Determination of the volume of activation of the key reaction steps in the oxidation of phenanthroline-copper(I) by molecular oxygen

Using the high-pressure pulse radiolysis technique, we have shown that the oxidation of (phen)₂Cu^I (phen = 1,10-phenanthroline) by molecular oxygen proceeds via a transient in which a copper-oxygen bond is formed, i.e., via Cu^I-O₂ as an intermediate. The volume of activation measured for this process is -22 ± 2 cm³ mol^-1. When high concentrations of (phen)₂Cu^II are present in the solution, this transient reacts with another (phen)₂Cu^I yielding (phen)₂Cu^II and H₂O₂. If the latter reaction involves the formation of Cu^IO₂Cu^I as an intermediate, it is produced with a rate constant close to the diffusion-controlled limit. In the presence of low concentrations of (phen)₂Cu^II, Cu^I-O₂ decomposes to (phen)₂Cu^II and O₂^-, where subsequently O₂^- reacts with another (phen)₂Cu^I to produce (phen)₂Cu^II and H₂O₂.