Competition Between Cα -S and Cα -Cß Bond Cleavage in ß-Hydroxysulfoxides Cation Radicals Generated by Photoinduced Electron Transfer.

A kinetic and product study of the 3-cyano-N-methyl-quinolinium photoinduced monoelectronic oxidation of a series of ß-hydroxysulfoxides has been carried out to investigate the competition between Cα -S and Cα -Cß bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λ max ≈ 520 nm) and that of 3-CN-NMQ• (λ max ≈ 390 nm). Steady-state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo Cα -S bond cleavage, the presence of a ß-hydroxy group makes, in some cases, the Cα -Cß scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the ß-OH group with -OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the Cα -Cß bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission.

Gold nanoparticle catalysis of the cis-trans isomerization of azobenzene.

Ablated, "pseudo-naked" gold nanoparticles (AuNPs) catalyze the cis-trans isomerization of substituted azobenzenes. para-Substitution was found to affect the rate of isomerization, suggesting the participation of AuNP-mediated electron transfer in the isomerization mechanism.

Reactions of the phthalimide N-oxyl radical (PINO) with activated phenols: the contribution of π-stacking interactions to hydrogen atom transfer rates.

The kinetics of reactions of the phthalimide N-oxyl radical (PINO) with a series of activated phenols (2,2,5,7,8-pentamethylchroman-6-ol (PMC), 2,6-dimethyl- and 2,6-di-tert-butyl-4-substituted phenols) were investigated by laser flash photolysis in CH(3)CN and PhCl in order to establish if the reactions with PINO can provide a useful tool for evaluating the radical scavenging ability of phenolic antioxidants. On the basis of the small values of deuterium kinetic isotope effects, the relatively high and negative ρ values in the Hammett correlations and the results of theoretical calculations, we suggest that these reactions proceed by a hydrogen atom transfer (HAT) mechanism having a significant degree of charge transfer resulting from a π-stacked conformation between PINO and the aromatic ring of the phenols. Kinetic solvent effects were analyzed in detail for the hydrogen transfer from 2,4,6-trimethylphenol to PINO and the data obtained are in accordance with the Snelgrove-Ingold equation for HAT. Experimental rate constants for the reactions of PINO with activated phenols are in accordance with those predicted by applying the Marcus cross relation.

Ultraclean derivatized monodisperse gold nanoparticles through laser drop ablation customization of polymorph gold nanostructures.

We report a novel nanosecond laser ablation synthesis for spherical gold nanoparticles as small as 4 nm in only 5 s (532 nm, 0.66 J/cm(2)), where the desired protecting agent can be selected in a protocol that avoids repeated sample irradiation and undesired exposure of the capping agent during ablation. This method takes advantage of the recently developed synthesis of clean unprotected polymorph and polydisperse gold nanostructures using H(2)O(2) as a reducing agent. The laser drop technique provides a unique tool for delivering controlled laser doses to small drops that undergo assisted fall into a solution or suspension of the desired capping agent, yielding monodisperse custom-derivatized composite materials using a simple technique.

One-electron oxidation of ferrocenes by short-lived N-oxyl radicals. The role of structural effects on the intrinsic electron transfer reactivities.

A kinetic study of the one electron oxidation of substituted ferrocenes (FcX: X = H, COPh, COMe, CO(2)Et, CONH(2), CH(2)OH, Et, and Me(2)) by a series of N-oxyl radicals (succinimide-N-oxyl radical (SINO), maleimide-N-oxyl radical (MINO), 3-quinazolin-4-one-N-oxyl radical (QONO) and 3-benzotriazin-4-one-N-oxyl radical (BONO)), has been carried out in CH(3)CN. N-oxyl radicals were produced by hydrogen abstraction from the corresponding N-hydroxy derivatives by the cumyloxyl radical. With all systems, the rate constants exhibited a satisfactory fit to the Marcus equation allowing us to determine self-exchange reorganization energy values (λ(NO˙/NO(-))) which have been compared with those previously determined for the PINO/PINO(-) and BTNO/BTNO(-) couples. Even small modification of the structure of the N-oxyl radicals lead to significant variation of the λ(NO˙/NO(-)) values. The λ(NO˙/NO(-)) values increase in the order BONO < BTNO < QONO < PINO < SINO < MINO which do not parallel the order of the oxidation potentials. The higher λ(NO˙/NO(-)) values found for the MINO and SINO radicals might be in accordance with a lower degree of spin delocalization in the radicals MINO and SINO and charge delocalization in the anions MINO(-) and SINO(-) due to the absence of an aromatic ring in their structure.