RADIOLYTIC STUDIES OF THE MECHANISM OF AUTOXIDATION OF TRIPHENYLPHOSPHINE AND RELATED-COMPOUNDS

Triphenylphosphine (Ph(3)P) undergoes one-electron oxidation in irradi ated CH2Cl2 solutions to yield the radical cation Ph(3)P(.+). This sho rt-lived species exhibits intense absorption with a maximum at 325 nm and an extended shoulder at 400-500 nm and decays by a second-order pr ocess in the absence of O-2. In the presence of O-2, however, the radi cal cation reacts with O-2 with a rate constant of 7 x 10(6) L mol(-1) s(-1) to yield a peroxyl radical, Ph(3)P(+)OO(.), which exhibits no s ignificant absorbance at lambda > 300 nm. Similar results, but with sl ightly different rate constants, were obtained also in CCl4 solutions and in CH3CN and cyclohexane solutions containing 1% CCl4. Tris(2,4,6- trimethoxyphenyl)phosphine in CH2Cl2 exhibits a behavior similar to th at of Ph(3)P, but the reaction of its radical cation with O-2 is an or der of magnitude more rapid. On the other hand, the perfluorinated Ph( 3)P(.+) in CH2Cl2 reacts with O-2 much more slowly, if at all. Dipheny l-2-pyridylphosphine yields a radical cation which exhibits a slightly narrower absorption band but reacts with O-2 With the same rate const ant as Ph(3)P(.+). The peroxyl radicals propagate a chain reaction by further oxidation of phosphine molecules to Ph(3)P(.+) either directly or indirectly. The final radiolysis product is the phosphine oxide, P h(3)PO. The radiolytic yields for oxidation of the phosphines were muc h higher than the radiolytic yield of the solvent derived radicals, ex cept for the case of the perfluoro derivative, and were dependent on t he concentrations of phosphine and O-2 and on the dose rate. At low do se rates and high solute concentrations the chain lengths exceeded 100 0.