Electron paramagnetic resonance studies of some ternary oxides of copper (II)

The electron paramagnetic resonance (EPR) of ternary oxides of Cu(II) has been studied between 4.2 and 300 K. The systems include those with 180 degrees Cu-O-Cu interactions (such as Ln2CuO4, Sr2CuO2Cl2, Sr2CuO3 and Ca2CuO3) or 90 degrees Cu-O-Cu interactions (such as Y2Cu2O5 or BaCuO2) as well as those in which the Cu2+ ions are isolated (such as Y2BaCuO5, La1.8Ba1.2Cu0.9O4.8 and Bi2CuO4). The change in the EPR susceptibility as a function of temperature is compared with that of the DC magnetic susceptibility. Compounds with extended 180 degrees Cu-O-Cu interactions which have a low susceptibility also do not give EPR signals below room temperature. For compounds such as Ca2CuO3 with one-dimensional 180 degrees Cu-O-Cu interactions a weak EPR signal is found the temperature dependence of which is very different from that of the DC susceptibility. For Y2BaCuO5 as well as for La1.8Ba1.2Cu0.9O4.8 the EPR susceptibility as well as its temperature variation are comparable with those of the static susceptibility near room temperature but very different at low temperatures. Bi2CuO4 also shows a similar behaviour. In contrast, for Y2Cu2O5, in which the copper ions have a very distorted nonsquare-planar configuration, the EPR and the static susceptibility show very similar temperature dependences. In general, compounds in which the copper ions have a square-planar geometry give no EPR signal in the ground state (0 K) while those with a distortion from square-planar geometry do give a signal. The results are analysed in the light of recent MS Xalpha calculations on CuO46- square-planar clusters with various Cu-O distances as well as distortions. It is suggested that in square-planar geometry the ground state has an unpaired electron in anionic orbitals which is EPR inactive. Competing interactions from other cations, an increase in Cu-O distance or distortions from square-planar geometry stabilise another state which has considerably more Cu 3d character. These states are EPR active. Both these states, however, are magnetic. For isolated CuO46- clusters the magnetic interactions seem to involve only the states which have mainly anionic character.

Mechanical and Chemical Thresholds in IV-VI Chalcogenide Glasses

It has been possible to identify two critical compositions in the IV-VI chalcogenide glassy system GexSe100-x by the anomalous variations of the high-pressure electrical resistivity behavior. The first critical composition, the chemical threshold, refers to the stoichiometric composition. The second critical composition, identified recently as the mechanical percolation threshold, is connected with the structural rigidity of the material.

Effect of grain size and texture on the mechanical properties of warm worked cadmium-1.5 Pct zinc alloy

The structural changes occurring during warm working of Cd-1.5 pct Zn alloy and their effect on the subsequent mechanical properties are studied. It is observed that changes in grain size and preferred orientation are important to a large extent in controlling the mechanical strength. The Hall-Petch slope,R decreases in the warm worked material while the friction stress, σo increases. The lowerR values are attributed to the development of a (101l) texture and the higher σo values are interpreted on the basis of changes in the basal texture.

Effect of Storage Temperatures on the Mechanical Properties of the Composite Solid Propellants

Accelerated ageing studies for three composite propellant formulations, namely polystyrene (PS)/ ammonium perchlorate (AP), polymethylmethacrylate (PMMA)/AP and poly phenol formaldehyde (PPF)/AP have been carried out in the temperature range of 55-125°C. Measurements of the ultimate compression strength (Uc) and isothermal decomposition rate (TD rate) were monitored as a function of storage time and temperature. The change in Uc was found to be linearly dependent on the change in TD rate irrespective of the propellant systems. Analysis of the results further revealed that the cause of ageing for both Uc and burning rate (r) is the thermal decomposition of the propellant. The safe-life for the change in mechanical properties was found to be higher compared to the change in r for PS and PMMA based propellants.

Mechanical Anisotropy in Crystalline Saccharin: Nanoindentation Studies

The nanoindentation technique has been employed to relate the mechanical properties of saccharin single crystals with their internal structure. Indentations were performed on (100) and (011) faces to assess the mechanical anisotropy. The load-displacement (P-h) curves indicate significant differences in the nature of the plastic deformation on the two faces. The P-h curves obtained on the (011) plane are smooth, reflecting homogeneous plasticity. However, displacement bursts (pop-ins) are observed in the P-h curves obtained on the (100) plane suggesting a discrete deformation mechanism. Marginal differences exist in the hardness and modulus on the two faces that may, in part, be rationalized, although one notes that saccharin has a largely three-dimensional close-packed structure. The structural origins of the fundamentally different deformation mechanisms on (100) and (011) are discussed in terms of the dimensionality of the hydrogen bonding networks. Down the (100) planes, the saccharin dimers are stacked and are stabilized by nonspecific van der Wants interactions mostly between aromatic rings. However, down the (011) planes, the molecules are stabilized by more directional and cross-linked C-H ... O hydrogen bonds. This anisotropy in crystal packing and interactions is reflected in the mechanical behavior on these faces. The displacements associated with the pop-ins were found to he integral multiples oldie molecule separation distances. Nanoindentation offers an opportunity to compare experimentally, and in a quantitative way, the various intermolecular interactions that fire present in a molecular crystal.

Conformations of synthetic alamethicin fragments. Evidence for 310 helical folding from 270-MHz hydrogen-1 nuclear magnetic resonance and circular dichroism studies

IH NMR studies at 270 MHz on the synthetic alamethicin fragments Z-Aib-Pro-Aib-Ala-Aib-Ala-OMe (1-6), Boc-Gln-Aib-Val-Aib-Gly-Leu-Aib-OMe (7-1 3), Boc-Leu-Aib-Pro-Val-Aib-OMe (1 2-16), and Boc-Gly-Leu- Aib-Pro-Val-Aib-OMe (1 1-16) have been carried out in CDC13 and (CD3)2S0. The intramolecularly hydrogen bonded amide hydrogens in these peptides have been delineated by using solvent titration experiments and temperature coefficientsof NH chemical shifts in (CD3)+30. All the peptides adopt highly folded structures, characterized by intramolecular 4 - 1 hydrogen bonds. The 1-6 fragment adopts a 310 helical conformation with four hydrogen bonds, in agreement with earlier studies (Rao, Ch. P., Nagaraj, R., Rao, C. N. R., & Balaram, P. (1980) Biochemistry 19, 425-4311. The 7-13

Antimicrobial activity of highly stable silver nano particles embedded in agar-agar matrix as a thin film

Highly stable silver nanoparticles (Ag NPs) in agar-agar (Ag/agar) as inorganic-organic hybrid were obtained as free-standing film by in situ reduction of silver nitrate by ethanol. The antimicrobial activity of Ag/agar film on Escherichia coli (E. coil), Staphylococcus aureus (S. aureus), and Candida albicans (C albicans) was evaluated in a nutrient broth and also in saline solution. In particular, films were repeatedly tested for antimicrobial activity after recycling. UV-vis absorption and TEM studies were carried out on films at different stages and morphological studies on microbes were carried out by SEM. Results showed spherical Ag NPs of size 15-25 nm, having sharp surface plasmon resonance (SPR) band. The antimicrobial activity of Ag/agar film was found to be in the order, C. albicans > E. coil > S. aureus, and antimicrobial activity against C. albicans was almost maintained even after the third cycle. Whereas, in case of E. coil and S. aureus there was a sharp decline in antimicrobial activity after the second cycle. Agglomeration of Ag NPs in Ag/agar film on exposure to microbes was observed by TEM studies. Cytotoxic experiments carried out on HeLa cells showed a threshold Ag NPs concentration of 60 mu g/mL, much higher than the minimum inhibition concentration of Ag NPs (25.8 mu g/mL) for E. coli. The mechanical strength of the film determined by nanoindentation technique showed almost retention of the strength even after repeated cycle. (C) 2010 Elsevier Ltd. All rights reserved.

High-resolution methyl edited GFT NMR experiments for protein resonance assignments and structure determination

Three-dimensional (3D) structure determination of proteins is benefitted by long-range distance constraints comprising the methyl groups, which constitute the hydrophobic core of proteins. However, in methyl groups (of Ala, Ile, Leu, Met, Thr and Val) there is a significant overlap of C-13 and H-1 chemical shifts. Such overlap can be resolved using the recently proposed (3,2)D HCCH-COSY, a G-matrix Fourier transform (GFT) NMR based experiment, which facilitates editing of methyl groups into distinct spectral regions by combining their C-13 chemical shifts with that of the neighboring, directly attached, C-13 nucleus. Using this principle, we present three GFT experiments: (a) (4,3)D NOESY-HCCH, (b) (4,3)D H-1-TOCSY-HCCH and (c) (4,3)D C-13-TOCSY-HCCH. These experiments provide unique 4D spectral information rapidly with high sensitivity and resolution for side-chain resonance assignments and NOE analysis of methyl groups. This is exemplified by (4,3)D NOESY-HCCH data acquired for 17.9 kDa non-deuterated cytosolic human J-protein co-chaperone, which provided crucial long-range distance constraints for its 3D structure determination.

Hyperfine effects in the nuclear magnetic resonance of paramagnetic molecules

Paramagnetic, or open-shell, systems are often encountered in the context of metalloproteins, and they are also an essential part of molecular magnets. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for chemical structure elucidation, but for paramagnetic molecules it is substantially more complicated than in the diamagnetic case. Before the present work, the theory of NMR of paramagnetic molecules was limited to spin-1/2 systems and it did not include relativistic corrections to the hyperfine effects. It also was not systematically expandable. --- The theory was first expanded by including hyperfine contributions up to the fourth power in the fine structure constant α. It was then reformulated and its scope widened to allow any spin state in any spatial symmetry. This involved including zero-field splitting effects. In both stages the theory was implemented into a separate analysis program. The different levels of theory were tested by demonstrative density functional calculations on molecules selected to showcase the relative strength of new NMR shielding terms. The theory was also tested in a joint experimental and computational effort to confirm assignment of 11 B signals. The new terms were found to be significant and comparable with the terms in the earlier levels of theory. The leading-order magnetic-field dependence of shielding in paramagnetic systems was formulated. The theory is now systematically expandable, allowing for higher-order field dependence and relativistic contributions. The prevailing experimental view of pseudocontact shift was found to be significantly incomplete, as it only includes specific geometric dependence, which is not present in most of the new terms introduced here. The computational uncertainty in density functional calculations of the Fermi contact hyperfine constant and zero-field splitting tensor sets a limit for quantitative prediction of paramagnetic shielding for now.

Nuclear-quadrupole resonance in coordination chemistry- identification of bridiging and therminal chlorines in the mercuric chloride-4-picoline n-oxide complex

The temperature dependence of the chlorine-35 n.q.r. in the mercuric chloride-4-picoline N-oxide complex has been studied from 77 K to room temperature, and the results are used to assign the observed frequencies to terminal and bridging chlorines.

Infrared and Nuclear Quadrupole Resonance Studies of the Phase Transitions of Para-dichlorobenzene

The α→γ→α→β transitions of para-dichlorobenzene have been studied by employing infrared and n.q.r. spectroscopy as well as differential scanning calorimetry. The γ phase is associated with considerably higher values of some of the intramolecular vibration frequencies. The α→γ transition shows athermal nucleation behaviour as in martensitic transitions. Intermolecular vibration bands around 46 and 85 cm–1 present in γ and α phases disappear in the β phase. The α→β transition seems to be associated with some orientational disorder.

Time-Resolved Resonance Raman Studies on Proton-Induced Electron-Transfer Reaction from Triplet Excited State of 2-Methoxynaphthalene to Decafluorobenzophenone

In this paper time-resolved resonance Raman (TR3) spectra of intermediates generated by proton induced electron-transfer reaction between triplet 2-methoxynaphthalene ((ROMe)-R-3) and decafluorobenzophenone (DFBP) are presented The TR3 vibrational spectra and structure of 2-methoxynaphthalene cation radical (ROMe+) have been analyzed by density functional theory (DFT) calculation It is observed that the structure of naphthalene ring of ROMe+ deviates from the structure of cation radical of naphthalene