Jai P. Mittal

Excited states and electron transfer reactions of C60(OH)18in aqueous solution

Dynamic light scattering of fullerenol solutions [C60(OH)18] reveals evidence for the formation of fullerene aggregates at high solute concentration (up to 3.85×10-2 mol dm-3). This hydrophilic fullerene derivative emits very weak fluorescence regardless of its concentration. Photolysis (35 ps; λex=355 nm) of C60(OH)18 in aqueous solution yields the immediate formation of a transient singlet excited state with broad absorption in the 550–800 nm region with e670nm=2130 d mol-1 cm-1. The energetically higher-lying singlet excited state transforms via intersystem crossing (i.e., with τ1/2=500 ps) to the also broadly absorbing (550–800 nm), triplet excited state. In contrast, at low solute concentration, the features of the (*T1→*Tn) absorption differ significantly exhibiting an absorption maximum at 650 nm concomitant to a shoulder at 570 nm. The π-radical anion of fullerenol, [C60(OH)18]-, generated by electron transfer from hydrated electrons and (CH3)2C(OH) radicals, absorbs with λmax at 870, 980 and 1050 nm. Based on electron transfer studies with suitable electron donor/acceptor substrates, the reduction potential of the C60(OH)18/[C60(OH)18]-couple was estimated to be in the range between -0.358 and -0.465 V vs. NHE.

Formation of radical adducts of C60 with alkyl and halo-alkyl radicals. Transient absorption and emission characteristics of the adducts

C60 is known to undergo addition reactions with several radicals. Laser flash photolysis and pulse radiolysis studies have been carried out on C60 solutions containing various halocarbons. Transient absorption peaks observed in pulse radiolysis experiments at 460 and 640 nm are attributed to different radical adducts between C60 and halocarbon radicals. No evidence is obtained for the radical cation C60˙+. On a longer timescale (ca. 30 µs), a unique strong emission is observed with a maximum at 745 nm which is attributed to the excited state of C60 radical adducts generated owing to absorption of the strong probe light. Rate constants for the formation and decay of the radical adducts have been estimated and compared with some reported values.

Kinetics and spectral properties of electron adducts of 2′-deoxyinosine: a comparison with other purine nucleosides

The heteroatom-protonated electron adducts of 2′-deoxyinosine formed following rapid protonation of the initially-produced radical anions in neutral solution exhibit a broad peak around 310 nm. In neutral solutions, these radicals transform spontaneously, in a slow process (k∼ 2 × 104 s–1), into C-protonated adducts. This reaction is catalysed by OH– and the absorption changes at 350 nm vs. pH consist of two types of pKa curve. The transformation rates in 2′-deoxyinosine are somewhat higher than those found for inosine and are in accord with the yields of MV˙+. The spectrum at pH 13.5 closely resembles the H-adduct spectrum recorded at neutral pH. This spectrum with Iµ315= 5900 and Iµ350= 4400 dm3 mol–1 cm–1 is assigned to the C-8 protonated adducts. This study suggests that the heteroatomprotonated and C-2 protonated electron adducts of 2′-deoxyinosine are probably less stable than the corresponding radicals of inosine.

Hydroxyl radical induced reactions in aqueous solutions of halogenated benzenes: effect of electronegativity of halogen

The OH radicals, generated by radiolysis, are found to react only in acidic conditions with halogenated benzenes by an electron transfer mechanism. The concentration of acid, at which solute radical cation of halogenated benzenes appear, is observed to depend strongly on the nature and number of halogen atoms in halogenated benzenes. A linear increase in the acid concentration required for solute radical cation formation is observed with electronegativity of halogen.

Kinetics, spectral and redox behaviour of OH adducts of methylxanthines: a radiation chemical study1

The reactions of ˙OH, O˙– and SO4˙– with mono- (MX), di- (DMX) and tri- (TMX) methylxanthines were investigated by pulse radiolysis. The second order rate constants for the ˙OH reaction range from 8.5xa0×xa0109 dm3 mol–1 s–1 for 1,3,7-TMX to 2.4xa0×xa0109 dm3 mol–1 s–1 for 1-MX, the order being 1,3,7-TMXxa0>xa01,3-DMXxa0≈xa03,7-DMXxa0≈ 7-MXxa0>xa03-MXxa0>xa01-MX. The second order rate constants (kxa0≈xa03.2–4.5xa0×xa0109 dm3 mol–1 s–1) for the SO4˙– reaction are comparable to those obtained in the ˙OH reaction which are considerably reduced in the case of O˙– reaction (kxa0=xa00.33–1xa0×xa0109 dm3 mol–1 s–1). The transient absorption spectra of the OH adducts of methylxanthines (X–4OH˙ and X–8OH˙) have exhibited maxima at 320–330 nm and a broad peak around 490 nm whereas the latter peak was not observed in the spectra obtained in the O˙– and SO4˙– reactions. The rates of decay of absorption around 500 nm due to the dehydration of the X–4OH˙ adducts are in the range 1.5–4xa0×xa0104 s–1. In contrast to the behaviour observed with 1,3,7-TMX, the X–8OH˙ adducts of mono- and dimethylxanthines undergo ring opening with kxa0=xa0(4–11)xa0×xa0104 s–1. The addition of O2 (N2O∶O2 (4∶1 v/v)) to the X–4OH˙ adduct of methylxanthines (kxa0=xa01xa0×xa0109 dm3 mol–1 s–1) is much more effective than its dehydration. While O2 addition predominates over the ring opening in 1-MX, 3-MX and 1,3-DMX, the X–8OH˙ adducts of 7-MX and 3,7-DMX were found to be relatively unreactive.