Flash spectroscopy laser

Sporadic reports of the 2//-thiopyran transformation are still available. The electrocyclic ring-opening of 2-benzyl-2,4,6-triphenyl-2//-thiopyran (131, X = PhCH2) was detected by laser flash spectroscopy (86MI1). 2-Amino-2//-thiopyran was predicted to be more stable than its valence-bond isomer on the basis of semiempirical MINDO/3 and MNDO calculations (84JPR955). 

Nano- and microsecond laser flash spectroscopy was employed to clarify photophysical effects on benzyl group migration in 2//-thiopyran 60 (R = PhCH2, 86MI1). 

In a recent paper, Soumillion and co-workers [49] were able to identify CIP and SSIPin the P-naphtholate anion/alkali cation/tetrahydrofuran system. They found out that with lithium, a CIP is formed whereas with sodium/crown ether, a SSIP results. Using uncomplexed sodium or potassium counterion, mixtures of CIPs and SSIP s were detected. All their conclusions are based on spectral shifts in the transient absorption and emission spectra which were gained using laser flash spectroscopy. 

Using nanosecond laser flash spectroscopy, it has been shown that excited ketones can abstract hydrogen of hydroperoxide groups and generate free radicals as follows [18] ... 

The five spiroimidazodihydroquinoline compounds shown in Figure 1.3 exhibit reverse photochromism and were studied in detail by both stationary illumination and nanosecond time-resolved laser flash spectroscopy. 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

Fig. 16. Absorbance changes measured at 480 nm and associated with ferredoxin reduction induced at 660-nm by 1-r s dye laser flashes. (A) In the absence and presence of spinach ferredoxin baseline for each trace is shifted for clarity (B) plot of rate constant vs. concentration of ferredoxins from spinach and the green alga Monoraphidium braunni. Figure source Herv s, Navarro and Toltin (1992) A laser flash spectroscopy study of the kinetics of electron transfer from spinach photosystem I to spinach and algal ferredoxins. Photochem Photobiol 56 321.

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

Nanosecond and microsecond laser flash spectroscopy was employed to identify a photophysical path in the photochemistry of 2-methyl- and 2-benzyl-2,4,6-triphenyl-2//-pyrans (86JP187). 

As already shown by many of the above examples, physical methods, especially spectroscopic ones, are having a very strong impact on the development of physical organic chemistry, and it is only for lack of space that such important techniques as laser flash spectroscopy can only be mentioned here in passing. Having established itself as a highly efficient method for the study of carbenes and carbocations [63] laser flash spectroscopy has also been used to generate carbanions like the trityl anion recently, [64] and promises to become a useful method for the determination of carbanion reactivity as well. 

If an intermediate is not sufficiently stable to be isolated, it might nevertheless be formed in sufficient concentration to be detected spectroscopically. Techniques used for this purpose include UV—vis spectroscopy in stopped-flow kinetics experiments for relatively stable intermediates or IR spectroscopy in matrix isolation spectroscopy for more reactive species. For photochemical reactions, we can detect transient spectra of intermediates in the millisecond to microsecond ( conventional" flash spectroscopy) or nanosecond to picosecond or femtosecond (laser flash spectroscopy) time scale. In all cases we must be certain that the spectra observed are indeed indicative of the presence of the proposed intermediate and only the proposed intermediate. Theoretical calculations have been useful in determining the spectroscopic properties of a proposed intermediate, whether it is likely to be sufficiently stable for detection, and the t)q e of experiment most likely to detect it. In addition, kinetic studies may suggest optimum conditions for spectroscopic detection of an intermediate. ... 

P800 quantitation Steady state photobleaching was assayed on a Varian Cary 219 spectrophotometer. Actinic illumination was fiom a 12 V projector lamp filtered through water and a Coming 4-96 filter. The photomultiplier was protected by a Hoya R72 filter. Chemically oxidized-minus-reduced spectra were done essentially as described in (14) with the modification that samples were adjusted to +300 mV by addition of 600 mM ferricyanide to a final concentration of 11 mM (note buffer is 5 mM ascoihate) and reduced to 0 mV by addition of a 10% excess of 300 mM ascorbate. Laser flash spectroscopy was done as described in (6). 

Either direct or triplet-sensitized irradiation of diene ( , )-14 in methanol affords the a s-cyclobutene 15 and the methyl ethers 16 and 17. > Laser flash spectroscopy revealed the formation of a ground-state intermediate having = 360 nm and a lifetime of 0.8 isec at 23°C in methanol. Based on the change of the quantum yields for the formation of ethers 16 and 17 with for a variety of sensitizers, it was concluded that cyclobutene 15 and ether 16 arise from the s-Z-conformation of isomer ( ,Z)-14 and ether 17 from the s- -conformation. In view of large deuterium isotope effects, k /kjy, of 8 1 and 10 2, that were observed for the formation of ethers 16 and 17, respectively, in CH3OD, concerted formation of the CH and CO bonds in the transition states for ether formation was proposed. 

Adam, W., Schneider, K., Stapper, M., and Steenken, S., Multiple-photon chemistry of 9-(phe-noxymethyl)- and 9,10-bis(phenoxymethyl)anthracenes in the laser-jet generation, photochemistry and time-resolved laser-flash spectroscopy of anthracenyl radicals and pulse radiolysis of 9-(bromomethyl)anthracene,/. Am. Chem. Soc, 119, 3280-3287, 1997. 

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