Search

Absolute rate constants ( k ) for reduction of substituted methylperoxyl radicals by ascorbate ions and by TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) in aqueous solutions have been determined by pulse radiolysis. The rate constants vary from lo6 to lo9 M-I s-l, increasing as the electron-withdrawing capacity of the substituent on the peroxyl group increases. Linear correlations are observed between log k and the Taft substituent constants u* for a wide variety of substituents, but not all substituents fit the same line. In the case of ascorbate as reductant, the points for peroxyl radicals that contain halogens on the a-carbon lie on a different line ( p * = 0.41) than that for the other substituents ( p * = 1.25). In the case of TMPD there are also two families of peroxyl radicals: those comprising the electron-donating groups Me through t-Bu ( p * = 5.6) and those containing electron-withdrawing substituents ( p * = 0.64).

ESR experiments were carried out in the temperature range 80–300 K on the crystalline powders of [TMPD + ] 2 [Cu x Ni 1− x (mnt) 2 ] with various copper concentrations below x = 0.06. At low temperatures, all the solid solutions prepared yielded an ESR spectrum consisting either of a fine structure arising from TMPD + dimers or of a hyperfine structure due to copper. The ESR spectra of the solid solution x = 0.02 exhibited both the fine and hyperfine structures between 150 and 200 K. The spin-Hamiltonian parameters were determined as | D | = 21.6 mT and | E | = 2.7 mT for TMPD + dimers and as g | = 2.082, g ⊥ = 2.022, | A | ( 63 Cu)|=17.0 mT, and | A ⊥ ( 63 Cu)| = 4.2 mT for Cu(II) ions. These values were independent of copper concentration. The interaction energy 2 J within TMPD + dimers was evaluated to be equal to −0.22 eV from the temperature dependence of the signal intensity. The interdimer-interaction energy J ′ and the copper-TMPD + interaction energy J ″ were estimated as | J ′ | ≌ 0.06 | J | and | J ″ | ≌ 0.01| J | on the basis of line broadening of component signals.

Role important du radical cationique du compose du titre et de l'electron solvate. Le comportement photophysique est interprete a l'aide d'un etat transitoire forme a partir de l'etat relaxe de fluorescence et agissant comme un precurseur commun de la paire ion, et des etats triplet et fondamental