Structural and spectroscopic investigations on Eu3+-doped alkali fluoroborate glasses

Eu³⁺-doped alkali fluoroborate glasses B₂O₃–XCO₃–NaF–Eu₂O₃ (where X = Li₂, Na₂, K₂, and Ca, Mg) have been prepared using the conventional melting technique and their structural and optical properties have been evaluated. The XRD pattern of the glasses confirmed the amorphous nature and the FTIR spectra reveal the presence of BO₃ and BO₄ units as their local structures along with the strong OH^? groups. From the absorption spectra the bonding parameters have been calculated and confirmed that the Eu–O bonds in the studied glasses are of covalent nature. Judd–Ofelt (JO) analysis has been carried out from the emission spectra. The JO parameters have been used to calculate transition probabilities (A), lifetime (τ_R) and branching ratios (β_R) and peak stimulated emission cross-section (σ_P^E) for the ⁵D₀ → ⁷F_J (J = 1, 2, 3 and 4) transitions of the Eu³⁺ ions. The decay from the ⁵D₀ level of Eu³⁺ ions in the title glasses has been measured and analysed. The lifetime of the ⁵D₀ level is found to be shorter than the reported glasses which may be due to the presence of OH^? groups.     

Photocatalytic and bactericidal activities of hydrothermally synthesized nanocrystalline Cd-doped ZnO

Nanocrystalline Cd-doped ZnO has been obtained by hydrothermal synthesis and characterized by powder X-ray diffraction, energy dispersive X-ray spectrum, high resolution scanning electron micrographs and UV–visible diffuse reflectance, photoluminescence and electrochemical impedance spectra. Cd-doping by hydrothermal method decreases the grain size and destroys the microstructure. Although doping does not modify the band gap it suppresses the deep level emission, decreases the charge-transfer resistance and increases the capacitance. Cd-doping enhances the photocatalytic and bactericidal activities.     

Kinetics and Mechanism of meso‐Tetraphenyl‐porphyriniron (III) Chloride Catalyzed Oxidation of Aniline and Its Substituents by Oxone in Aqueous Acetic Acid Medium

Metalloporphyrins are present naturally in cytochromes and hemoglobins. Perhaps the most interesting feature of heme‐enzymes, such as cytochromes P450, is their ability to perform extremely difficult oxidations with high selectivity. The present work is undertaken with the aspiration of designing the mechanistic studies on meso‐tetraphenylporphyriniron(III) chloride catalyzed oxidation of anilines by potassium peroxymonosulfate (oxone) in aqueous acetic acid medium. The study of oxidation with respect to the catalyst reveals that there is degradation of the catalyst. The concentration‐protonated aniline (a less reactive species) increases due to the increase in [H⁺], which inhibits the rate of the reaction. The thermodynamic parameters for the oxidation have been determined and discussed. It confirms the Exner relationship and also the activation parameters to the isokinetic relationships. The oxone oxidation with 12 meta‐ and para‐substituted anilines complies with the isokinetic relationship but not with any of the linear free energy relationships. The solvent interaction plays a major role in governing the reactivity. A suitable mechanism is proposed for this reaction.     

Kinetics of the Oxidation of Fluoren-9-olsby 4-Nitro-1-chlorobenzotriazole

Summary.  The oxidation of fluoren-9-ols by 4-nitro-1-chlorobenzotriazole in aqueous acetic acid is a second order reaction with a reaction constant of −1.4. The oxidation exhibits a kinetic deuterium isotope effect indicating the cleavage of the 9-C*H bond in the rate-determining step. The title oxidation is compared with that by 1-chlorobenzotriazole. Received October 23, 2000. Accepted (revised) January 22, 2001     

Vinyl polymerization

1:1 Poly (styrene-co-ethylmethacrylate) (PSEM) was prepared by bulk polymerization and fractionated using methylethyl ketone/petroleum ether system. IR and NMR analyses of the copolymer substantiated the composition kinetically expected. As the composition heterogeneity of the compolymer was narrow, the characterization of the copolymer was effected in a single solvent by light scattering as well as viscosity measurements at 35° C. The molecular weight dependence of [η], the limiting viscosity number;\(\left( {\bar r^2 } \right)w^{_2^1 } \), therms end to end distance and A₂ the second virial coefficient were established. The solvent ethylacetate was found to be thermodynamically more ideal for PSEM, than for polyethylmethacrylate. In an attempt to evaluate K, the Flory’s constant, application of various methods appeared to give almost similar values. In order to explain the low value ofΦ, the Flory’s universal parameter, rough estimates of\(\left( {\bar r_0 ^2 } \right)^{_2^1 } \), the unperturbedrms end to end distance;b, the statistical chain element;q, the persistence length and\({{\bar r_0 ^2 } \mathord{\left/ {\vphantom {{\bar r_0 ^2 } {l\bar r_{\max .} }}} \right. \kern-\nulldelimiterspace} {l\bar r_{\max .} }}\), the semiflexibility parameter were also made.     

Vibrational coherence spectroscopy of the heme domain in the CO-sensing transcriptional activator CooA

Femtosecond vibrational coherence spectroscopy was used to investigate the low-frequency vibrational dynamics of the heme in the carbon monoxide oxidation activator protein (CooA) from the thermophilic anaerobic bacterium Carboxydothermus hydrogenoformans (Ch-CooA). Low frequency vibrational modes are important because they are excited by the ambient thermal bath (k(B)T = 200 cm(-1)) and participate in thermally activated barrier crossing events. However, such modes are nearly impossible to detect in the aqueous phase using traditional spectroscopic methods. Here, we present the low frequency coherence spectra of the ferric, ferrous, and CO-bound forms of Ch-CooA in order to compare the protein-induced heme distortions in its active and inactive states. Distortions take place predominantly along the coordinates of low-frequency modes because of their weak force constants, and such distortions are reflected in the intensity of the vibrational coherence signals. A strong mode near ~90 cm(-1) in the ferrous form of Ch-CooA is suggested to contain a large component of heme ruffling, consistent with the imidazole-bound ferrous heme crystal structure, which shows a significant protein-induced heme distortion along this coordinate. A mode observed at ~228 cm(-1) in the six-coordinate ferrous state is proposed to be the ν(Fe-His) stretching vibration. The observation of the Fe-His mode indicates that photolysis of the N-terminal α-amino axial ligand takes place. This is followed by a rapid (~8.5 ps) transient absorption recovery, analogous to methionine rebinding in photolyzed ferrous cytochrome c. We have also studied CO photolysis in CooA, which revealed very strong photoproduct state coherent oscillations. The observation of heme-CO photoproduct oscillations is unusual because most other heme systems have CO rebinding kinetics that are too slow to make the measurement possible. The low frequency coherence spectrum of the CO-bound form of Ch-CooA shows a strong vibration at ~230 cm(-1) that is broadened and up-shifted compared to the ν(Fe-His) of Rr-CooA (216 cm(-1)). We propose that the stronger Fe-His bond is related to the enhanced thermal stability of Ch-CooA and that there is a smaller (time dependent) tilt of the histidine ring with respect to the heme plane in Ch-CooA. The appearance of strong modes at ~48 cm(-1) in both the ferrous and CO-bound forms of Ch-CooA is consistent with coupling of the heme doming distortion to the photolysis reaction in both samples. Upon CO binding and protein activation, a heme mode near 112 ± 5 cm(-1) disappears, probably indicating a decreased heme saddling distortion. This reflects changes in the heme environment and geometry that must be associated with the conformational transition activating the DNA-binding domain. Protein-specific DNA binding to the CO-bound form of Ch-CooA was also investigated, and although the CO rebinding kinetics are significantly perturbed, there are negligible changes in the low-frequency vibrational spectrum of the heme.     

Solvent effects and photophysical studies of a new class of acridine(1,8)dione dyes

Spectroscopic properties of a new family of acridinedione dyes are reported. The absorption and fluorescence spectra of the different substituted acridinediones have been recorded in different solvents and the difference in the dipole moment between ground and excited state has been obtained by solvatochromic shift method. The value of the Onsager cavity radius was calculated from the total surface area using software PCMODEL. Fluorescence quantum yield and fluorescence lifetime were determined. Radiative and non-radiative constants have been calculated. The triplet-triplet absorption maxima and triplet lifetime show variation depending on the substitution.     

New polymorphs of alumina: Part II μ and λ alumina

Abstract

We had reported observation of polymorphs of alumina having rare earth sesquioxide type structures in xerogel γ alumina (XGA) at high pressures and temperatures (High Pressure Research, 1998). In this paper we report observation of two additional phases at somewhat different pressures and temperatures. The XGA quenched from 5.2 GPa and 1450°C showed, besides α Al₂O₃, a new hexagonal metastable polymorph μAl₂O₃. Over a period of 10 to 12 weeks μ A1₂O₃ transforms to a stable hexagonal phase λ Al₂O₃. The cell parameters of λ Al₂O₃ are comparable with those of k Al₂O₃ found by Saalfield on dehydration of gibbsite A1₂O₃3H₂O in air at 800°C. The XGA containing 1 wt% of Cr₂O₃ yielded similar results at 4.56GPa and 1100°C. Further the XGA containing Cr showed complete transformation to n Al₂O₃ at 5.2 GPa and 1520°C.     

High Pressure and High Temperature Studies on Manganese Oxides

High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature. The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C. Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure. f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO 2 is identified to be a low-density polymorph f -MnO 2 . This unusual result of formation of low-density f -MnO 2 , having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu's (1976) laser-heated DAC experiment.