Dioxetanes activation energy

The PM3 calculations of the So and the vertical (Franck-Condon) Ti energies as a function of the 0-0 bond length [<7(0-0)] have successfully reproduced the experimental activation energy for the dioxetane thermolysis . However, an unusual shape has been found for the energy profile A flat plateau, in which the ground-state energy... 

Energy transfer to photoreactive acceptors has also been widely utilized for excitation quantum yield determination (chemical titration), mainly in the decomposition of dioxetanes ° . The quantum yields are calculated from the photoproduct yield obtained at infinite energy acceptor concentrations ( = 1.0) by extrapolation of the double-reciprocal relationship between the photochemically active energy acceptor concentration and the photoproduct yield ( Lp ). H the quantum yield of the photochemical reaction (excitation quantum yield (< > ) can be calculated (equation 8) . ... 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

Besides the isothermal kinetic methods mentioned above, by which activation parameters are determined by measuring the rate of dioxetane disappearance at several constant temperatures, a number of nonisothermal techniques have been developed. These include the temperature jump method, in which the kinetic run is initiated at a particular constant initial temperature (r,-), the temperature is suddenly raised or dropped by about 15°C, and is then held constant at the final temperature (7y), under conditions at which dioxetane consumption is negligible. Of course, for such nonisothermal kinetics only the chemiluminescence techniques are sufficiently sensitive to determine the rates. Since the intensities /, at 7 ,- and If at Tf correspond to the instantaneous rates at constant dioxetane concentration, the rate constants A ,- and kf are known directly. From the temperature dependence (Eq. 32), the activation energies are readily calculated. This convenient method has been modified to allow a step-function analysis at various temperatures and a continuous temperature variation.Finally, differential thermal analysis has been employed to assess the activation parameters in contrast to the above nonisothermal kinetic methods, in the latter the dioxetane is completely consumed and, thus, instead of initial rates, one measures total rates. 

In Table 3 the activation energies of tetramethyl-1,2-dioxetane by a variety of isothermal and nonisothermal kinetic methods are compared. The values range... 

TABLES. ACTIVATION ENERGIES OF TETRAMETHYL-1,2-DIOXETANE BY A VARIETY OF KINETIC METHODS... 

It has been suggested that the more easily the dioxetane ring can be puckered, the lower its thermal stability. This could be the reason for the high activation energies of cyclobutadienedioxetane (Entry 23 in Table 4) and diadamantylidenedioxetane (Entry 30 in Table 4). However, why should the tetraethyldioxetane (Entry 25 in Table 4) be more planar and more rigid than tetramethyldioxetane (Entry 19 in Table 4)7 Clearly, we do not as yet understand the thermal stability of 1,2-dioxetanes and much has yet to be learned. X-ray structure data would seem important on this problem. 

Most of the experimental evidence also points to the diradical mechanism as the preferred decomposition mode. Thus, the very earliest experimental evidence in support of the diradical mechanism rests on the fact that alkyl and phenyl substitution does not significantly alter the activation parameters for dioxetane decompositon. It was argued that if C-C bond cleavage occurs simultaneously with 0-0 bond cleavage, the incipient carbonyl group in the activated complex (23) should be stabilized in the relative order phenyl > alkyl > hydrogen. Thus, the activation energies should obey the relative order ii a(Ph)< fl(R)< a(H), that is, lowest for phenyl-substituted dioxetanes. Since this expectation was not borne out by the experimental data, the diradical (24) was proposed as an intermediate. 

As additional support for the diradical mechanism, it was shown that the 3,4-diethoxy-l,2-dioxetane (8) and the p-dioxene-l,2-dioxetane had identical activation energies, implying that the C-C bond is not significantly stretched in the activated complex. That these notions on substituent effects in dioxetane decomposition are grossly oversimplified has come clearly into focus in recent years (Table 4). The fact that little yet is understood about the correspondence between activation parameters and dioxetane structure has already been amply expounded in Section V.l.B. Nevertheless, a few additional comments seem appropriate on this subject in... 

Simple alkyl dioxetanes have an activation energy for decomposition in the range 20-26 kcal/mole, which means that they rapidly decompose at, or even below, room temperature (L6, T24). Clearly, such compounds are of little value... 

Although preliminary quantum-chemical calculations had predicted that the a-peroxylactones should be more stable than the simple 1,2-dioxetanes,32 the experimental data in Table III indicate the contrary.2,22 Moreover, thermokinetic calculations are in excellent accord with the experimental data.97b Thus, Richardson and co-workers97 predict activation energies of the order of 24-25 kcal for the 1,2-dioxetanes (1), 21-22 kcal for a-peroxylactones (2), and 17 kcal for the carbon dioxide dimer (3). 

Fig. 4.1. The photooxygenation of methyl sorbate (I) produces compounds III and IV, which are consistent with a dioxetane (II) intermediate that is thought to have a high activation energy. Abbreviation hv, ultraviolet radiation.

The activation energy in this case varied from 87 to 93 kJ/mol in different solvents. From the temperature dependence, several competitive reaction paths for this dimethyl-dioxetanone decomposition were deduced, all having a biradical as first intermediate. Heavy-atom effects often play a role in dioxetan chemiluminescence. If DBA is used as fluorescer, the quantum yield is markedly greater than that observed when DPA is used - although the latter has a fluorescence efficiency of 0.89, compared with 0.1 for DBA. In both cases triplet-singlet energy transfer is the origin of the chemiluminescence. 

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