Experimental Characterization of Reaction Intermediates Time-Resolved Methods

Experimental Characterization of Reaction Intermediates Time-Resolved Methods 

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). 

Ultraviolet-visible absorption has traditionally been the basis of detection in flash photolysis experiments. It offers a number of advantages in sensitivity and efficiency and has certainly delivered much vital information about the reactivity of transient species. On the other hand, the ultraviolet-visible absorption bands characteristic of any but the simplest molecules tend to be broad and relatively uninformative as regards identity and structure, and so we may run into problems not only with the overlap of absorptions due to different 

As a start, we return to that remarkable Lewis acid Cr(CO)5. Flash photolysis experiments have been carried out with Cr(CO)6 either in solution or in the gas phase and with either infrared or visible probes (9, 60). For example, photolysis of gaseous Cr(CO)6 with the output of an XeF laser yields a predominant transient that can be tracked by a 

The photochemistry of the binuclear iron carbonyl [( -CsHslFe-(CO)2]a, VIII, provides another outstanding example of the interplay between time-resolved and alternative strategies (76). According to the infrared spectrum of this compound in a suitable matrix at low temperatures (83), ultraviolet photolysis results in the elimination of free CO, but the pronounced cage effect of the medium entirely sup- 

An alternative technique for the rapid generation of transient species is that of pulse radiolysis triggered not by a light flash but by a short pulse (10 s) of high-energy electrons (1-5 MeV) this 

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IV. Experimental Characterization of Reaction Intermediates Time-Resolved Methods... 

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