Articles you may be interested inMolecular near-field antenna effect in resonance hyper-Raman scattering: Intermolecular vibronic intensity borrowing of solvent from solute through dipole-dipole and dipole-quadrupole interactions Electron spin relaxation of ditertiarybutyl nitroxide in supercooled water New measurements of the solvent effect on the nitrogen hyperfine coupling constant of di-tert-butyl nitroxide are reported. These, together with literature data, are used to test various models for the solvent effect. At the Huckel level of approximation, aN is a linear function of the applied electric field. Thus various reaction field theories may be considered. The widely used Onsager reaction field does not account for the effects of the more polar solvents or for the differences between polar and nonpolar solvents. The Wertheim and Block-Walker reaction fields are better, especially for very polar solvents. However none of these continuum reaction fields is entirely satisfactory theoretically or experimentally. We propose a dipole-dipole model for polar solvents which is superior to the continuum models. From the dipole-dipole model, we suggest that the quantity IJp/M is a convenient linear parameter for polar solvent effects, the factors being solvent dipole moment, density, and molecular weight. The dipole-dipole model should apply to a wide range of polar solutes. Some special situations are not explained by the model, including hydrogen-bonding solvents, halogenated aromatiCS, and solvents with more than one conformation. The temperature dependence of the solvent effect is also considered.

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One-electron reduction by γ-ray irradiation of cobalt(III) and manganese(III) tetraphenylporphyrins (ClCo(III)TPP, ClMn(III)TPP, and IMn(III)TPP) in ethanol and MTHF(2-methyltetrahydrofuran) solutions at 77 K results in the reduction of the central metals of these porphyrins. From the optical and ESR measurements, the reduced species are found to retain the axial anion, Cl− or I−, in MTHF but not in ethanol. The absorption spectroscopic studies of these porphyrins in ethanol solutions indicate that the axial halide ion is displaced by an ethanol molecule prior to γ-ray irradiation.

We have demonstrated in this and previous studies that the reactivity of the photoexcited triplet state of porphyrins and chlorophylls with radicals is substantially higher than that of the corresponding ground states. This behavior, not limited to the porphyrin moiety only, was found also with pyrene,1 2345 whose photoexcited triplet exhibits similar chemical reactivity toward transient radicals. The experimental results clearly indicate that PRAP spec-troscopy is most suitable in studying reactions of photoexcited states with short-lived radicals. With regard to stable radicals such as MV+•, the advantage of employing this method, over conventional laser photolysis, is less substantial. Nevertheless, in the PRAP mode the interference due to a possible absorption overlap between the photoexcited triplet and that of the stable radical can be avoided as the triplet state is prepared prior to the radical production.