Water Effect on Peroxyoxalate Kinetics and Mechanism for Oxalic Esters with Distinct Reactivities

Abstract

The peroxyoxalate reaction is being widely used for various analytical and bioanalytical applications, and however, few mechanistic studies are performed in aqueous media, important mainly for bioanalytical applications, where low chemiluminescence emission quantum yields are obtained. In this sense, we report here kinetic studies on the peroxyoxalate reaction, using two commercially available and widely utilized esters, bis(2,4‐dinitrophenyl) oxalate (DNPO) and bis(2,4,6‐trichlorophenyl) oxalate (TCPO), in 1,2‐dimethoxyethane:water mixtures. The reaction of the much more reactive DNPO, in anhydrous and aqueous media, occurs by a direct nucleophilic attack of H2O2 to the oxalic ester, not involving nucleophilic catalysis by imidazole. Contrary, in the reaction of the less reactive TCPO with H2O2, imidazole acts mainly as nucleophilic catalyst. For both esters, experimental conditions are established where precise kinetic data and emission quantum yields can be obtained. Interestingly, the quantum yields in 1,2‐dimethoxyethane water mixtures increase up to a water concentration of 0.7 mol L–1 and decrease significantly with higher concentrations.

An Update on General Chemiexcitation Mechanisms in Cyclic Organic Peroxide Decomposition and the Chemiluminescent Peroxyoxalate Reaction in Aqueous Media†

“…Additionally, it was observed that with both esters Φ S values increased up to [H 2 O] = 0.7 mol L −1 , reaching chemiexcitation yields as high as 5 × 10 −3 E mol −1 (Table 1), decreasing afterwards for superior water concentrations (70); constituting the first report of a systematic increase in Φ S with water concentration for the PO‐CL reaction. It was hypothesized that the chemiexcitation efficiency may be related to a solvation effect involved with the bimolecular interaction between the HEI and ACT, within the CIEEL sequence (70). This may occur due to a medium polarity effect, or even due to a specific molecular water effect that could stabilize the generated radical‐ion pair through hydrogen‐bonding (Scheme 5) (70).…”

“…For both TCPO and DNPO in DME/water mixtures, the recorded perhydrolysis rate constants are several orders of magnitude higher than the corresponding hydrolysis rate constants (Scheme 5) (70). Additionally, it was observed that with both esters Φ S values increased up to [H 2 O] = 0.7 mol L −1 , reaching chemiexcitation yields as high as 5 × 10 −3 E mol −1 (Table 1), decreasing afterwards for superior water concentrations (70); constituting the first report of a systematic increase in Φ S with water concentration for the PO‐CL reaction. It was hypothesized that the chemiexcitation efficiency may be related to a solvation effect involved with the bimolecular interaction between the HEI and ACT, within the CIEEL sequence (70).…”

“…Many reports have been made, particularly recently (70–77), regarding the determination of the reaction mechanism of the peroxyoxalate chemiluminescence (PO‐CL) transformation in aqueous environment. The determination of mechanisms relies heavily on kinetic measurements and in the interpretation of molecularity made from observed reaction rates (78–80).…”

“…Nonetheless, other oxalate derivatives with distinct reactivity have also been used (Fig. 3), either possessing electron‐withdrawing substituents, for example, bis(4‐nitrophenyl) oxalate (DNPO) (70), bis(3‐(methoxycarbonyl)phenyl) oxalate (3MCPO) (71) and bis(2‐(methoxycarbonyl)phenyl) oxalate (2MCPO) (72), as well as electron‐donating groups, such as bis(4‐methylphenyl) oxalate (MePO) (72, 77) and bis(4‐methoxyphenyl) oxalate (MeOPO) (77) (Fig. 3).…”

“…It was observed for the TCPO/H 2 O 2 /IMI‐H and DNPO/H 2 O 2 systems in DME/water mixtures, going from pure organic solvents to [H 2 O] up to 7 mol L −1 , that in higher water concentrations H 2 O 2 reacts directly with TCPO without involvement of nucleophilic IMI‐H catalysis, whereas no nucleophilic IMI‐H catalysis appears to operate for DNPO, even in anhydrous conditions (70). For both TCPO and DNPO in DME/water mixtures, the recorded perhydrolysis rate constants are several orders of magnitude higher than the corresponding hydrolysis rate constants (Scheme 5) (70). Additionally, it was observed that with both esters Φ S values increased up to [H 2 O] = 0.7 mol L −1 , reaching chemiexcitation yields as high as 5 × 10 −3 E mol −1 (Table 1), decreasing afterwards for superior water concentrations (70); constituting the first report of a systematic increase in Φ S with water concentration for the PO‐CL reaction.…”