Experimental Evidence Time-Resolved Studies

Investigation of the PFR of 2-naphthyl acetate in poly(methyl methacrylate) by means of time-resolved fluorescence has shown that the photoproducts act as long-range quenchers for the starting ester. This confirms singlet excited state involvement and suggests a nonrandom distribution of the generated chromophores [27]. 

A further step in the study of cyclohexadienones as PFR reaction intermediates has consisted in the secondary photolysis of these relatively long-lived species by using the two-laser-two-color technique, which gives rise to formation of a dienic ketene, absorbing at k =330nm (Fig. 31.1) [31]. 

FIGURE 31.1 Two-laser-two-color photolysis of a phenyl ester (Abs at 330 nm vs. time). The jump at 5 ps of the kinetic trace corresponds to the first laser shot (266 nm) and the jump at 15 ps to the second one (308nm), producing the cyclohexadienone ring opening. Reproduced from Jimenez et al. [31] with permission from the Royal Society of Chemistry. 

A full picture of the PFR of phenyl acetate has recently been obtained by UV-vis and IR transient spectroscopy in the (sub)picosecond range (Fig. 31.2). The combination of the two techniques provides accurate data of the involved ultrafast processes. The singlet lifetime is 28 ps, both from the decay of its Sj- S absorption, monitored at 524nm, and its vibration at 1807cm h The phenoxyl 

FIGURE 31.2 (a) Transient electronic absorption spectrum (Sj- -S ) of the phenyl acetate singlet excited state 

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Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

The effect of N-acetyl substitution in methionine on the nature of transients formed after one-electron oxidation was studied as a function of pH and NAM concentration. The observed absorption bands with X = 290 nm, 360 nm, and 490 nm were respectively assigned to a-(alkylthio)alkyl, hydroxysulfuranyl and dimeric radical cations with intermolecular three-electron bond between sulfur atoms. N-acetylmethionine amide (NAMA) (Chart 7) represents a simple chemical model for the methionine residue incorporated in a peptide. Pulse radiolysis studies coupled to time-resolved UV-Vis spectroscopy and conductivity detection of N-acetyl methionine amide delivered the first experimental evidence that a sulfur radical cation can associate with the oxygen of an amide function vide infra). ... 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

TR EPR is a powerful experimental technique in studying transient paramagnetic species on the nanosecond and microsecond time scales. The time-resolved EPR spectra of transient free radicals and radical-pairs are mostly influenced by spin polarization effects. These CIDEP effects provide unique experimental evidence of radical precursors and dynamic processes in primary radical pairs. The application of TR EPR to various photochemical and biochemical problems... 

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