| The one-electron transfer reactions of water soluble manganese porphyrins, MnP, (P = TPPS, TPPC, TMPyP or TPyP) have been investigated by steady-state and pulse radiolysis. Most porphyrins are reduced by e(,aq)('-) with diffusion-controlled rate constants ((TURN) 10('10) M('-1)s('-1)) and somewhat more slowly by the radical from isopropanol (k (TURN) 10('8)-10('9) M('-1)s('-1)). Thus reduction of Mn(III)P yields Mn(II)P which upon further reduction forms Mn(II)(')P('-) (ligand (pi)-radical anion). The one-electron oxidation of porphyrins is carried out by Br(,2)('-), Cl(,2)('-) and CH(,2)CHO radicals. The rate constants are in the range of 10('8)-10('10) M('-1)s('-1) depending on the redox potentials of the porphyrin and the radical used, and on their electrostatic charges. Oxidation of Mn(II)P by Br(,2)('-) involves a rapid reaction (k = 2 x 10('9) M('-1)s('-1)) to form an intermediate complex, Br-Mn(III)P, which hydrolizes to aqueous Mn(III)P in a first order process (k (TURN) 10('4) s('-1)). Oxidation of Mn(III)P by the above radicals yields Mn(IV)P. The latter is further oxidized by Br(,2)('-) at high pH to form either Mn(V)P or Mn(IV)(')P('+). Formation of the Mn(V)P is favored over that of Mn(IV)(')P('+) as the pH is increased (from 11 to 14) and in the order of porphyrins TPPS > TPPC > TMPyP.; Pulse radiolytic reduction of Mn(III)P to Mn(II)P in acidic solutions permitted the determination of the rate constants and mechanism for demetallation of Mn(II)P to Mn('2+) and H(,4)P('2+). Demetallation of MnTPPS was investigated at pH 1.5 to 3 and MnTMPyP and MnTPyP at pH 1V at pH 7 and that E(,ox)(MnTMPyP) > E(,ox)(MnTPPS).; The kinetics for the disappearance of Mn(IV)P and Mn(V)P were measured and the results are discussed in terms of the possibility of O(,2) production from water. |
