Method kinetic spectroscopy

Vary fast reactions, both in gaseous and liquid phases, can be studied by this method. In flash photolysis technique, a light flash of very high intensity and very short duration ( 10 6 sec) is produced in the neighborhood of the reaction vessel. This produces atoms, free radicals and excited species in the reaction system. These species undergo further reactions which can be followed by spectroscopic means. The method is also known as kinetic spectroscopy. The first order rate constant as large as 105 sec-1 and second order rate constants as large as 1011 mol dm sec-1 can be measured by this technique. 

Vibrationally excited nitric oxide has been produced flash photo-lytically at room temperature by Norrish and co-workers.22-24 They monitored the vibrationally excited nitric oxide by the method of kinetic spectroscopy developed in Norrish s laboratory. Information on relaxation processes was obtained as a result. 

To determine the chemical nature, concentration, and kinetics of reactive intermediates, time-resolved techniques are used. To detect short-lived species, an inert matrix at extremely low temperature [7], an extremely high-intensity light source, extremely sensitive detection method, or combination of these methods is used. The method using an intensive light source, called flash photolysis, is a technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce sufficient concentration of a transient species for spectroscopic observation. The method can be applied to follow concentrations of substrates, intermediates, and products as a function of time after the flash, which enables in the elucidation of photochemical reaction mechanisms (kinetic spectroscopy) [8,9],... 

Janata E. (2003) Instrumentation of kinetic spectroscopy-19. A method for the simulation of kinetics of reaction mechanisms. Radiat Phys Chem 66 219-222. 

The flash photolysis of CI2 in the presence of O2 was studied by Porter and Wright . Using the method of kinetic spectroscopy, they observed CIO radicals. Their observations were consistent with the mechanism, which was later corroborated by Burns and Norrish , viz. 

An important series of studies on the pyrolysis and oxidation of both ammonia and hydrazine was published by Husain and Norrish in 1963. These workers studied both systems by the method of flash photolysis and kinetic spectroscopy developed in Norrish s laboratory over a decade earlier. The major observations in the oxidation of NH3 were (1) the NH3-O2 explosion is preceded by an induction period of several milliseconds, at the end of which the spectra of NO, O2, and OH are observed in vibrationally excited states (but with a vibrational temperature equal to the translational temperature) (2) NH and OH spectra are observed before the end of the induction period and (3) the main nitrogenous product is NO, with some N2O. 

In their work on the explosive oxidation of ammonia, which has already been discussed earlier, Husain and Norrish also examined the hydrazine system. Their principal results, obtained by the method of flash photolysis and kinetic spectroscopy, were as follows (a) no induction period was observed (b) NH emission, observable in the photolysis of pure N2H4, was visible at the shortest delay times (c) NO and OH were produced as the NH was decaying d) NO added to the hydrazine-oxygen system did not disappear in the combustion ( ) NO represented only about 5 % of the final products. Since, unlike the oxidation of ammonia, the major nitrogenous product is N2 rather than NO, Husain and Norrish concluded that reaction (15 ) is not a part of the main reaction sequence. They felt that the N-N bond was not split in in the main chain propagation. They proposed a mechanism involving nitrosamine, NH2NO, as a chain carrier viz. 

Moreover, by means of the method of luminescent-kinetic spectroscopy we have shown that Eu(III) ion in excited state forms more stable complex (up to two orders of magnitude) with sulfoxides, sulfones, ketones and amines then that in ground state. Since it is known that excitation of the europium is caused by electron transitions in the inner metal-centered 4f-states, our findings testify that f-electrons participate in the chemical bonding. 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

Coherent anti-Stokes Raman scattering (CARS) provides a promising method for examining vibrationally excited intermediates formed in isomerization reactions of polyatomic molecules. The technique may permit kinetic spectroscopy of single vibrational levels during fast reactions (Luther and Wieters). 

Rate constants for the reactions of OH radicals with n-nitroalkanes, n-alkyl nitrates and n-alkyl nitrites have been determined at 298 K and 1 atmosphere total pressure using both pulse radiolysis combined with kinetic spectroscopy and a conventional relative rate method. In order to provide more mechanistic information for these reactions, rate constants for the reaction of Cl atoms with these compounds were determined using the relative rate method. The data indicate that the reaction of OH radicals with these nitrogen-containing compounds involves both an abstraction and an addition channel. These results are discussed in terms of reactivity trends and compared with the data from the literature. 

Rgure 5 Results of SKS measurements the signal intensities of desorbed species from an Ni(111) surface under irradiation of CD3OH as a function of the temperature. (Reproduced with permission from Gates SM, Russell JN Jr., and Yates JT Jr. (1985) Scanning kinetic spectroscopy (SKS) A new method for investigation of surface reaction processes. Surface Science 159 233-255 Elsevier.)... 

Gates SM, Russell JN Jr., and Yates JT Jr. (1985) Scanning kinetic spectroscopy (SKS) A new method for investigation of surface reaction processes. Surface Science 159 13,3,-155. 

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

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