to ketyl radical, the extent of radical anion formation is negligible in nonpolar solvents. Therefore, it is inferred that in nonpolar media ketyl radical is mainly generated by hydrogen-transfer reaction in the encounter complex between (FL)-F-3 and TMB. In solvents of medium polarity, the ion-pair decay leads to the formation of both ketyl radical and ketyl radical formed from the encounter between triplet state and the donor. Thus, competition between the formation of ketyl radical and ion pair is influenced by the solvent polarity. The nature of the ion pair in different solvent polarity has been investigated from the changes observed in the vibrational frequency of (fluoranil) FL part of the complex.

The effect of radiation on the electron pairing in a multiband system

The possible occurrence of a generalized (1-wave) nonequilibrium superconducting state in a multiband system under certain conditions is studied. In the model the radiation field causes interband mixing, and phonons of an appropriate mode (branch) are involved in the interband scattering of electrons of two conduction bands of the system. The strength of the generalized 1-wave pairing interaction between quasiparticles belonging to new radiation admixed states depends on the density (n o/V) of quanta in the system. The coupling constant has the form Xl= AiB(n o/V)/[C + B(no/V)], where A1, B, and C are parameters. For C > B(n0/V), the transition temperature T1* increases with (no/V) in the initial stages. It levels off with higher power. With further increase of power, the transition temperature is expected to drop sharply due to heating effects which cause pair breaking. Estimates show that p-wave (triplet state) pairing may be possible under radiation-induced nonequilibrium situations in appropriate systems. Estimates for lifetimes of various processes quasiparticle, phonon, pair relaxation, and photon-induced mixing) show that the coherence required for the mixing and pairing effects will be maintained for the temperature range and photon density considered.

Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires

Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced B2 -> body-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of similar to 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of similar to 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 angstrom, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Cr3+ electron paramagnetic resonance study of Sn(1-x)CrxO(2) (0.00 <= x <= 0.10)

This paper reports on the liquid-helium-temperature (5 K) electron paramagnetic resonance (EPR) spectra of Cr3+ ions in the nanoparticles of SnO2 synthesized at 600 degrees C with concentrations of 0%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 5.0%, and 10%. Each spectrum may be simulated as overlap of spectra due to four magnetically inequivalent Cr3+ centers characterized by different values of the spin-Hamiltonian parameters. Three of these centers belong to Cr3+ ions in orthorhombic sites, situated near oxygen vacancies, characterized by very large zero-field splitting parameters D and E, presumably due to the presence of nanoparticles in the samples. The fourth EPR spectrum belongs to the Cr3+ ions situated at sites with tetragonal symmetry, substituting for the Sn4+ ion, characterized by a very small value of D. In addition, there appears a ferromagnetic resonance line due to oxygen defects for samples with Cr3+ concentrations of <= 2.5%. Further, in samples with Cr3+ concentrations of >2.5%, there appears an intense and wide EPR line due to the interactions among the Cr3+ ions in the clusters formed due to rather excessive doping; the intensity and width of this line increase with increasing concentration. The Cr3+ EPR spectra observed in these nanopowders very different from those in bulk SnO2 crystals.

Kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on lead dioxide electrodes

The kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on titanium-substrate lead dioxide electrodes have been investigated experimentally and theoretically. It has been demonstrated that the ionic strength of the solution has a marked effect on the rate of perchlorate formation, whereas the pH of the solution does not influence the reaction rate. Experimental data have also been obtained on the dependence of the reaction rate on the concentration of chlorate ion in the solution at constant ionic strength. With these data, diagnostic kinetic criteria have been deduced and compared with corresponding quantities predicted for various possible mechanisms including double layer effects on electrode kinetics. It has thus been shown that the most probable mechanisms for anodic chlorate oxidation on lead dioxide anodes involve the discharge of a water molecule in a one-electron transfer step to give an adsorbed hydroxyl radical as the rate-determining step for the overall reaction.

Development of pseudo-two-dimensional turbulent wakes

Pseudotwo-dimensional wakes are generated by introducing spanwise cellular structures into an otherwise plane turbulent wake by means of the castellated blunt trailing edges of different configurations. The transverse growths of these coflowing cellular wakes are found to be independent of each other without any noticeable spanwise interaction. This wake growth is examined in the light of the plane equilibrium wake analysis. Though these wakes are not found to be exactly self-similar, their growth shows a nonmonotonous approach toward the asymptotic state appropriate to that of a plane wake. The dye emission in the wakes illustrated a coherent vortical structure in the transverse plane, similar to that of the usual two-dimensional wake, in spite of the initial spanwise irregularities.

Electron spin resonance studies of d5 ions (Fe3+ and Mn2+) in lead oxide-lead halide glasses

Electron spin resonance (ESR) of d5 ions (Fe3+ and Mn2+) has been investigated in PbO---PbF2 and PbO---PbCl2 glasses in wide ranges of composition. ESR spectra of d5 ions in these glasses exhibit significant differences which we have attributed to at least three important causes: (i) The ionic potentials of Fe3+ and Mn2+ are different. Hence Fe3+ ions tend to acquire their own environment while Mn2+ ions take up substitutional (Pb2+ ion) positions. (ii) The sizes and nephelauxetic behaviours of O2- and F- ions are similar. Thus even when there is a mixed anionic coordination, the environment of Mn2+ ions is highly symmetrical in oxyfluoride glasses. The Mn2+ spectra in oxychloride glasses are considerably different. (iii) Increase in halide ion concentration increases the ionicity of lead-ligand bonding and favours a more symmetrical environment around dopant ions in halide-rich glasses. The features in ESR spectra have been interpreted in the light of known behaviour of d5 ions in glasses and also in the context of known structural features of PbO---PbX2 glasses. Dopant ions appear to cluster at high concentrations although isolated low-symmetry sites are still observed. Effects of crystallization and annealing upon ESR spectra have also been investigated.

Interaction of Metal Ions with 2'-Deoxyribonucleotides. Crystal and Molecular Structure of a Cobalt(l1) Complex with 2'-Deoxyinosine 5'-Monophosphate

The crystal structure of the cobalt( 11) complex with 2'-deoxyinosine 5'-monophosphate (5'- dlMP), [Co(5'-dlMP) (H,0),]-2H20, has been analysed by X-ray diffraction. The complex crystallizes in the space group P2,2,2, with a = 6.877(3), b = 10.904(2), c = 25.421 (6) A, and Z = 4. The structure was solved by the heavy-atom method and refined to an R value of 0.043 using 1 776 unique reflections. The cobalt ion binds only to the 6-oxopurine base of the nucleotide at the N(7) position, the octahedral co-ordination of the metal being completed by five water oxygens. The phosphate oxygens are involved in hydrogen bonding with the co-ordinated water molecules. The structure is closely similar to that of the corresponding ribonucleotide complex. The nucleotide has the energetically preferred conformation: an anti base, a C(3') -endo sugar pucker, and a gauche-gauche conformation about the C(4')-C( 5') bond. The significance of sugar puckering in the monomeric complexes of general formula [ M (5'-nucleotide) (H20),] is explained in terms of the structural requirements for metal-water-phosphate bridging interactions.

Studies involving electrostatic potential of valinomycin

The electrostatic potential of valinomycin in various conformations as obtained by the crystal structures (uncomplexed, complexed) and theoretical considerations have been evaluated and compared. The potential energy profiles along the æ axis of the bracelet-like structures show a systematic variation from the uncomplexed to the complexed structure. This type of conformational change and the potential variation are probably associated with different states of ion transport, like the capture and release of ions by the ionophore. Also, the asymmetry of the molecule due to D-HyIV on one side and L-Lac on the other side is reflected in the potential values along the Z-axis, the magnitude of which, is considerable in the uncomplexed structure. The evaluation of the potential at the ab-initio level on smaller fragments indicate that the order of liganding capacity of oxygen is amide ether ester. Also, the inductive effects due to alkyl substitution is negligible as evidenced by the potential studies on the substituted amides and esters.

Coherent potential approximation, averaged T-matrix approximation and Lloyd's model

It is shown how the single-site coherent potential approximation and the averaged T-matrix approximation become exact in the calculation of the averaged single-particle Green function of the electron in the Anderson model when the site energy is distributed randomly with lorentzian distribution. Using these approximations, Lloyd's exact result is reproduced.

Interaction of carbon monoxide with surfaces of metglasses

UPS and XPS studies indicate that carbon monoxide preferentially adsorbs dissociatively on the surfaces of the metallic glasses, Ni76B12Si12 and Fe40Ni38Mo4B18, suggesting that such metglasses could be potential catalysts for some of the reactions involving CO.