A. Y. Khan

Kinetics and electrochemical studies on superoxide

Rate constants for the reaction of superoxide O-2 with various substrates were obtained through stationary electrode polarography theory and technique. In solvent acetonitrile, the substrate and the rate constants of the reaction O-2 + AH-k2→Product, are, AH = isopropanol (k2 < 0.01 M-1 s-1); ethanol (k2 = 1.42 × 102 M-1 s-1); methanol (k2 = 1.1 × 107 M-1 s-1), H2O (k2 = 1.0 × 105 M-1 s-1). In MeCN, O-2 was found to be rather unreactive towards glucose and acetone but it reacts with fructose and sucrose catalytically. However, in DMF2, O-2reacts with glucose and fructose with k2 order of 105 M-1 s-1. The mechanism of the reaction of O-2 with the substrates (AH) is proposed as O-3 + AH k2O, AHk2k-1 k′ [O2H + AH]-, k-2→O2H + A- with k1 = 109 M-1 s-1 and k-1 = 108 -109 s-1. With these values of k-1 and k1, k′ k2(obs). The reversible E1/2 for O2 + e O-2 in various solvents: MeCN, acetone, isopropanol, methanol, H2O were obtained either directly from the reversible voltammogram or from experimental voltammograms and the rate constants obtained (as above) using stationary electrode polargraphy theory; E1/2 being -0.82 (MeCN),-0.85 (acetone),-0.72 (isopropanol);-0.66 (MeOH),-0.56 (H2O) vs SCE.

Hydrogen Bonding Association in the Electroreduced Intermediates of Benzoquinones and Naphthoquinones

Keeping in view the importance of chemical and biological functions of quinone based couples; two different series of quinones, namely benzoquinones and naphthoquinones, are investigated electrochemically. Five compounds of each series are studied systematically in dichloromethane, acetonitrile, and propylene carbonate and from there the half-wave potentials of first and second reductions are obtained through cyclicoltammetry measurements. Four different alcohols are used with increasing concentrations as hydrogen bond donors on the basis of their increasing acidity. The hydrogen-bonding power is analyzed from the positive shifts in both the waves with increasing concentrations of alcohols. The quantitative comparison is made while calculating the thermodynamic association constants and number of alcohol molecules bonded to both anion and dianion of quinones. The qualitative behavior and quantitative data both indicate the quinone-alcohol interaction as hydrogen bonding and the strength of hydrogen bond is dependant upon the nature of species involved in this couple. From the cyclic voltammetric data the relative effects of hydroxylic additives and different substituted quinones on equilibrium constant are also observed. Solvent effect is rationalized in favor of hydrogen bonding in terms of solvent polarity parameters.

Protonation of anion radicals and dianions of some dinitro aromatics

A simple electrochemical method developed to study reactions of reactive intermediates (neutralfree readical, cation-, anion- radicals etc.) was employed to study the protonation reaction of anion radicals and dianions of some dinitroaromatics. Rate constants thus obtained and the mechanism of protonation reaction invesitgated, are reported. It is suggested that the reactions do not involve a direct electron transfer. The possible role of various ion-pairs was investigated. Positions of nucleophilic attack were investigated through M.O. calculations.

Are pyridinyl radicals really the reactive intermediates

Abstract The behaviour of the reduction products of the herbicide paraquat (1,1′-dimethyl-4,4′-bipyridilium dichloride) and 1-ethyl-4-carbomethoxy pyridinium salt, vis a vis proton donors and oxygen were investigated. The rate constants for the protonation reaction of these reduction products, with various protonating agents, were evaluated. Also, the effect of solvent and electrode surface on the reaction of these reduction products with oxygen were investigated. It was found that the nature of the electrode surface had a profound effect on the reaction with oxygen, which could have a bearing on the herbicidal activity of paraquat. It was also found that the higher reduction products are 10 4 –10 5 times more reactive than the initial reduction products 1-ethyl-4-carbomethoxypyridinyl radical and 1,1′-dimethyl-4,4′-bypiridilium cation radical. Hence, the possible role of more reactive higher reduction products in the herbicidal properties of paraquat is pointed out.

Superconductivity in Y-Ba-Na-Cu-O

The Y-Ba-Ca-Cu-O superconductor (Ca0.2Y0.3Ba0.5CuO3−z) was prepared under different heat treatments and sintering conditions. Electrical resistance measurements from ambient temperature to 77 K show that four of the five samples were single-phase superconductors withTc in the range 82 to 85 K. The X-ray diffraction measurements revealed an orthorhombic perovskite type structure for all samples.

Electrochemical studies of the interactional mechanism and scavenging activity of antioxidants towards dinitroaromatics

In the work discussed in this paper, cyclic voltammetric results obtained for the interaction of ascorbic acid, β-carotene, and three structurally related flavonoids (quercetin, rutin, and morin) with the anion radical of 1,3-dinitrobenzene were used to determine their antioxidant activity. The extent of the antioxidant–anion radical interactions was measured as the antioxidant activity coefficient. Higher values of this coefficient obtained for the three flavonoids in DMF are indicative of their greater antioxidant activity than ascorbic acid and β-carotene in this solvent. On the basis of cyclic voltammetric responses, a possible mechanism of the reaction of the reduction product of 1,3-dinitrobenzene with the antioxidants is proposed. Hyperchem PM3 quantum mechanical semi-empirical calculations of charges on reactive sites of the antioxidants and the 1,3-dinitrobenzene anion radical were also carried out; the results obtained supported the proposed mechanism of interaction.Graphical abstract