Photolysis experiments

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

Direct proof of an oxaziridine intermediate was achieved in photolysis experiments in an organic glass at 77 K (80JA5643). Oxaziridine (75), formed by photolysis of A/-oxide (74) and evidenced by UV spectroscopy under the above conditions, decomposed at higher temperature to form the imino ether (76) by N—O bond cleavage and C -> O migration of an aryl group. 

Two kinds of flash photolysis experiments on the interfacial reactions of electrons and holes have been reported. In the first case, the electrons and holes themselves were detected by their optical absorptions, and in the second case the products of their reactions were traced. 

The photolysis experiments have also received theoretical attention, with electronic structure methods used to calculate the nature of the excited states (44), as well as the potential energy curves for loss of CO (45). Theoretical models for the excited-state dynamics leading to dissociation have also been proposed (46). 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

The H20 exchange mechanism was studied by Hunt et al. (32) who reported that exchange between aqueous solvent and Fein(TPPS)(H20)2 occurs with a first-order rate constant (kex = 1.4xl07s-1 in water at 298 K) far exceeding the k0 s values determined at any [NO]. If the steady state approximation was applied with regard to the intermediate Fem(Por)(H20), the kohs for the exponential relaxation of the non-equilibrium mixture generated by the flash photolysis experiment would be,... 

Quantitative Photolysis. Photolysis experiments were performed by exposing samples to a 450 Watt medium pressure mercury lamp. A shutter was placed between the samples and the lamp so that the exposure time could be accurately controlled. Unless otherwise stated, the samples were placed 4 inches from the lens. Filters (254 nm, 280 nm and 366 nm) when used, were placed immediately in front of the shutter. Sample holders were available for 1" x 1" quartz plates, NaCl windows and quartz UV cuvettes, and samples of each type were utilized. 

In flash photolysis experiments at pressure <0.1 torr reactions (1) and (2) presumably still occur, but the subsequent reactions involve vibrationally excited ethyl radicals91, viz. 

Future work will be concerned with quantitative measurement of hydrogen abstraction rates of labile hydrogens in the carbamate moieties of several aromatic diisocyanate based polyurethanes. It is expected that experimental conditions will alter significantly the hydrogen abstraction rate. Emphasis will also be placed on measurement of transient intermediates in polyurethane films. Finally, extensive laser flash photolysis experiments will be conducted on polyurethanes based on both 2,A-toluenediisocyanate and 2,6-toluenediisocyanate. Preliminary data suggest that the placement of the methyl substituent can alter the nature of the transient intermediates formed. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

By using pulsed laser sources and fast measuring devices, direct observation of redox products in flash photolysis experiments provides evidence regarding oxidative and reductive quenching mechanisms in Ru(bpy)3+. 

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. 

The time course of the charge-separated intermediate I can be measured in a flash photolysis experiment that monitors the (I — A) transient absorbance difference at a ground state/triplet state isosbestic point (e.g., 432 nm for Mg, and 435 nm for Zn). We have observed this intermediate for the [M, Fe] hybrids with M = Mg, Zn representative kinetic progress curves are shown in Fig. 3 [7a]. In a kinetic scheme that includes Eqs. (1) and (2) as the only electron-transfer processes, when the I A step is slow (kb < kp) the intermediate builds up (exponentially) during the lifetime of A and exponentially disappears with rate constant kb (Fig. 4A). This behavior is not observed for the hybrids, where the I - A process is more rapid than A - I, with kb > kp. In this case, I appears exponentially at early times with rate-constant kb and is expected to disappear completely in synchrony with A in an exponential fall with rate-constant kp (Fig. 4B). 

The following data were obtained for the decay of methyl radicals in the presence of argon diluent in a flash photolysis experiment. 

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