Kinetics of two pathways in peroxyoxalate chemiluminescence

It has been shown that 1,1'-oxalyldiimidazole (ODI) is formed as an interme diate in the imidazole-catalyzed reaction of oxalate esters with hydrogen p eroxide. Therefore, the kinetics of the chemiluminescence reaction of 1,1'- oxalyldiimidazole (ODI) with hydrogen peroxide in the presence of a fluorop hore was investigated in order to further elucidate the mechanism of the pe roxyoxalate chemiluminescence reaction. The effects of concentrations of OD I, hydrogen peroxide, imidazole (ImH), the general-base catalysts lutidine and collidine, and temperature on the chemiluminescence profile and relativ e quantum efficiency in the solvent acetonitrile were determined using the stopped-flow technique. Pseudo-first-order rate constant measurements were made for concentrations of either H2O2 or ODI in large excess. All of the r eaction kinetics are consistent with a mechanism in which the reaction is i nitiated by a base-catalyzed substitution of hydrogen peroxide for imidazol e in ODI to form an imidazoyl peracid (Im(CO)(2)OOH). In the presence of a large excess of H2O2, this intermediate rapidly decays with both a zero- an d first-order dependence on the H2O2 concentration. It is proposed that the zero-order process reflects a cyclization of this intermediate to form a s pecies capable of exciting a fluorophore via the "chemically initiated elec tron exchange mechanism" (CIEEL), while the first-order process results fro m the substitution of an additional molecule of hydrogen peroxide to the im idazoyl peracid to form dihydroperoxyoxalate, reducing the observed quantum yield. Under conditions of a large excess of ODI, the reaction is more tha n 1 order of magnitude more efficient at producing light, and the quantum y ield increases linearly with increasing ODI concentration. Again, it is pro posed that the slow initiating step of the reaction involves the substituti on of H2O2 for imidazole to form the imidazoyl peracid. This intermediate m ay decay by either cyclization or by reaction with another ODI molecule to form a cyclic peroxide that is much more efficient at energy transfer with the fluorophore. The reaction kinetics clearly distinguishes two separate p athways for the chemiluminescent reaction.