Rate constants, chemiluminescence intensity

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

This model permits a determination of the rate constants for the rise of the chemiluminescence intensity and its subsequent decay and, more importantly, allows a quantitative assessment of the effects of reaction conditions, such as solvent variation, temperature, or additives, on the rates (r and f), the time required (t... 

Figure 13, indicates that the first mole of phenol is released in <30 s, the same elapsed time for the chemiluminescence to reach a maximum intensity. In fact, the measured rate constant r, for the rise in the chemiluminescence emission, is identical to the rate of the first phenol s release from the oxalate ester. Furthermore, the slower rate of release of the second phenol ligand has a rate constant that is identical to the chemiluminescence decay rate f. Thus, the model allows a quantitative analysis of the reaction mechanism, heretofore not available to us. We intend to continue this avenue of investigation in order to optimize the chemiluminescence efficiencies under HPLC conditions and to delineate further the mechanism for peroxy-oxalate chemiluminescence. 

The effect of oxygen concentration is thus included in a constant m, which modifies both the resulting maximum of the chemiluminescence intensity and the apparent rate constant k of hydroperoxide decomposition. 

Finally, in activated chemiluminescence, an added compound also leads to an enhancement of the emission intensity however, in contrast with the indirect CL, this compound, now called activator (ACT), is directly involved in the excitation process and not just excited by an energy transfer process from a formerly generated excited product (Scheme 5). Activated CL should be considered in two distinct cases. In the first case, it involves the reaction of an isolated HEI, such as 1,2-dioxetanone (2), and the occurrence of a direct interaction of the ACT with this peroxide can be deduced from the kinetics of the transformation. The observed rate constant (kobs) in peroxide decomposition is expected to increase in the presence of the ACT and a hnear dependence of kobs on the ACT concentration is observed experimentally. The rate constant for the interaction of ACT with peroxide ( 2) is obtained from the inclination of the linear correlation between obs and the ACT concentration and the intercept gives the rate constant for the unimolec-ular decomposition ( 1) of this peroxide (Scheme 5). The emission observed in every case is the fluorescence of the singlet excited ACT" ° . ... 

The rate constants for recombination of radicals, k, were determined using the chemiluminescence method, which consists in recording the intensity of luminescence induced by recombination of R02 radicals, when the system passes from one steady state to another (6). A change in steady state was induced by introducing an initiator (Figure 1). The k6 value was obtained from... 

Final evidence for the involvement of a ground-state complex with [21] on the catalytic chemiluminescence pathway comes from the inhibition of the special catalysis by the addition of donor molecules capable of competitive complexation. Both the rate constant for the reaction of [21] catalyzed by MgTPP and the initial chemiluminescence intensity are decreased markedly by the addition of diethyl ether and even more dramatically by the addition of pyridine. This inhibition of catalysis apparently derives from complexation of the diethyl ether or pyridine to MgTPP. The added donor competes with [21] for the formation of the weak ground state complex, thereby inhibiting the otherwise effective catalysis. The special catalysis of ZnTPP and MgTPP is thus fully consistent with and readily accommodated by the CIEEL mechanism. 

In addition to absorption and stimulated emission, a third process, spontaneous emission, is required in the theory of radiation. In this process, an excited species may lose energy in the absence of a radiation field to reach a lower energy state. Spontaneous emission is a random process, and the rate of loss of excited species by spontaneous emission (from a statistically large number of excited species) is kinetically first-order. A first-order rate constant may therefore be used to describe the intensity of spontaneous emission this constant is the Einstein A factor, Ami, which corresponds for the spontaneous process to the second-order B constant of the induced processes. The rate of spontaneous emission is equal to Aminm, and intensities of spontaneous emission can be used to calculate nm if Am is known. Most of the emission phenomena with which we are concerned in photochemistry—fluorescence, phosphorescence, and chemiluminescence—are spontaneous, and the descriptive adjective will be dropped henceforth. Where emission is stimulated, the fact will be stated. 

The potential diagram for NO is shown in Figure 1.8. Baulch et al. [112] have recently reviewed the rate data and recommend a value for the third-order rate constant for recombination at 298°K with N2 as the third body of 1.03 x 10 32 cm6/molecule2-sec. The /8 (2f2II-X2n), y (A 2 +-X 2I I), d (C ari—A 2TI), and Ogawa (b 4S -a 4II) bands have all been identified in the complex chemiluminescence that accompanies the recombination. Young and Sharpless [113, 114] determined the total intensities of the first three of these systems at room temperature, and the temperature dependences of these processes have since been measured by Gross and Cohen [115]. 

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. 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

The experimental procedure to determine 4>wa is quite analogous to that discussed for The experimental definition is given by Eq. 38, in which all the terms have been already defined. Again the dioxetane decomposition rate constant kj) is determined by following the first-order kinetics of the DPA-enhanced chemiluminescence decay. The initial or total DPA fluorescence intensity is standardized with a suitable light standard, usually with luminol or the scintillation cocktail. The photomultiplier tube should be corrected for wavelength response. ... 

The intensity of chemiluminescence, /CL, as it is stated in Equation (h) of Scheme 3.1, is proportional to the rate of the bimolecular reaction of (POO ) species, which has a rate constant, kb, and considering an efficiency factor, /. This factor of proportionality depends on different factors such as the yield of formation and emission of the excited state responsible for the chemiluminescence and the inherent apparatus constant. 

Table I. Chemiluminescent Intensities and Rate Constants from the Reaction of 0-atoms with CO in the Presence of Added Gases ...

Steady-state analysis gives the intensity of chemiluminescence / in terms of the rate constants for combination, fci, redissociation A i, electronic quenching and radiation k ... 

No other primary emitting states of Kr were detected in this reaction and two-body combination would be too slow to account for more than a negligible fraction of the total quenching rate. Piper et al., therefore, were able to ascribe rate constants to channels (22) and (23) of respectively S.6 X 10 and 6.5 X 10" cm molecule" s" . These workers have used this reaction as a standard to obtain rates of energy transfer and chemiluminescent reaction with other reagents, BC, by comparing Kr, BC and ArB emission intensities when... 

One of the chemiluminescence curves in ttie case when an oxygen-saturated solution of ethylbenzene was placed in the reaction vessel, is cited in Fig. 2. After introduction of the initiator, the chemiluminescence intensity rose rapidly, after which constant intensity of the luminescence was observed for a long time (curve 1). When the oxygen was almost entirely consumed, the concentration of radical RO began to decrease rapidly, which led to a decrease in the rate of recombination of these radicals and produced a decrease in the chemiluminescence intensity. At the lowest portion of curve 1, recombination of radicals R was observed, related to the smaller luminescence yield. 

Light induced PP chemiluminescence arises due to the termination of PP peroxy macroradicals formed under the action of light and oxygen [22], It is known that, at low concentrations of radicals, this reaction is first order in the concentration of radicals (linear termination of oxidation chains), resulting in the chemiluminescence intensity I from PP at room temperature being proportional to the product of the rate constant of peroxy macroradicals decay and their concentration [22] ... 

Figure 8.5 shows typical kinetic curves in the coordinates of Eq. (9) for chemiliuninescence decay upon a short irradiation of PP. The behavior of the initial portions of the curves is determined by the conditions of photoinitiation, being dependent on the initial nonequilibrium distribution of the radicals over their reactivity and on the characteristics of the relevant relaxation processes [23]. The progress of the relaxation processes leads to the establishment of the exponential decay of the chemiluminescence intensity with a fixed rate constant k, which can be determined from the slope of the as5miptote of the kinetic curve in the ln l/lj-t coordinates. 

The mechanism of oxidation in the liquid phase is more difficult to study. In spite of numerous works published in this domain, it is not yet possible to define exactly the basic processes or their relative importance in oxidation reactions of liquid organic substances. However, the chemiluminescence method offers some promising applications in this field [2]. It has been used to show the participation of R- and ROj radicals in these oxidation reactions [3]. This method measures the intensity of chemiluminescence produced by the combination of ROi radicals. Absolute rates of different oxidizing reactions, efficiencies of initiators, and rate constants of radical reactions involving some antioxidants have been measured with this experimental technique [4]. A stepwise mechanism based on results from many works [1,5,6] can be suggested for the oxidation of hydrocarbons in the liquid phase. The same mechanism can be applied to thermal oxidation and to light-induced oxidation of polymers [7,8] the main steps are as follows ... 

The physical significance of 6a is that under defined experimental conditions it is the constant of proportionality between ICl, the observed intensity of chemiluminescence, and the rate of consumption of the initial luminophore (reactant L) i.e.,... 

Sharma et al. measured the rate of the chemiluminescent step, reaction (2), in a flow system. For this measurement they produced their SO by the reaction of O atoms (produced from the electrical discharge of O2 or 02-Ar mixtures) with OCS. They also conducted experiments in which the SO was produced by the reaction of O atoms with H2S. The details of this reaction are discussed below here we note only the conclusions pertinent to the OH-SO reaction. They found that the initial intensity of the afterglow in both systems (OH-OCS and O-I-H2S) increased linearly with time at constant reactant pressures, was proportional to [O] when the other reactant pressure was held constant and was linearly proportional to [H2S] or [OCS] when [O] was held constant. 

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