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ionization potentials, electron affinities, and charge distribution, and that of Mulliken,25 which is related to ionization potentials and electron affinities. None provides a linear free energy relationship for the reactions under discussion here. Typically, the enhancement of the rate constant caused by CH3 substitution is underestimated compared to the predicted reduction of reactivity caused by the substitution of the electron-withdrawing halogen atoms.The reaction transition state in the R + HI reactions, whether formed directly from the reactants or formed from a rearrangement of an R.1-H complex,' involves an H atom with a partial positive charge located in the region between the I atom and the methyl radical. Enhanced electron density (as provided by methyl (25) Mullikcn, R. S. J . Chem. Phys. 1934, 2, 782.substitution for H) at the methyl radical carbon would stabilize such a transition state and facilitate reaction, while diminished electron density (such as that caused by halogen atom substitution for H) would destabilize such a transition state. This is the behavior that is observed.Additional studies of the kinetics of R + HI and R + HBr reactions are in progress to understand more fully the factors controlling reactivity. Acknowledgment.The kinetics and mechanism of the sonochemical reactions of p-nitrophenol have been investigated in oxygenated aqueous solutions. In the presence of ultrasound (20 W z , 84 W) pnitrophenol was degraded primarily by denitration to yield NO2-, NOf, benzoquinone, hydroquinone, 4nitrocatecho1, formate, and oxalate. These reaction products and the kinetic observations are consistent with a model involving high-temperature reactions of pnitrophenol in the interfacial region of cavitation bubbles. The main reaction pathway appears to be carbon-nitrogen bond cleavage. Reaction with hydroxyl radical provides a secondary reaction channel. The average effective temperature of the interfacial region surrounding the cavitation bubbles was estimated to be T 800 K.

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Illumination of air-free aqueous solutions containing sulfonated poly(ether ether ketone) and poly(vinyl alcohol) with 350 nm light results in benzophenone ketyl radicals of the polyketone. The polymer radicals form with a quantum yield 0.02 and decay with a second-order rate constant 6 orders of magnitude lower than that of typical alpha-hydroxy radicals. Evidence is presented that the polymeric benzophenone ketyl radicals reduce Ag+, Cu2+, and AuCl4- to metal particles of nanometer dimensions. Decreases in the reduction rates with increasing Ag(I), Cu(II), and Au(III) concentrations are explained using a kinetic model in which the metal ions quench the excited state of the polymeric benzophenone groups, which forms the macromolecular radicals. Quenching is fastest for Ag+, whereas Cu2+ and AuCl4- exhibit similar rate constants. Particle formation becomes more complex as the number of equivalents needed to reduce the metal ions increases; the Au(III) system is an extreme case where the radical reactions operate in parallel with secondary light-initiated and thermal reduction channels. For each metal ion, the polymer-initiated photoreactions produce crystallites possessing distinct properties, such as a very strong plasmon in the Ag case or the narrow size distribution exhibited by Au particles.

Cross-linking of sulfonated poly(ether-ether)ketone-poly(vinyl alcohol) (SPEEK-PVA) materials yields flexible polymer films, possessing high light-sensitivity and ion-exchange capabilities. Adsorbed Ag+ ions are photoreduced in the film under illumination (lambda = 350 nm), leading to metal nanoparticle formation in places where the film has been exposed to the light. Nanoparticles form via reduction of Ag+ by the polymeric alcohol radicals, generated in the system as a result of photochemical H-abstraction from PVA molecules by the excited carbonyl triplet state of SPEEK. Use of the films for direct metal photopatterning is demonstrated.