Molecular and impedance spectroscopy of Na 2 Mo 2 O 7 ceramics

The fine (i.e. 38 nm) powder of polycrystalline Na₂Mo₂O₇ was prepared by the high-temperature solid-state reaction technique. The formation of the compound in orthorhombic system is confirmed by preliminary structural analysis using X-ray diffraction (XRD) data. Spectroscopic studies of the compound have been carried out by vibration spectroscopy (Raman/FTIR) to understand its molecular structure at microscopic level. The complex impedance spectroscopy (CIS) technique has been used to study the electrical properties of the material as a function of frequency (10²–10⁶ Hz) at different temperatures (23–450^°C), and also to investigate the fundamental mechanism involved in the material. Impedance analysis also indicates that below 300^°C, the electrical conduction in the material is due to grain interior only. At and above 325^°C, the contribution of grain boundary is clearly evident. The electrical processes in the material are found to be temperature-dependent and are due to the relaxation phenomena in it. A frequency-dependent maximum of the imaginary electrical impedance is found to obey the Vogel–Fulcher law.     

Room-temperature ferromagnetism in nanoparticles of superconducting materials

Nanoparticles of superconducting (YBCO) () exhibit ferromagnetism at room temperature while the bulk YBCO, obtained by heating the nanoparticles at high temperature (940 ^°C), shows a linear magnetization curve. Across the superconducting transition temperature, the magnetization curve changes from that of a soft ferromagnet to a superconductor. Furthermore, our experiments reveal that not only nanoparticles of metal oxides but also metal nitrides such as NbN () and δ-MoN () exhibit room-temperature ferromagnetism.     

Signals in solid-state photochemically induced dynamic nuclear polarization recover faster than signals obtained with the longitudinal relaxation time

During the photocycle of quinone-blocked photosynthetic reaction centers (RCs), photochemically induced dynamic nuclear polarization (photo-CIDNP) is produced by polarization transfer from the initially totally electron polarized electron pair and can be observed by 13C magic-angle spinning (MAS) NMR as a strong modification of signal intensities. The same processes creating net nuclear polarization open up light-dependent channels for polarization loss. This leads to coherent and incoherent enhanced signal recovery, in addition to the recovery due to light-independent longitudinal relaxation. Coherent mixing between electron and nuclear spin states due to pseudosecular hyperfine coupling within the radical pair state provides such a coherent loss channel for nuclear polarization. Another polarization transfer mechanism called differential relaxation, which is based on the long lifetime of the triplet state of the donor, provides an efficient incoherent relaxation path. In RCs of the purple bacterium Rhodobacter sphaeroides R26, the photochemical active channels allow for accelerated signal scanning by a factor of 5. Hence, photo-CIDNP MAS NMR provides the possibility to drive the NMR technique beyond the T1 limit.     

Cucurbituril adamantanediazirine complexes and consequential carbene chemistry

Adamantanediazirines, precursors of adamantanylidenes, form 1:1 complexes (guest to host) with cucurbit[7]uril and cucurbit[8]uril and a 3:1 complex with a Pd nanocage in water. (1)H NMR spectra suggested that these complexes are stable in water on the NMR time scale. While photolysis of adamantanediazirines in water gave mainly adamantanone and adamantanol via adamantanylidene as intermediate, the 1:1 complexes of adamantanediazirine with cucurbiturils gave intramolecular C-H insertion products of adamantanylidene in >90% yield. The study establishes that significant control of carbene reactivity can be achieved when the precursor is encapsulated within a tight inert cavity. While the general characteristics of molecular containers can be understood on the basis of concepts such as "confinement" and "weak interactions", each one is unique and deserves careful scrutiny.     

Magnetic field dependence of photo-CIDNP MAS NMR on photosynthetic reaction centers of Rhodobacter sphaeroides WT

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by (13)C solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The good agreement of simulations with the experiments raises expectations that artificial solid reaction centers can be tuned to show photo-CIDNP in the near future.     

Experimental Determination of a Third Derivative of G. (III): Differential Pressure Perturbation Calorimetry (II)

Earlier we constructed a differential pressure perturbation calorimeter, in order to determine directly a third derivative of the Gibbs energy G, the partial molar entropy-volume cross fluctuation density of solute B, ^SVd_[(B)\dot]^{\mathit{SV}}\delta_{\dot{\mathrm{B}}}, in a binary aqueous solution of B. Our attempt was basically successful, as described elsewhere (J. Solution Chem. 39:431–440, 2010). However, the uncertainty of the results for ^SV δ _B was several percent. One of the purposes of our direct experimental determination was to obtain a fourth derivative quantity graphically with some confidence. For this purpose, data of the third derivative quantity are necessary with uncertainties of less than one per cent. Various difficulties that we had on the first generation apparatus were attended to, and a new apparatus was constructed. The new experimental results for aqueous 2-butoxyethanol are found to be at least several-fold better. Hence, we were able to evaluate the fourth derivative quantity for the first time. The physical meaning of the fourth derivatives may become clearer in comparative studies when such data become available for aqueous solutions of other solutes including mono-ols and poly-ols. A project is underway to that effect.     

Dissociation constants of some derivatives of 5-amino isophthalic acid in mixed solvents

The dissociation constants of some derivatives of 5-amino isophthalic acid have been measured by Calvin Bjerrum pH titration method in DMF-water and DMSO-water (60: 40 v/v) systems at 308.15 K. It is observed that dissociation constant depends on the solvent and substituents group present in the compound.     

Dynamic internal cavities of dendrimers as constrained media. A study of photochemical isomerizations of stilbene and azobenzene using poly(alkyl aryl ether) dendrimers

Dendritic microenvironments defined by dynamic internal cavities of a dendrimer were probed through geometric isomerization of stilbene and azobenzene. A third-generation poly(alkyl aryl ether) dendrimer with hydrophilic exterior and hydrophobic interior was used as a reaction cavity in aqueous medium. The dynamic inner cavity sizes were varied by utilizing alkyl linkers that connect the branch junctures from ethyl to n-pentyl moiety (C(2)G3-C(5)G3). Dendrimers constituted with n-pentyl linker were found to afford higher solubilities of stilbene and azobenzene. Direct irradiation of trans-stilbene showed that C(5)G3 and C(4)G3 dendrimers afforded considerable phenanthrene formation, in addition to cis-stilbene, whereas C(3)G3 and C(2)G3 gave only cis-stilbene. An electron-transfer sensitized trans-cis isomerization, using cresyl violet perchlorate as the sensitizer, also led to similar results. Thermal isomerization of cis-azobenzene to trans-azobenzene within dendritic microenvironments revealed that the activation energy of the cis- to trans-isomer was increasing in the series C(5)G3 < C(4)G3 相似文献   

A thermal study of some Schiff bases derivative of α-napthylamine

Thermal analyses of some Schiff bases, derivative of α-napthylamine, were performed by the DSC, TG and DTA techniques. The thermograms were used to determine various kinetic parameters, such as the order of degradation (n), energy of activation (E), frequency factor (A), and entropy change (ΔS), by the Freeman-Carroll method. The text was submitted by the authors in English.