Chemiluminescence reaction parameters

To understand how these parameters affected the efficiency of the chemiluminescent reaction, we examined the mechanism originally proposed by Rauhut (26). As shown in Scheme 2, hydrogen peroxide reacts with an oxalate ester, such as 2,4,6-trichlorophenyl oxalate (TCPO), in a two-step process to form a reactive intermediate for which Rauhut suggested structure 1, the 1,2-dioxetanedione. The dioxetanedione then interacts with an acceptor (ACC) to produce two molecules of COj and the excited state of the acceptor. The last stage of the sequence is fluorescence emission from the acceptor. 

CL emissions can be characterized by four parameters, including color, intensity, rate of production, and decay of intensity. The properties of several organic, chemiluminescent reactions known to produce emissions of light are shown in Table 1. 

Herschbach [58] noted a striking similarity between the recoil energy distribution of Cl atoms in the H + CI2 reaction and that observed in the photodissociation of CI2. This suggests that the electron attachment to the molecule is essentially a vertical process, hence he proposed the DIP extension to the model, which makes the AB repulsion after the electron jump analogous to that encountered in photodissociation experiments. This provided the necessary empirical basis for estimating the parameter of the repulsive interaction. All the mathematical expressions relevant to the model were given by Truhlar and Dixon [62]. Zare and co-workers extended the model to chemiluminescent reactions and a full account of the new model is given in Ref. [81]. It was used to predict successfully the product state distribution in the reaction Ca( So) -I- F2 —> CaF(B ) + F. 

The DIPR model is often used to help in understanding the stereodynamics of direct reactions [82-85]. The important parameter of the model is thus the electron-transfer probability as a function of the molecular orientation. The same parameter, which actually defines the best geometry of the system in the electron transfer step, also plays a role in determining the product alignment in chemiluminescent reactions [86, 87]. A new model has been introduced recently, the anisotropic impulsive model [88]. It is conceptually close to the DIPR model, and also helps to determine the preferred angle of approach between the reactants. 

In the worked examples above, there was a tacit assumption that 1 Einstein (1 mole of photons) is produced by 1 mole of chemiluminescent reactant. However, a reference to Fig. 11 shows that this cannot be the case. Only a fraction of the energy supplied to ground-state reactant molecules will finally be converted into photon emission. A parameter of major importance, then, in the field of chemiluminescence is the quantum yield of the process. Stated simply, the quantum yield, Q relates the photon output to some reference input such as the number of ground-state molecules in the chemiluminescent reaction i.e., its absolute value will be in Einsteins per mole of reactant. Clearly, Q must reflect the variety of means whereby molecules are able to dissipate the excess energy absorbed in the chemical reaction (Fig. 11). It is generally accepted that Q is the product of three separate components, viz.. 

The dimensions (particularly the diameter) of the flow tube are restricted to those given above by the requirement for plug flow, when kinetic measurements are required. The range of total pressures over which reactions may be studied is then similarly restricted (about 0-5 to 10 torr). For investigations of the kinetics of formation of excited states in chemiluminescent reactions, where time discrimination is not needed, the range of experimental parameters may be considerably extended. For example, the formation of electronically excited NO in the reaction O + NO -F M - NOj -F M has been studied over the pressure range 10 /imHg to 0-25 torr total pressure. ... 

R. Bezman and L. R. Faulkner 189> developed methods for defining a concise set of parameters which quantitatively describe the efficiencies of chemiluminescent electron-transfer reactions (see Section VIII. A.) by means of analysis of chemiluminescence decay curves. 

Flow rate The limitations associated with the volume of flow cell can be overcome by accurately controlling the flow rate of each stream entering into the manifold. This experimental parameter controls the residence time of the chemiluminescent solution within the cell and can be easily optimized by the operator. How rates are directly proportional to the rate of the CL reaction. As the rate of the reaction increases, the flow rate should be increased but, at the same time, consumption of reagents increases. The flow rate also affects the shape and the height of the peak as well as the measurement rate (number of sample or standard solutions injected per hour). 

An indirect method has been used to determine relative rate constants for the excitation step in peroxyoxalate CL from the imidazole (IM-H)-catalyzed reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide in the presence of various ACTs . In this case, the HEI is formed in slow reaction steps and its interaction with the ACT is not observed kinetically. However, application of the steady-state approximation to the reduced kinetic scheme for this transformation (Scheme 6) leads to a linear relationship of l/direct measure of the rate constant of the excitation step. 

Part of the radiation from the reaction Zone of this flame is nonthermal, being chemiluminescent in origin. In determination of populations of species such as C2 and CH in low-pressure diffusion or premixed flames at 1-15 torr, these authors developed a simple model of a flame which reflects the characteristic requirements for start of laser action. They show that by choice of the proper experimental parameters in the model, laser action should be attainable... 

A slightly different application is where species produced electrochem-ically lead to photon emission in the visible spectrum, via the formation of organic radicals by homogeneous reaction from electrochemically generated precursors. The electrode controls the quantity of precursor, enabling quantitative parameters of the homogeneous reaction to be elucidated. This is known as electrogenerated chemiluminescence or electrochemiluminescence (ECL). 

The rates of thermolysis (37) of the peroxyesters in argon-purged benzene can be followed conveniently by the direct, indirect, or activated chemiluminescence. In all of the cases reported peroxyesters in benzene solution show clean first-order reaction for low initial peroxide concentrations (10-5-10-3 M). The activation parameters for the peroxyester thermolyses reveal some important details of the reaction mechanism. The activation enthalpy obtained for peroxyester [28] is quite similar to that reported by Hiatt... 

In the presence of oxygen, the chemiluminescence intensity (/CL) is significantly enhanced with respect to the emission produced under nitrogen. As the samples are highly oxidized in a diffusion-controlled reaction simultaneous to the emission, reaction (b) in Scheme 3.1 is very fast and the relative concentration of [POO ] will be larger in proportion to that of [P ]. The rate of oxidation (R,) in Equation 3.2 increases under these conditions, the bimolecular termination of peroxy radical, reactions (f) and (g) in Scheme 3.1, is, therefore, predominant. All these parameters can be used to evaluate the degradation in different materials and the effectiveness of antioxidants in the polymer stability. 

The electrogenerated chemiluminescence (ECl) of five l-amino-3-anthryl-9-propane derivatives has been studied in tetrahydrofuran. Emission from intramolecular exciplexes in ECl spectra and weak emission from the locally excited anthracene moiety were observed. The influence of triplet state interaction in ECl emission is discussed. The chemiluminescent decomposition of three a-peroxy-lactones gives CO2 and the corresponding ketone in high yield. The chemiluminescent species produced has been investigated in some detail by measurements of lifetime, energy-transfer activation parameters, and photochemical reactions. 

The reaction of oxygen atoms with carbon monoxide is an important reaction in many combustion systems. Although there is an extensive literature on this reaction Q) there is disagreement and uncertainty on the molecularity of the reaction, on the kinetic parameters and on the mechanism of the chemiluminescence. We have investigated this reaction using 0-atoms from the thermal decomposition of ozone. This has advantages compared to systems where... 

---