Chemiluminescent Reactions Spectroscopic Studies

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. 

In addition to the use of spatially-resolved concentration measurements for the determination of rate constants for reactions of ground state atoms, the discharge-flow method has been extensively applied to kinetic and spectroscopic studies of chemiluminescent phenomena. In these cases, the flow parameters in the flow tube are of no great importance, as time resolution is not obtained from axial displacements consequently, the total pressures and flow rates, and tube diameters may be varied over wide limits, since it is unnecessary to ensure adherence to the conditions for plug flow. 

This reaction mechanism was first proposed by Halsted and Thrush (30) when studying the kinetics of elementary reactions involving the oxides of sulfur. Visible and UV spectroscopic studies (31) confirmed that the chemiluminescent emission was from SO2. Recently, it has also been confirmed that the sulfur- analyte molecule from the GC effluent is converted to SO in the flame of the SCD (32). Even though SO is a free radical, it can be sufficiently stabilized in a flow system under reduced pressure (33,34) to be sampled and transferred to a vessel to react with introduced O. Based on these operational principles. Burner and Stedman (33) concluded that SO produced in a flame could be easily detected. They modified a redox chemiluminescence detector (36) to produce what was termed a Universal Sulfur Detector (USD). A linear response between 0.4 ppb and l.S ppm (roughly equal to 3 to 13,000 pg of S/sec) was demonstrated with equal response to the five sulfur compounds tested. This detection scheme has been utilized as the basis for the commercially available GC detector. 

These studies have allowed the spectroscopic identification of a number of electronically excited states of the metal oxides, but there appear to have been no analytical applications of the reactions to date. The emitting states, as summarized by Toby [14], are CaO(A n), SrO(ATl), PbO(a32+, b32+), ScO(C2II), YO(C2n), FcO(C ), A10(A2ni B2X+), and BaO(A i)1, D 2+). Nickel carbonyl reacts with ozone to produce chemiluminescence from an excited electronic state of NiO, which is probably produced in the Ni + 03 reaction [42, 43],... 

Dioxetanes have been the sole subject of several specialized reviews in recent years (Bartlett and Landis, 1979 Horn et al., 1978-79 Adam, 1977 T. Wilson, 1976 Turro et al., 1974a Mumford, 1975). These articles cover with depth which is not possible here such topics as (1) preparation, (2) physical and spectroscopic characterization, (3) experimental techniques, especially for the study of chemiluminescence, (4) mechanisms of decomposition and chemiexcitation, (5) ground state transformations, and (6) reactions involving dioxetanes as postulated intermediates. The interested reader is referred to these articles for details on these specialized topics, and for some interesting historical perspectives. 

As these remarks indicate, chemical lasers employ infrared chemiluminescence. As a method for obtaining kinetic information, they have to be looked at in relation to other spectroscopic techniques having the same goal. The study of spontaneous vibrational-rotational emission has been most fruitfully applied to fast reactions in the gas phase. This method has experimental limitations due to the relaxation processes competing with spontaneous emission. A very authentic discussion of this method has been given in a recent review by J. C. Polanyi 3>. As opposed to this steady-state technique, chemical lasers permit observations in the pulsed mode. 

Hundreds of ECL reactions have been reported, and many are spectroscopically simple enough to be understood in these terms. Others offer emission bands due to excimers [excited dimers such as (DMA), where DMA is 9,10-dimethylanthracene], exciplexes [excited-state complexes, such as (TPTA BP ), where TPTA is tri-p-tolylamine and BP is benzophenone], or simply decay products of the radical ions. More complicated mechanisms are obviously needed to describe such situations. Many studies involve radical ions of aromatic compounds, but others have dealt with metal complexes such as Ru(bpy)3 [bpy = 2,2 -bipyridine], superoxide, solvated electrons, and classical chemiluminescent reagents, such as lucigenin (1-5). 

The diffusion cloud (flame) technique developed by Hartel and Polanyi in the 1930s is one of the early methods of studying rapid bimolecular chemical reactions imder pseudo-first-order, steady-state conditions. This method is the source of most measured rates for reactions of alkali metals with halogenated compounds and still serves as a basis for experimental and theoretical studies. In most applications of the technique, sodium metal is heated in an oven, mixed with an inert carrier gas, and allowed to diffuse into a backgroimd of a reactant gas. In very exothermic reactions the sodium flame is chemiluminescent otherwise the cloud is illuminated with a sodium resonance lamp. The reaction rate can be measured either by determining the distance the sodium diffuses until it all reacts or by spectroscopically measuring the total amount of sodium in the cloud. ... 

Experiments carried out in bulk systems in which the energy states of the reaction products are determined by spectroscopic techniques. The reaction itself may occur in a static or flow system, and often the reactant molecules have been put into particular energy states. The term chemiluminescence is applied to such studies, since they are concerned with radiation emitted by the products. 

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