Flash photolysis hydration

A surprising observation was made in the first experiments on the flash photolysis of CdS and CdS/ZnS co-colloids Immediately after the flash from, a frequency doubled ruby laser (X = 347.2 nm photon energy, = 3.57 eV) the absorption spectrum of the hydrated electron was recorded. This species disappeared within 5 to 10 microseconds. More recent studies showed that the quantum yield increased... 

Photolysis of 2,1,3-benzothiadiazole-l-oxide produces l,3-dihydro-2,l,3-benzothiadiazole-2,2-dioxide, shown by flash photolysis to be formed via hydration of the 2-oxide intermediate... 

The enol form of mandelic acid (101) has been generated by flash photolysis of phenyldiazoacetic acid in aqueous solution.101 The enol forms by hydration of the intermediate carbene (102). The reaction of chloramine-T (TsNClNa O) with methyl p-tolyl sulfide to give the corresponding sulfimide (103) appears to proceed via a nitrene-transfer mechanism in the presence of copper(I) and a second nitrogen ligand (such as acetonitrile).102... 

The experimental methods used in the investigation of the hydrated electron include competition kinetics and product analysis, as well as pulse-radiolysis and flash-photolysis techniques. All these methods have... 

The formation of the transient species Ag° by reduction of Ag+ from hydrated electrons using a double flash photolysis technique has been observed.199 This species may be photodissociated at 315 nm, probably via charge-transfer to solvent, to produce Ag+ and solvated electrons. 

Photolysis of 2,1,3-benzothiadiazole 1-oxide produces l,3-dihydro-2,l,3-benzothia-diazole 2,2-dioxide, shown by flash photolysis to be formed via hydration of the 2-oxide intermediate <78ACS(B)625). Independent of this process 2-thionitrosobenzene is generated reversibly as a short-lived intermediate, analogous to the thermal and photochemical formation of a 1,2-dinitroso intermediate from benzofuroxans. Preliminary flash photolysis and spectrometric results point to a nitroselenanitroso pathway in the photolysis of 2,1,3-benzoselenadiazole 2-oxide to benzofurazan (76ACS(B)675>. 

Fig. 3 shows the spectra of short lived species produced in the laser flash photolysis of PN+ in water, NaLS, and in silica particles. Previous work enable us to identify the excited triplet and singlet states of the pyrene moiety, together with the pyrene cation and hydrated electron, e g q... 

The guanine moiety has the lowest ionization potential of any of the DNA bases or of the sugar-phosphate backbone. As a result, radiation-produced holes are stabilized as dG for hydrated DNA irradiated at 77 K There is an extensive literature describing the role of dG in the radiation chemistry of DNA as studied by pulse radiolysis, flash photolysis, and product analysis. In order to explicate the oxidative reaction sequence in irradiated DNA and to more firmly identify the relevant radical intermediates, ESR spectroscopy was employed to investigate y-irradiated hydrated DNA (T = 12 2). Some experiments were also performed on hydrated (T = 12 2) DNA in which an electron scavenger [thallium(ni) (TP )] was employed to isolate the oxidative path. Oxygen-17 isotopically enriched water was also used to confirm a proposed water addition step to G and the subsequent transformations that follow These experiments were run in oxygen-free samples under conditions for which indirect effects were unimportant. 

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

A series of p-aryloxy- and p-alkoxyphenylnitrenium ions have been generated in aqueous solutions by photolysis of the parent azides, whereupon the resulting nitrenes are protonated. Hydration of these cations at the para position leads via hemiacetal or halohydrin intermediates to quinone imines, which finally hydrolyse to the ultimate quinone products. In flash-photolysis studies of these reactions it was shown that nitrenium ion hydration occurs on the ps timescale, hemiacetal or halohydrin breakdown on the MS timescale, and the final imine hydrolysis over minutes. 

Upper triplet states of biphenyls in micelles ionize to produce hydrated electrons . Diffuse reflective laser flash photolysis has been used to characterise the triplet states of p-terphenyl generated in powder systems. Other triplet states which have been characterised include those of naphthalene and acenaphthene in the solid state", those involved in photoreactions of tetracene with anthracene and 9-bromoanthrene", phenanthrene and biphenylene, highly excited triplet states... 

Sortino, S., Giuffrida, S., and Scaiano, J.C., 1999b, Phototoxicity of naphazoline evidence that hydrated electrons, nitrogen-centered radicals, and OH radicals trigger DNA damage a combined photocleavage and laser flash photolysis study, Chem. Res. Toxicol. 12, 971-978. 

Whilst thus reaffirming that basic surface hydroxyl groups are essential for all photoprocesses, Munuera et al. [143] express the view that the degree of hydration of the surface determines, through the ease of protonation, whether 02 becomes H02 or, instead, produces O3- surface species on extensively dehydroxylated surfaces [cf. eqn. (29)]. This latter process may also account for the secondary uptake of 1802 shown in Fig. 8(b) (iii) after flash photolysis of 1802 /Ti02 interfaces. 

In recent years this approach has been extensively employed and there is substantial evidence for the production of e aq. When steady state ultraviolet light sources were used, hydrated electrons were detected by competitive scavenging techniques (9, 21, 22, 51, 63, 71). When flash photolysis techniques were employed, e q was identified by its absorption spectrum and its reactivity followed spectrophotometrically (28, 38, 60). 

A new flash photolysis instrument, especially designed to generate hydrated electrons and study their reactions, is described in detail. With a new three-dimensional multiple reflection cell and its capacity to produce up to 10 7M e aq in a single 40-fisec. light pulse, this instrument provides adequate sensitivity for determining e aq rate constants and for use in analytical chemistry. With this instrument less than 10 9M e aq may be detected. 

Continuous irradiation of these frozen solutions gave cation radicals which were identifiable because they were of fairly stable types. Less easily formed and much shorter-lived cation radicals can be obtained by a high-intensity, short-duration flash technique. Here, unfortunately, the term flash photolysis, a general one for the flash technique, is often used. For our purposes the term connotes flash ionization. Simple phenol and arylamine (Land and Porter, 1963) cation radicals have been made in this way and characterized by absorption spectroscopy. The rapidity of the technique allows the use of liquid solvents at room temperature, such as water (the hydrated electron was detected by Joschek and Grossweiner, 1966 Feitelson and Hayon, 1973) and saturated hydrocarbons, as well as glasses at low temperature. Three primary processes may occur in the flash irradiation of arylamines [(47)-(49)], and which of them... 

Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. One of the early discoveries utilizing pulse radiolysis was the spectrophotometric detection of the hydrated electron by Boag and Hart (35,36). Since then thousands of rate constants, absorption spectra, one-electron redox potentials and radical yields have been collected using the pulse radiolysis technique. The Radiation Chemistry Data Center at the University of Notre Dame accumulates this information and posts it (at www.rcdc.nd.edu/) for the scientific community to use. They cover the reactions of the primary radicals of water and many organic radicals and inorganic intermediates. 

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