Flash photolysis time-resolved Raman

The phenol ArOH is always a side product, resulting from some ArO that leaks from the solvent cage and abstracts a hydrogen atom from a neighboring molecule. When the reaction was performed on phenyl acetate in the gas phase, where there are no solvent molecules to form a cage (but in the presence of isobutane as a source of abstractable hydrogens), phenol was the chief product and virtually no o- or p-hydroxyacetophenone was found. Other evidence for the mechanism is that CIDNP has been observed during the course of the reaction and that the ArO radical has been detected by flash photolysis and by nanosecond time-resolved Raman spectroscopy. ... 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

One advantage of Raman spectroscopy is that it is relatively easy to perform time-resolved spectra on a sub-millisecond time scale. Therefore it is possible to obtain spectra of a reaction intermediate that decays rapidly. For example, coupling resonance Raman to flash photolysis resulted in the detection of the much-hypothesised ferryl intermediate in cytochrome c oxidase [207]. 

The radical anions of tram- and c/s-stilbene can be distinguished by ESR [503]. A radical ion pair has been observed by this method using trial-kylamines in acetonitrile [497], Electron back transfer (i.e., a reaction of the radical anion of frans-stilbene with the radical cation of the donor) opens a new pathway for intersystem crossing to the tram triplet state. Time-resolved resonance Raman spectroscopy of photoinduced electron transfer from amines to frans-stilbene has been reported [497,504], In the photooxidation of frans-stilbene the radical cation has been observed by flash photolysis using cyanoanthracenes [505-507], The radical cations of cis-and frans-stilbene were also produced by electron transfer from biphenyl to excited 9,10-dicyanoanthracene and subsequent electron transfer from stilbene to the radical cation of biphenyl [508]. External magnetic field effects... 

Tinkler JH, Tavender SM, Parker AW McGarvey DJ, Mukoy L and Truscott TG (1996) Investigation of carotenoid radical cations and triplet states by laser flash photolysis and time-resolved resonance Raman spectroscopy Observation of... 

Biological Aspects.—The lowest excited triplet states of all-rran.s- -carotene produced by pulse radiolysis has been studied by time-resolved resonance Raman spectroscopy.Six transient Raman bands at 965, 1009, 1125, 1188, 1236, and 1496cm were observed and assigned to the triplet state of ) -carotene. The authors conclude that the molecule may be substantially twisted, presumably at the 15,15 band in the triplet state. Further work has also been carried out by the same workers on the triplet state of all-rran -retinal. The results indicate increased 7r-electron delocalization in the triplet state and propose that the relaxed excited triplet-state exists in either sAX-trans or 9-cis conformation. Das and Becker" have also employed pulse radiolysis and laser flash photolysis to study several photophysical properties of the triplet states of the series of polyenals (29)—(33) related to retinal (31) as homologues (Table 35). Results are presented... 

Surface Raman techniques have been used in applications such as in situ ink analysis (cfr also Chps. 1.2.3.1.1-2). Nanosecond laser flash photolysis and time-resolved resonance Raman spectroscopy have been used to study reactions between the AOs a-tocopherol and ascorbate and the triplet excited states of duroquinone (DQ) and ubiquinone (UQ). 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

Bisby, R. and Parker, A. W. (1995) Reactions of excited triplet duroquinone with alpha-tocopherol and ascorbate a nanosecond laser flash photolysis and time-resolved resonance Raman investigation. J. Am. Chem. Soc., 117, 5664-5664. 

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