Pulse radiolysis of trans-1,2-bis(4-pyridyl)ethylene in aqueous solution

Pulse radiolysis with optical and conductometric detection techniques was used to study the reactions of hydrated electrons (e{sub aq}{sup {minus}}), H atoms, 2-hydroxy-2-propyl radicals, and OH radicals with trans-1,2-bis(4-pyridyl)ethylene (neutral form: M) and its conjugate acids HM{sup +} and H{sub 2}M{sup 2+} in aqueous solution. Electron attachment to M leads to a transient with {lambda}{sub max} = 455 nm ({epsilon} = 4.0 {times} 10{sup 4} dm{sup 3} mol{sup {minus}1} cm{sup {minus}1}) at pH 11-12 and to another transient ({lambda}{sub max} = 500 nm, {epsilon} = 5.2 {times} 10{sup 4} dm{sup 3} mol{sup {minus}1} cm{sup {minus}1}) at pH 6-9. The 455-nm transient is assigned to a neutral H-adduct radical (HM{sup {sm bullet}}, H attached to an N atom), formed via reaction of water with the radical anion. The 500-nm transient is assigned to H{sub 2}M{sup {sm bullet}+}, the conjugate acid of HM{sup {sm bullet}}, with both H's at the N's; pK{sub a} = 9.8. At pH 4-6 H{sub 2}M{sup {sm bullet}+} is also formed by reactions of e{sub aq}{sup {minus}} or H atoms with HM{sup +} and H{sub 2}M{sup 2+} (at pH 1-5) quantitatively to H{sub 2}M{sup {sm bullet}+} but does not react with neutral M. Three OH-adduct radicals were observedmore » in different pH regions. M{sup {sm bullet}}OH, a neutral radical ({lambda}{sub max} = 392 nm), and its conjugate acid (HM{sup {sm bullet}+}OH, {lambda}{sub max} = 410 nm) have a pK{sub a} of about 6; at pH {le} 4.5 the latter radical is converted by acid catalysis into an isomer with {lambda}{sub max} = 510 nm. Structural assignments are presented for all radicals observed. Some results with trans-1,2-bis(n-pyridyl)ethylene (n = 2 or 3) are also reported.« less