Kinetic studies of the oxidation reactions of o-,m-,and p-benzenediols with tris(1,10-phenanthroline)iron(III). An estimation of the redox potentials of the organic radicals by application of the marcus theory.
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概要
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Kinetic studies of the oxidation of <I>o</I>-, <I>m</I>-, and <I>p</I>-benzenediols (H<SUB>2</SUB>A) by tris(1,10-phenanthroline)iron(III) were made in dilute perchloric acid solution at an ionic strength of 1.0 mol dm<SUP>−3</SUP> at four temperatures between 10 and 30 °C. The second-order rate constants (<I>k</I><SUB>0</SUB>) which follow the observed rate law of –d [Fe(phen)<SUB>3</SUB><SUP>3+</SUP>]/d<I>t</I>=<I>k</I><SUB>0</SUB>[Fe(phen)<SUB>3</SUB><SUP>3+</SUP>][H<SUB>2</SUB>A] were determined under varied conditions. The order of the rate constants was <I>m</I>-<<I>o</I>-<<I>p</I>-benzenediols, with the ratio being approximately 1 : 10<SUP>3</SUP> : 10<SUP>4</SUP>. By the application of the Marcus theory to the kinetic parameters obtained, the standard redox potentials of the dihydroxyphenyl radicals (H<SUB>2</SUB>A<SUP>\underset.+</SUP>) were estimated in 0.11 mol dm<SUP>−3</SUP> perchloric acid solution to be 1.41, 1.10, and 0.97 V for <I>m</I>-, <I>o</I>-, and <I>p</I>-benzenediols, respectively. The energies of the highest occupied molecular orbitals for the H<SUB>2</SUB>A molecules (ε<SUB>HOMO</SUB>) were calculated to be −0.4468, −0.4332, and −0.4224 a.u. for <I>m</I>-, <I>o</I>-, and <I>p</I>-benzenediols, respectively. The differences of the total electronic energies between H<SUB>2</SUB>A<SUP>\underset.+</SUP> and H<SUB>2</SUB>A molecules (Δ<I>E</I><SUB>T</SUB>) were calculated to be 0.4183, 0.4033, and 0.3957 a.u. for <I>m</I>-, <I>o</I>-, and <I>p</I>-benzenediols, respectively. Thus, the standard redox potentials of the free-radicals estimated are found to be well correlated with the HOMO energies and also with the differences of the total electronic energies between the cation radicals and the parent molecules.
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