Optical absorption spectroscopy reaction kinetics

Hydroperoxo-ferric intermediate, termed also Compound 0, is the immediate precursor of the main catalytic intermediate Compound I in peroxidase enzymatic cycle. Attempts to study this intermediate directly in reactions of hydrogen peroxide with HRP using fast kinetic methods have been inconclusive, possibly because it is not accumulated in sufficient concentrations.90,91 However, Compound 0 could be prepared and studied by EPR and optical absorption spectroscopy via cryoreduction of... 

ESR (electron spin resonance) and optical absorption spectroscopy at low temperatures were used to analyse the individual reaction steps of the optical and thermal polymerization reactions and their kinetics. The reaction steps are the photoinitiation, the chain propagation and chain termination reactions. 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

Academic kinetic investigations are generally performed in stationary solutions, typically in a cuvette. Continuous reactors are much more common in industrial situations. Using fiber-optic probes, absorption spectroscopy is routinely performed in flow reactors. The flow of reagents into a reactor or of a reaction mixture out of a reactor is also quantitatively modeled by appropriately modifying the set of differential equations. Refer to the engineering literature for details that are beyond the scope of this chapter [40],... 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

Typically, the reactions illustrated in the above scheme are complete in less than 1 hr, often in only 10-15 min., progress being usually monitored by both absorption spectroscopy and other optical methods. In kinetic experiments in the presence of ultrasounds, using visual methods based on color changes, we have found that sonication affords a significant amelioration in the preparation of the catalyst because the rate of formation of the tungsten carbene-species is increased. Thus, formation of the active... 

It has in fact been anticipated for many years that the CT free energy surfaces may deviate from parabolas. A part of this interest is provoked by experimental evidence from kinetics and spectroscopy. Eirst, the dependence of the activation free energy, Ff , for the forward (/ = 1 ) and backward i = 2) reactions on the equilibrium free energy gap AFq (ET energy gap law) is rarely a symmetric parabola as is suggested by the Marcus equation,Eq. [9]. Second, optical spectra are asymmetric in most cases and in some cases do not show the mirror symmetry between absorption and emission.In both types of experiments, however, the observed effect is an ill-defined mixture of the intramolecular vibrational excitations of the solute and thermal fluctuations of the solvent. The band shape analysis of optical lines does not currently allow an unambiguous separation of these two effects, and there is insufficient information about the solvent-induced free energy profiles of ET. 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

Picosecond spectroscopy provides a means of studying ultrafast events which occur in physical, chemical, and biological processes. Several types of laser systems are currently available which possess time resolution ranging from less than one picosecond to several picoseconds. These systems can be used to observe transient states and species involved in a reaction and to measure their formation and decay kinetics by means of picosecond absorption, emission and Raman spectroscopy. Technological advances in lasers and optical detection systems have permitted an increasing number of photochemical reactions to be studied in. greater detail than was previously possible. Several recent reviews (1-4) have been written which describe these picosecond laser systems and several applications of them... 

For fast reactions, the simplest kinetics experiment is to resolve the disappearance of the reactants, for example, by transient emission if the reaction can be photoinitiated and a reactant is luminescent. If there are multiple reaction channels available for reactant decay, the kinetics are described by a mono-exponential decay according to the sum of these rates of which PCET is only one. A more powerful experiment is to observe the disappearance of PCET reactants and growth of PCET products directly. In photoinitiated optical experiments, this means probing by transient absorption (TA) spectroscopy rather than transient emission. If PCET proceeds in a concerted fashion then concomitant mono-exponential disappearance of reactant and growth of product will be observed. If a stepwise mechanism operates, the growth of the products will be delayed (and fit by a bi-exponential function), however, this observation does not reveal the sequence in which the electron and proton were transferred. Moreover, in the limit where one of the steps is significantly faster then the other, the bi-exponential character of the kinetics trace will not be discernible, and the reaction may appear as if it were concerted. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

UV-Vis spectra are generally highly sensitive but less informative, because they typically consist of a few broad absorption peaks. Chemical reaction monitoring using UV-Vis spectroscopy is less common than using other spectroscopic techniques. Two major devices have been developed for supercritical fluids the fiber-optic and the cell device. Hunt et al. [9] reported the development of a fiber-optic-based reactor connected directly to a CCD array UV-Vis spectrometer for in situ determination of reaction rates in SCCO2. The cell can be configured either to study the kinetics of chemical reactions or to determine the rate of dis-... 

---