Kinetics and Mechanism of Oxidation of Methionine by Chromium(VI): Edta Catalysis

The kinetics of the oxidation of DL-methionine to sulfoxide by CrVI has been studied in aqueous acetic acid in the presence of sodium salt of ethylenediaminetetraacetic acid (edta). It appears that CrVI-edta is the active electrophile. The low dielectric constant of the medium favours the reactivity. Ionic strength and MnII have no significant effect on the conversion. The ratio k(D2O) / k(H2O) ≈ 1 does not seem to indicate any significant solvent isotope effect. Highly negative ΔS# values indicate a structured transition state. A ternary intermediate has been envisaged for describing the enhanced reactivity

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KHP crystals as investigated by high-resolution XRD and SEM

To reveal the influence of complexing agents on crystalline perfection, tristhiourea zinc(II) sulfate (ZTS), ammonium dihydrogen phosphate (ADP) and potassium hydrogen phthalate (KHP) crystals grown by slow-evaporation solution growth technique using low concentrations (5 x 10(-3) M) of dopants like ethylenediamminetetraacetic acid (EDTA) and 1,10-phenanthroline (phen) were characterized by high-resolution X-ray diffractometry (XRD) and scanning electron microscopy (SEM). High-resolution diffraction curves (DCs) recorded for ZTS and ADP crystals doped with EDTA show that the specimen contains an epilayer, as observed by the additional peak in the DC, whereas undoped specimens do not have such additional peaks. On etching the surface layer, the additional peak due to the epilayer disappears and a very sharp DC is obtained, with full width at half-maximum (FWHM) of less than 10 arcsec, as expected from the plane wave dynamical theory of X-ray diffraction for an ideally perfect crystal. SEM micrographs also confirm the existence of an epilayer in doped specimens. The ZTS specimen has a layer with a rough surface morphology, having randomly oriented needles, whereas the ADP specimen contains a layer with dendric structure. In contrast to ADP and ZTS crystals, the DC of phen-doped KHP shows no additional peak, but it is quite broad (FWHM = 28 arcsec) with a high value of integrated intensity, rho (area under the DC). The broadness of the DC and the high value of rho indicate the formation of a mosaic layer on the surface of the crystal. However, similar to ADP and ZTS, the DC recorded after etching the surface layer of the KHP specimen shows a very sharp peak with an FWHM of 8 arcsec. An SEM photograph of phen-doped KHP shows deep cracks on the surface, confirming the mosaicity. After removing the surface layer, the SEM pictures reveal a smooth surface. A similar trend is observed with other complexing agents, like oxalic acid, bipy and picolinic acid. However, only typical examples are described in the present article where the effects were observed prominently. The investigations on ZTS, ADP and KHP crystals, employing high-resolution XRD and SEM studies, revealed that some organic dopants added to the solution during the growth lead to the formation of a surface layer, due to complexation of these dopants with the trace metal ion impurities present in the solution, which prevents the entry of impurities, including the solvent, into the crystal, thereby assisting crystal growth with high crystalline perfection. The influence of organic dopants on the second harmonic generation efficiency is also investigated

Effect of doping cations Li(I)-, Ca(II)-, Ce(IV)- and V(V)- on the properties and crystalline perfection of potassium dihydrogen phosphate crystals: A comparative study

The effect of doping metal ions with varied ionic charges (ranging from +1 to +5), Li(I)-, Ca(II)-, Ce(IV)- and V(V)on the growth process and properties of potassium dihydrogen phosphate (KDP) crystals, grown by slow evaporation solution growth technique, has been investigated. Incorporation of metal ion into the KDP crystalline matrix is well confirmed by energy dispersive X-ray spectroscopy and atomic absorption spectroscopy. Interesting to observe that the incorporation is comparatively less in doping the higher valent metal. The powder XRD pattern and Fourier transform-IR analysis confirm the slight distortion in the structure of the KDP crystals as a result of metal ion doping. Slight changes in cell parameter values of doped KDP crystals are observed by single crystal XRD analysis. The high-resolution X-ray diffraction (HRXRD) studies used to evaluate the crystalline perfection reveal many interesting features on the ability of accommodating the dopants by the crystalline matrix. Surface morphological changes because of foreign metal ion incorporation are observed by scanning electron microscopy. UV-Vis spectroscopy reveals that the transparency is not affected much by the dopants and the cut-off wavelengths of all the doped specimens lie in a close range. Band-gap energies are estimated using optical transmittance data. Enhanced second harmonic generation efficiency is observed

Chromium(VI) catalyzed oxidation of indole in aqueous acetic acid

1778-1781The catalytic activity of picolinic acid, 1,10-phenanthroline, oxalic acid, 2,2'-bipyridyl and ethylenediamine tetracetic acid have been investigated in the chromium(VI) oxidation of indole in aqueous acetic acid medium. The catalysts increase the electrophilic activity of the oxidant and the Cr(VI)-Cat complex is believed to be the reactive electrophile. The proposed reaction pathway involves the electrophilic attack of the Cr(VI)-Cat complex at the ethylenic bond of the indole ring to give an indoleninic intermediate. The reaction is routed through via a termolecular complex, Cr(VI)-Cat-IND, the decomposition of which in slow rate determining step is envisaged to explain the reactivity. In presence of the catalyst, indole is oxidized by chromium(VI) to give oxindole as the major product, under most conditions. The conversion is favoured in a medium of low dielectric constant. The failure to observe inhibition in the presence of acrylonitrile suggests the non-radical mechanistic pathway. The ratio, k(D2O)/k(H2O) ≈ 1, does not seem to indicate any significant solvent isotope effect. Cobalt(III) enhances the reactivity while pyridine and chromium(III) depress the reactivity. The relative catalytic activity for indole oxidation in the presence of different catalysts reveals that picolinic acid is the best catalyst under the experimental conditions while activity of EDTA is minimum. A detailed interpretation of the redox mechanism based on the available kinetic data has been proposed

Effect of alkaline earth and transition metals doping on the properties and crystalline perfection of potassium hydrogen phthalate (KHP) crystals

The influence of alkaline earth metal (Mg) and transition metal (Hg) doping on the properties and crystalline perfection of potassium hydrogen phthalate (KHP) crystals has been described. Incorporation of dopant into the crystalline matrix even at the low concentrations was well confirmed by energy dispersive X-ray spectroscopy (EDS). Further, high-resolution X-ray diffraction (HRXRD) studies indicate predominant substitutional site occupancy for Mg. Heavy doping results in internal structural grain boundaries due to stress aroused in the lattice caused by the entry of dopants into the crystalline matrix. The transition metal (Hg) doping results in multi-peaks in the diffraction curve (DC) with a wide angular spread and the site occupancy seems to be predominantly interstitial positions in the crystal lattice, quite likely due to its bigger size in comparison with alkaline earth metal. The reduction in the intensity observed in powder X-ray diffraction (XRD) for both types of doped specimens and slight shifts in vibrational frequencies reveal minor structural variations. It is observed that the doping with high concentrations of metal facilitates nonlinearity and enhances the second harmonic generation (SHG) efficiency to a significant extent

Os(VIII) - catalysis on the oxidation of some diaryl sulphoxides with bromamine-B in alkaline medium

71-75The effect of substituents reveals the electronic character of Os(VIII)- catalysed N-bromobenzene sulphonamide (bromamine-B) oxidation of some diaryl sulphoxides to sulphones in a strongly alkaline (p͠H~.5) <i style="mso-bidi-font-style: normal">t-butanol-water (1:1 v/v) medium and the conversion appears to be nucleophilic for bromamine-B (BAB). High dielectric constant of the medium favours the reactivity. The kinetic studies reveal that electron-withdrawing groups enhance the reactivity while electron-releasing ones inhibit the rate. It appears that the anion RNBr is the reactive species of bromamine-B. The possibility of the formation of Os(VI)-BAB complex causing a retarding effect on the pseudo-First-order rate coefficients is discussed. Contrary to expectations there is a marked depression in the catalytic activity of Os(VIII) in the presence of other transition metals and pyridine bases such as 2, 2'-bipyridyl and 1, 10-phenanthroline. The probable reaction mechanisms are discussed on the basis of above results

An interesting correlation between crystalline perfection and second harmonic generation efficiency on KCI- and oxalic acid-doped ADP crystals

The effect of dopants (over a concentration range from 1 to 10 mol %), namely KCl and oxalic acid (C2H2O4·2H2O, Oxa), on the growth process, crystalline perfection, and nonlinear optical (NLO) properties of ammonium dihydrogen phosphate (NH4H2PO4, ADP) single crystals grown by a slow evaporation solution growth technique has been investigated. The high-resolution X-ray diffraction (HRXRD) studies used to evaluate crystalline perfection reveal some interesting features on the ability of accommodating the dopants by the crystalline matrix. The remarkable and systematic increase in the broadness and asymmetry of the diffraction curves as the concentration increases clearly indicates that the dopants predominantly occupied the interstitial positions in the crystalline matrix. Due to the larger size of the Oxa molecule, the crystalline matrix could not accommodate these dopants, which led to the formation of a very low angle (tilt angle <1′) internal structural grain boundaries at high concentrations. The relative second harmonic generation (SHG) efficiency measurements revealed that both KCl and Oxa dopants enhance the SHG efficiency. However, at higher concentrations of Oxa, SHG efficiency of ADP is not increased but rather decreased from its undoped condition. This may be due to deterioration in the crystalline perfection as observed by HRXRD. Powder XRD and FT-IR spectral analyses confirm the slight distortion of the structure of the crystal in the presence of a high concentration of dopants (10 mol %). UV–vis study shows that the transparency is not affected much by the dopants. The surface morphology of the as-grown specimens, which is changed with the nature and concentration of dopants, was studied by scanning electron microscopy. Presence of dopants was confirmed by energy-dispersive spectrometry

Rare earth cerium doping effects in nonlinear optical materials: potassium hydrogen phthalate (KHP) and tris(thiourea)zinc(II) sulfate (ZTS)

The influence of Ce(IV) doping on ZTS and KHP crystals over a concentration range from 1 to 10 mol% in the solution during crystallization, which leads to a true concentration range from few ppm to few tens of ppm in the crystals has been investigated. The XRD and FT-IR analyses indicate that the crystal undergoes considerable stress as a result of doping. Incorporation of the Ce(IV) dopant into the crystal lattice was well confirmed by energy dispersive X-ray spectroscopy (EDS) and quantified by inductively coupled plasma (ICP) technique. The high-resolution X-ray diffraction (HRXRD) studies reveal that Ce doping in KHP leads to degradation of crystal quality whereas ZTS can accommodate Ce predominantly at the substitutional sites without any degradation of crystalline perfection. The second harmonic generation (SHG) efficiency is not influenced by Ce doping in the KHP crystals while in ZTS crystals, it is enhanced to a considerable extent correlated with moderately improved crystalline perfection

Crystal growth, structure, crystalline perfection and characterization of zinc magnesium ammonium sulfate hexahydrate mixed crystals ZnxMg(1-x)(NH4)(2)(SO4)(2) center dot 6H(2)O

Mixed crystals ZnxMg(1-x)(NH4)(2)(SO4)(2) center dot 6H(2)O of the two well-known Tutton's salts Zn(NH4)(2)(SO4)(2) center dot 6H(2)O and Mg(NH4)(2)(SO4)(2) center dot 6H(2)O were grown with varying molar proportions (x=0.10-0.90) by slow evaporation solution growth technique. The mixed crystal Zn0.54Mg0.46(NH4)(2)(SO4)(2) center dot 6H(2)O is crystallizing in monoclinic system with space group P2(1)/c and cell parameters a=6.2217(4) angstrom, b=12.5343(7) angstrom, c=9.2557(6) angstrom, beta=106.912(3)degrees. The coexistence of zinc and magnesium ions in the mixed crystal was confirmed by inductively coupled plasma (ICP), atomic absorption spectroscopy (AAS) and energy dispersive X-ray spectroscopy (EDS). Compositional dependence of lattice parameters follows Vegard's relations. Slight variations are observed in FT-IR and XRD of pure and mixed crystals. Comparison of crystalline perfection as evaluated by high-resolution X-ray diffraction (HRXRD) for mixed crystals of various proportions reveals a reasonably good crystalline perfection for the mixed crystal with nearly equimolar ratio of Zn and Mg. The surface morphology of the mixed crystals changing with composition was studied by scanning electron microscopy (SEM). UV-vis studies reveal that the transparency of the mixed crystals was not much affected