Stopped-flow kinetics investigation of the imidazole-catalyzed peroxyoxalate chemiluminescence reaction

The stopped-now technique was used to study the temperature-dependent kinetics of the imidazole-catalyzed peroxyoxalate reaction in order to further elucidate the reaction mechanism. Pseudo-first-order rate constants were obtained from the chemiluminescence intensity vs time profiles for the sequential reaction model X --> Y --> Z over a wide range of initial concentrations of each of the following reagents: bis(2,4,6-trichlorophenyl) oxalate (TCPO), imidazole (ImH), and hydrogen peroxide. These measurements were complemented by UV absorbance measurements of the kinetics of the step X --> Y. For both reaction conditions pseudo-first-order in TCPO ([ImH], [H2O2] >> [TCPO]) and pseudo-first-order in H2O2 ([ImH]>>[TCPO]>>[H2O2]), the first step of the reaction is nucleophilic substitution by two imidazole molecules to form 1,1'-oxalyldiimidazole (ODI). Under conditions of excess TCPO in the concentration range 0.075-0.25 mM, the Y --> Z reaction probed the subsequent reaction of ODI with H2O2 to form the imidazoyl peracid intermediate, ImC(O)C(O)OOH. For excess H2O2 concentrations in the range 2.5-15 mM, the reaction of H2O2 with ODI is fast, and the Y --> Z step of the sequential reaction model describes subsequent reactions of the imidazoyl peracid. An important unexpected finding necessary for interpreting the kinetics of this reaction is that under conditions of a large excess of H2O2 the faster rise of the chemiluminescence signal corresponds to the second step of the reaction (Y --> Z), and the slower fall of the signal corresponds to the first step (X --> Y). Lutidine and collidine, amine bases of similar aqueous pK(a) as imidazole, displayed very Little catalytic effect on the PO-CL reaction in comparison to imidazole, corroborating the conclusion that nucleophilic catalysis with formation of ODI as an intermediate constitutes the principal reaction pathway under conditions of both excess oxalate ester and excess H2O2. Imidazole quenches the quantum yield of the reaction, a result that can be well explained by catalysis of the decomposition of the key energy-transfer intermediate.