Peroxyoxalates chemiluminescence, mechanism

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Though we and others (27-29) have demonstrated the utility and the improved sensitivity of the peroxyoxalate chemiluminescence method for analyte detection in RP-HPLC separations for appropriate substrates, a substantial area for Improvement and refinement of the technique remains. We have shown that the reactions of hydrogen peroxide and oxalate esters yield a very complex array of reactive intermediates, some of which activate the fluorophor to its fluorescent state. The mechanism for the ester reaction as well as the process for conversion of the chemical potential energy into electronic (excited state) energy remain to be detailed. Finally, the refinement of the technique for routine application of this sensitive method, including the optimization of the effi-ciencies for each of the contributing factors, is currently a major effort in the Center for Bioanalytical Research. 

Figure 6 Proposed formation of an alternative C204 structure in the excitation mechanism for peroxyoxalate chemiluminescence.

Peroxyoxalate chemiluminescence, continued) high-energy intermediate, 1188-9, 1257, 1261-6, 1267, 1269 mechanism, 1257-61... 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

G. Orosz, R.S.Givens and R.L. Schowen, A model for mechanism of peroxyoxalate chemiluminescence as applied to detection in liquid chromatography, Crit. Rev. Anal. Chem., 26, 1-27 (1996). 

Bos R, Barnett NW, Dyson GA, Lim KF, Russell RA, Watson SP. Studies on the mechanism of the peroxyoxalate chemiluminescence reaction part 1 confirmation of 1,2-dioxetanedione as an intermediate using C nuclear magnetic resonance spectroscopy. Anal Chem Acta. 2004 502 141-7. 

The mechanism of peroxyoxalate chemiluminescence centers on the nature of the postulated key intermediate and its mode of interaction with, and excitation of, the fluorophore. An early proposal for this key intermediate, the highly strained 1,2-dioxetanedione, was confirmed more than three decades later using low-temperature nuclear magnetic resonance spectroscopy in combination with ab initio calculations. As shown in Scheme 2, the formation of the key intermediate is subject to both nucleophilic and general-base catalysis by concurrent mechanisms. 

Spectroscopic analysis of peroxyoxalate chemiluminescence also revealed several other, as yet unknown, transient entities. The next step in the proposed mechanism was the formation of a charged transfer complex (between 1,2-dioxetanedione and the fluorophore) that decomposes to yield the... 

In order to optimize the chemiluminescence response, we have investigated the mechanism of the complex reactions leading to chemical generation of chemiluminescence. A new peroxyoxalate-hydrogen peroxide reaction mechanism has emerged from our preliminary studies on the five contributing factors listed above. Two kinetic models are discussed, one for the... 

In this part of the chapter, we will focus essentially on mechanistic aspects of the peroxyoxalate reaction. For the discussion of the most important advances in mechanistic aspects of this chemiluminescent system, covering mainly literature reports published in the last two decades, we will divide the sequence operationally into three main parts (i) the kinetics of chemical reactions that take place before chemiexcitation, which ultimately produce the high-energy intermediate (HEI) (ii) the efforts to elucidate the structure of the proposed HEIs, either attempting to trap and synthesize them, or by indirect spectroscopic studies and lastly, (iii) the mechanism involved in chemiexcitation, whereby the interaction of the HEI with the activator leads to the formation of the electronically excited state of the latter, followed by fluorescence emission and decay to the ground state. 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

A simplifled mechanism for oxalate ester chemiluminescence is shown in Fig. 46, which is essentially a two-step reaction. Attack by peroxide on the ester produces an excited-state, 1,2-dioxetaneone via a peroxyoxalate intermediate, which breaks down in the presence of a suitable fluorophore to give carbon dioxide plus an excited-state fluorophore, which, in turn, emits light. The scheme depicted in Fig. 46 is an oversimplification and the true reaction mechanism continues to attract much attention (AlO, Cll, Cl la, G8, 05). 

Fig. 46. Possible mechanism for chemiluminescence of oxalate esters. Attack by peroxide ions leads to a peroxyoxalate intermediate which, in turn, breaks down to an unstable excited-state dio-xetaneone. In the presence of an appropriate fluorophore, energy transfer from the dioxetaneone leads to luminescence from the fluorescent sensitizer.

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