H. Mohan

Preparation and characterization of DABCOδ+x-Cδ−60(y) charge transfer complex

Abstract A stable complex of C 60 with an organic donor (tertiary amine, DABCO) has been prepared in the solid state at room temperature. The charge transfer complex, DABCO δ+ x -C δ− 60( y ) is characterised by powder X-ray diffraction and FTIR methods. The complex is soluble in an aqueous medium and shows weak paramagnetic properties.

Reaction of sulphate radical anion (SO.4-) with hydroxy-and methyl-substituted pyrimidines: a pulse radiolysis study

Reactions of sulphate radical anion (SO·4-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU) and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis at pH 3 and at pH 10. The transient intermediate spectra were compared with those from the reaction of hydroxyl radical (·OH). It is proposed that SO·4- produces radical cations of these pyrimidines in the initial stage. These radical cations are short-lived except in the case of DMHP where a relatively longer lived radical cation is proposed to be formed. When there is a hydrogen atom attached to the N(1) or N(3) position, a deprotonation from these sites is highly favored. When there is no hydrogen attached to these sites, deprotonation from a substituted methyl group is favored. At acidic pH, deprotonation from nitrogen is observed for DHMP, MU and DMU. At basic pH, the radical cation reacts with OH- leading to the formation of OH adducts.

RADIATION CHEMICAL STUDY OF OH, O- AND SO4- REACTIONS WITH PURINES

The reactions of OH, O .- and SO 4 .- with purine, adenine, adenosine and guanosine were studied by pulse radiolysis with optical detection. The second order rate constants for both O .- and SO 4 .- with these compounds range from (0.25 - 4.2) × 10 9 dm 3 mol -1 s -1 and the k values for the O .- reaction are lower by an order of magnitude than those obtained for the corresponding OH reaction except in the case of purine where the rates are nearly the same. The transient absorption spectra measured in the reaction of . OH with purine, adenine and adenosine have characteristic peaks at 335 and 450 nm while absorption only in the lower wavelength region with peaks at 300 and 335 nm was observed in the O .- reaction. The rate of ring opening of the (C-8-O) .- adduct of adenine is lower than that found for the OH reaction with practically no ring opening in the case of purine. In contrast, a single peak at 300 nm was observed in the O .- reaction with the nucleosides adenosine and guanosine. The spectral nature and the product analysis confirm that the H-abstraction by the O .- from the sugar moiety leading to the base release is an important reaction channel. The transient absorption spectra obtained in the SO 4 .- reaction with adenine and adenosine showed maxima at 290 and 335 nm with weak and broad absorption around 580 nm which are assigned to the neutral nitrogen centered radical.

Reaction of oxide radical ion (O.-) with substituted pyrimidines

Pulse radiolysis technique has been used to investigate the reaction of oxide radical ion (O.–) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 5,6-dimethyl uracil (DMU) and 6-methyl uracil (MU) in strongly alkaline medium. The second-order rate constants for the reaction of O.– with these compounds are in the range 2-5 × 108 dm3 mol–1 s–1. The transient absorption spectra obtained with DHMP have two maxima at 290 and 370 nm and with DMHP have maxima at 310 and 470 nm. The transient spectrum from DMU is characterized by its absorption maxima at 310 and 520 nm and that of MU by its single maximum at 425 nm. The intermediate species were found to react with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) with high G(TMPD.+) values ranged between 3.9 × 10–7 molJ–1 and 4.8 × 10–7 molJ–1. These radicals undergo decay by second-order kinetics (2k/∈ = 1.0-1.7 × 106 s–1). The reaction of O.– with the selected pyrimidines is proposed to proceed through a hydrogen abstraction from the methyl group forming allyl type radicals. These are mainly oxidizing radicals and hence readily undergo electron transfer reactions with TMPD.

Redox chemistry of o- and m-hydroxycinnamic acids: A pulse radiolysis study

Radiation chemical reactions of•OH, O•−, N3•and eaqt- witho- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of•OH with ortho and meta isomers in buffer solution at pH7 are 3.9±0.2 × 109 and 4.4 ± 0.3 × 109 dm3 mol-1 s-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 109 dm3 mol-1 s-1) but it was unaffected in the case of meta isomer (k = 4.2±0.6 × 109dm3mol-1 s-1). The rate constant in the reaction of N3• with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 108 dm3 mol-1s-1). The rates of the reaction of eaqt- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the•OH witho-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of Obb−with theo-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370–380 nm in both the isomers. The underlying reaction mechanism is discussed.

Radiolytically generated benzene triplets as sensitizers for energy and combined electron/proton transfer processes

Abstract Pulse radiolysis technique has been employed to investigate energy and electron transfer reactions involving triplets of naphthols and hydroxybiphenyls. The transient absorption spectra obtained on pulse radiolysis of N2-saturated solution of naphthols and hydroxybiphenyls in benzene are assigned to triplet–triplet absorption. It was found that biphenyl triplets undergo energy transfer to naphthols and hydroxybiphenyls forming the acceptor triplets. On the other hand, benzophenone triplets, favor electron transfer followed by H+ transfer reaction forming benzophenone ketyl radical and phenoxyl radical of the acceptor. An analogous sequence mimics with 2-naphthol triplets and using benzophenone, acetophenone or chloranil as electron acceptor.

Pulse radiolytic reduction studies of 1,4,4a,8a-tetrahydro-endo–1,4-methano-naphtha-5,8-dione (THMND): effect of tertiary structure

Radiolytic reduction of a substituted 5,8-naptha dione (THMND), synthesized in our laboratory, has been investigated by pulse radiolysis and steady-state γ-radiolysis in pure aqueous, aqueous-formate and in aqueous-2-propanol-acetone mixed solvent systems. The rate constants of formation of the semi-dione radicals were approx. 109 dm3 mol-1 s-1 in aqueous-formate and aqueous-2-propanol-acetone mixed solvent. The semi-dione radicals decay by second order kinetics with rate constants (2k) of about 109 and 108 dm3 mol-1 s-1 in the above two solvents, respectively. The pKa value of the radical was found to be 5.0 in aqueous-formate solution and 5.8 in the aqueous-2-propanol-acetone mixed solvent. The one-electron reduction potential (E1) value at pH 7, determined from the pulse-radiolysis experiment, was found to be –420 ± 20 mVvs. NHE at 298 K and was independent of solvent. Ab initio calculations on its one-electron reduction reaction suggest the formation of a radical, which is different from a semiquinone where the electron density is delocalised over the two oxygen atoms. Experimental absorption maxima of the radical in aqueous solution also agree very well with the ab initio calculated values. Steady-state γ-radiolysis of THMND produces the corresponding two-electron reduced species.

Investigations on the nature of the transient species formed on pulse radiolysis of a aqueous solution of 4-(methylthio)benzoic acid

The transient absorption spectrum (λmax = 320, 400 and 550 nm) obtained on reaction of OH radicals with 4-(methylthio)benzoic acid is assigned to a solute radical cation with a positive charge on the benzene ring. The reaction with specific one-electron oxidants also produced similar spectrum and the oxidation potential for the formation of solute radical cation is estimated to be between 1.4 and 1.6 V vs NHE. The reaction of eaq- with the solute showed the formation of a transient absorption band at 320 nm and is assigned to solute radical anion with reduction potential more negative than-1.5 V.

Anomalous high reactivity of formyl and acetone ketyl radicals with uracil and its derivatives

Abstract CO 2 · − and ( CH 3 ) 2 · COH radicals apparently react with uracil and its derivatives, containing two carbonyl groups, with high rate constants ( k =10 10 dm 3 mol −1 s −1 ). Various mechanistic aspects of such reactions have been probed. The effect of pH and buffer concentration on the initial formation of absorbance points to some keto-enol tautomerism-type fast-reaction between OH − and the substrate, followed by a slower relaxation to the original equilibrium. The radical anions of these compounds react with methyl viologen mainly via an electron transfer reaction with a high rate constant value (4–9)×10 9 dm 3 mol −1 s −1 .

Study of radiation chemical reactions of oxidising and reducing radicals with furazan derivatives

Abstract Reactions of OH, O −, N3 and SO4 − and eaq− radicals with furazan and its dimethyl and diamino derivatives were studied by radiation chemical methods. The OH radical reaction is selective with the rates following the order furazan OH radical reaction with 3,4-DMF and 3,4-DAF exhibited maxima at 270 and 290xa0nm, respectively, which are assigned to the benzylic and anilino type radicals. The same intermediates are also formed in the O − reaction by H abstraction. All three derivatives showed lack of reactivity with N3 radical and was only 3,4-diaminofurazan found reactive with SO4 − (k=3.6×109xa0dm3xa0mol−1xa0s−1). The rates of eaq− with all the three derivatives are diffusion controlled k=(0.8–1.9)×1010xa0dm3xa0mol−1xa0s−1 and its reaction mechanism involves very fast protonation of the initially formed radical anions by water (k≥107xa0s−1) followed by tautomerisation of the N-atom protonated C-centred radical to the C-atom protonated N-centred radical. The transformation is catalysed by OH− and the rates for the spontaneous transformation in neutral solution are 5.4×104 and 1.5×105xa0s−1 with furazan and 3,4-diaminofurazan, respectively.