P.N. Moorthy

One-electron reduction of thionine studied by pulse radiolysis

One-electron reduction of thionine has been studied by using the technique of nanosecond pulse radiolysis and kinetic spectrophotometry. H,eaq− as well as radicals derived from methanol, ethanol, isopropanol, THF, dioxane andt-butanol by H atom abstraction were used as reductants. The rate constants for the transfer of electrons from these radicalts to thionine were directly determined from the pseudo first-order formation rates of the product, semithionine and the one-electron reduction potential of thionine estimated. The absorption spectrum of semithionine in its different conjugate acid-base forms was found to be in agreement with previously reported spectra and the decay of the species was second order. By monitoring transient absorbance changes as a function of pH, twopKa values were observed and, based on the effect of ionic strength on the second-order decay constants of the species were assigned to the equilibria described.

Nonradiative energy transfer from 7-amino coumarin dyes to thiazine dyes in methanolic solutions

Abstract Nonradiative energy-transfer processes between 7-amino coumarin and thiazine dyes in methanolic solutions have been studied using a single-photon counting technique. The critical transfer distances calculated from the experimantal rate constants are in the range 30–70 A and exhibit the same trend as the values calculated from Forsters formula.

Studies on the transient species formed in the pulse radiolysis of benzotriazole

Abstract Rate constants for the reaction of benzotriazole (BTZ) with the primary species of water radiolysis (e.g. eaq−, OH, H and O−) have been determined at different pHs using the pulse radiolysis technique. Absorption spectra of the transient intermediates formed have been recorded in the region 220–650 nm. Semireduced radicals formed by eaq− reaction with benzotriazole are found to be strongly reducing in nature and they transfer electrons to various acceptors such as thionine, methylene blue, paraquat with diffusion controlled rate constants. Only SO4− radicals were found to oxidize benzotriazole. Reactivity of BTZ towards eaq− and specific oxidants has been compared with that of indole. Evidence for characterization of both semireduced and semioxidized benzotriazole as three electron bonded species are given.

A study of the triplet state of 7-amino coumarin laser dyes by the nanosecond pulse radiolysis technique

Abstract The triplet characteristics of two 7-amino coumarin laser dyes (7-diethylamino 4-methyl coumarin (C 1) and 7-diethylamino 4-trifluoromethyl coumarin (C 1F)) were studied in benzene solution using the nanosecond pulse radiolysis technique. Their T—T absorption spectra, rate constants for triplet to ground state transition and triplet—triplet annihilation and G values were determined. The energy transfers from coumarin triplets to various acceptor molecules such as pyrene, t -stilbene and anthracene and from benzene, biphenyl and naphthalene triplets to these dyes as acceptors were studied. Estimates of their energy levels were made.

Kinetic and spectral characteristics of transients formed in the pulse radiolysis of phenylthiourea in aqueous solution

Abstract The reactions of primary species such as e - aq , H-atom and OH radicals as well as some one electron reductants and oxidants with phenylthiourea have been studied at various pHs. The kinetic, spectral, acid-base and redox properties of the transients have been determined using the pulse radiolysis technique.

Kinetic and Spectral Properties of the Intermediates Formed in the Pulse Radiolysis of 2-Mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e-aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λmax) at 330 and 590 nm. The transient species (pKa = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e-aq at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λmax at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.

Triplet state of coumarin 153 studied by nanosecond pulse radiolysis

Abstract The triplet—triplet absorption spectrum of the rigidized 7-amino coumarin dye C 153 has been investigatd in benzene solution in the region 450 – 750 nm by the pulse radiolysis technique. The triplet decay was first order with a half-life of 5 μs. Its formation was confirmed by energy transfer from the pulse radiolytically generated biphenyl triplet. The extinction coefficient of C 153 triplet in benzene was calculated to be 10 000 ± 500 dm 3 mol −1 cm −1 . Using various triplet donors and acceptors its energy level was estimated to be 50 ± 2 kcal mol −1 . The energy transfer rate constants from benzene, biphenyl and naphthalene triplets to C 153 and from C 153 triplet to anthracene were found to be diffusion controlled.

Reactions of pyridin-3-ol with pulse-radiolytically generated reducing species

Rate constants for reactions of hydrated electron, hydrogen atom, and other reducing radicals such as (CH3)2COH and CO2– with pyridin-3-ol at different values of pH have been determined using the pulse-radiolysis technique. The absorption spectra of the transient product species and kinetic parameters pertaining to them have been investigated. Of the above reducing species all except e–aq lead to transient species other than the one-electron reduction product of pyridin-3-ol. (CH3)2ĊOH and CO2– radicals are found to add on to the pyridin-3-ol ring. Pyridin-3-ol radicals formed by reaction with e–aq are found to be highly reducing in nature and transfer electrons to thionine, methylene blue, safranine, anthraquinone 2-sulphonate, paraquat, and benzophenone with almost diffusion-controlled rate constants.

Redox reactions of V(III) and Cr(III)picolinate complexes in aqueous solutions

Abstract Reactions of e aq − , H-atoms, OH, (CH 3 ) 2 COH, and CO 2 − radicals with V(III)picolinate and Cr(III)picolinate have been studied by the pulse radiolysis technique. The spectra of V(II)picolinate, V(IV)picolinate, Cr(II)picolinate, OH adduct of Cr(III)picolinate and Cr(IV)picolinate have been obtained and the rate constants of the reactions of various radicals with V(III) and Cr(III)picolinate have been determined. The implications of these results to the chemical decontamination of nuclear reactor systems are discussed.

Pulse radiolysis studies of 4-pyridinol: Evidence for dimer anion formation

Abstract Rate constants for the reactions of 4-pyridinol with e-aq and OH/O- species were determined at different pHs. The spectral and kinetic characteristics of product transient species formed have been assessed. It has been found that semireduced 4-pyridinol forms dimer anions on reaction with the parent molecule. Equilibrium constant for the dimer formation has been determined to be 97 dm3 mol-1. Reactions of 4-pyridinol with radiolytic species have been compared with those of 2- and 3-pyridinols investigated and reported earlier.