Probing Reactivity of Intermediates

We and others have introduced an important methodology based on the isolation of species in the gas phase assumed to participate in the reaction mechanism and perform ion/molecule reactions with neutral substrates (in the collision cell) of the reaction solution. This methodology seems to be very useful for discarding side-products and to assure the reliability of the analyses. 

Initially, the traditional methods used for gas-phase ion/molecule reactions were limited to neutral compounds that were sufficiently volatile and thermostable to be transferred to the gas phase by heating. Some years ago it was shown that using API conditions (ESI or APCI), the range of neutral molecules participating in ion/ molecule reactions could be extended toward those of lower volatility and thermal stability. 

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A comprehensive compilation of data derived from feeding and other biosynthetic studies performed by Kirby [21], Sammes [22], and Taylor [23], genomic studies conducted by Howlett [lc, 24], as well as our own synthetic studies probing the innate reactivity of various intermediates (vide infra) culminated in our formulation of the biosynthetic hypothesis delineated below (Scheme 9.1). The biosynthesis features two distinct phases an oxidative dimerization phase and a thiolation phase in which diketopiperazine structures are elaborated to afford the defining epipolysulfide motifs (Scheme 9.1) [25, 26]. 

We consider first how surfaces which are themselves not photosensitive can perturb chemical reactivity. First, the surface can influence diffusional motion of adsorbed substrates, intermediates or products. With preadsorbed substrates, one can probe the nature of motion of intermediates generated on the surface and search for differences in reactivity caused by surface confinement . When several photochemical precursors to benzyl radicals, e.g., benzyl phenylacetate, a dibenzyl ketone, or a dibenzyl sulfone, are irradiated as adsorbates on dry silica gel, singlet and triplet radical pairs are generated, Eq. (7). The extent of radical recombination observed requires... 

A direct comparison of the abilities of Angeli s salt and synthetic ONOO- to affect intracellular targets was probed with the fluorophore diaminofluorescein (DAF) (239). Exposure of cells to synthetic ONOO- at a rate comparable to Angeli s salt decomposition resulted in only 4% of the intracellular signal from Angeli s salt. These data show that the reactivity of HNO-derived species in an aerobic cellular milieu cannot be readily reconciled with an intermediacy involving ONOO-. Importantly, cells acted as a sink for HNO-derived species, effectively sequestering intermediates from reaction with DAF when the probe was present in the extracellular medium. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

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