Irradiated aqueous systems

In the discussion above it has been shown that the lipid can been polymerized through UV irradiation of its aqueous suspension. The polymerization of the system improves the stability of the synthetic liposomes. Since there is an acetal linkage introduced between the polymer chain and the amphiphilic structure, this linkage can be slowly hydrolyzed in aqueous systems to separate the polymer chain from the lipid. 

Three component, aqueous-base developable, positive-tone resists utilizing the chemical amplification principle have also recently been reported (81,82). In these systems, irradiation of a phenolic resin/inhibitor/acid generator resist generates an acid which upon mild heating, catalyzes either depolymerization or deblocking of a... 

These metal and metal oxide catalysts must work as a kind of electron pool which brings about multi-electron process for H2 and 02 generation. Silver colloids were studied as electron pool for H2 formation under y-irradiation in the aqueous system composed of Ag° colloids, acetone, 2-propanol and SDS S9). The colloids (average diameter 140 A) of 2.5 x 10 4 M can store 1 coulomb/1, corresponding to the storage of 450 electrons/particle 60 ... 

Paramagnetic species formed by reactions of the radiation-produced transient species in ice and frozen aqueous systems have been studied by ESR technique. The radiation-produced electrons have been found to react e.g. with acidic solutes to form H-atoms and with group 11(b) metal ions to give the corresponding univalent radical ions, while the holes can react with anions such as S04 2 and H2P04 giving the radical ions S04 and HP04. Evidence that the electron and hole are coupled to each other, and may in fact exist in irradiated pure ice primarily in an (exciton-like) bound state has been discussed. The present work provides evidence for the reactions of the radiation-produced positive holes apart from the reactions of the electrons. 

Figure 1. ESR spectra of y-irradiated frozen aqueous systems at 77°K. (a) H20 (b) 0.5M H2SOa ...

Figure 11. ESR spectra of y-irradiated frozen aqueous systems at 77°K. (a) 0.2M CiOa 2 (6) H2O (c) 0.1M CrO4-2, 2M NaOH after annealing (d) 2M NaOH after annealing (e) 0.1M CtOa 2, 2M HCIOa after annealing (f) 2M HCIOa after annealing.

The photocatalytic activity of Ti02 toward a specific reaction depends on both, its physicochemical properties such as primary particle size, degree of aggregation, surface area, crystalline structures, etc. and external conditions such as irradiation intensity, pH of the aqueous system, the presence/absence of elec-tron/hole scavengers, and the bias potential if applied for Ti02 film photoelectrodes. These factors are often interactively affecting the overall photocatalytic activity (Hoffmann et al. 1995). 

Concomitant strand-break studies on frozen aqueous systems on irradiation suggest that both radical cations and radical anions can lead to strand breaks. They also show that DSBs are remarkably frequent under these conditions. This was interpreted in terms of proximal trapping of positive and negative radical centres, each leading to a strand break (Boon et al., 1984). 

Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e.g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for microwave photochemistry experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f Klan et al., 2001, 1999). 

Again, the challenge involves a suitable detection system. As illustrated in Fig. 1, many radicals produced upon pulse irradiation of an aqueous system have UV/Vis absorption bands that can be monitored. Clearly, it is suitable to directly follow absorbance changes in the hydrated electron and the carbonate radical anion, with absorption maxima of 715 nm and 600 nm respectively, through the use of an UV/vis spectrophotometer with fast response time. The OH radical. 

A very important point occurs in the transmission of acoustic power into a liquid which is termed the cavitation threshold. When very low power ultrasound is passed through a liquid and the power is gradually increased, a point is reached at which the intensity of sonication is sufficient to cause cavitation in the fluid. It is only at powers above the cavitation threshold that the majority of sonochemical effects occur because only then can the great energies associated with cavitational collapse be released into the fluid. In the medical profession, where the use of ultrasonic scanning techniques is widespread, keeping scanning intensities below the cavitation threshold is of vital importance. As soon as the irradiation power used in the medical scan rises above this critical value, cavitation is induced and, as a consequence, unwanted even possibly hazardous chemical reactions may occur in the body. Thus, for both chemical and medical reasons there is a considerable drive towards the determination of the exact point at which cavitation occurs in liquid media, particularly in aqueous systems. Historically, therefore, the determination of the cavitation threshold was one of the major drives in dosimetry. 

The reports in the organic sections of this review are now considered. Irradiation of valerophenone is well known to yield both acetophenone and cyclobutanols by a Norrish Type II process but Zepp et al. report that the latter product (cis trans ratio 2.4 1) is more efficient in aqueous systems than hydrocarbons. Such ketones as 1 readily undergo the Type II process in the solid phase and from a detailed study involving the use of chiral auxiliaries as counter ions of its carboxylate derivative, Leibovitch et al. conclude that the ionic chiral auxiliary approach is a viable general method for asymmetric synthesis. Crystals of the ketone 2 are apparently photostable at room temperature but when finely ground or at elevated temperatures intramolecular hydrogen abstraction and formation of the benzocyclobutene 3 occurs (Ito et al), and the same workers also note that irradiation of S-4 at 4 °C in the solid state and at 34% conversion gives the SS product 5 with a diastereoselectivity of 99%. 

Hydrogen Transfer - Irradiation of valerophenone (29) in aqueous solution has been studied. The reaction follows the same path as that in hydrocarbon solution and yields acetophenone and cyclobutanols. The reaction in water arises from the triplet state. Interestingly, the formation of the cyclobutanols cis. trans ratio is 2.4 1) is more efficient in the aqueous system than in hydrocarbons. Cyclobutanols are also formed on irradiation of the butanoate derivatives (30). Hydrogen abstraction by the triplet excited state carbonyl group occurs from the alkyl groups on C2 of the butanoate chain. 

Irradiation of a methanolic solution of l-methyltricyclo[4.1.0.0 ]heptane (22) in the presence of naphthalene-l-carbonitrile gave 6-methoxy-7-methylbicyclo[3.1.1]heptane (23a, 93Vo, isolated yield 56%). In an aqueous system, the corresponding hydroxy derivative 23b was isolated in 70% yield. The orientation of addition was anti-Markovnikov and the substituents were located in the less hindered positions. The same bond was cleaved when the 2-/er/-butyl derivative 24 was submitted to this reaction. With two methyl substituents, i.e. 26, a mixture of two stereoisomers 27A,B and a dehydrogenated product 28 was obtained whose formation could be explained by the occurence of a tertiary carbon radical. ... 

Unsaturated Steroids.—Irradiation of cholesterol or cholest-4-en-3/ -ol (719) in benzene-ether, or in aqueous-organic solvent mixtures containing xylene as sensitiser, affords the oxetan (722 ca. 20%) in the aqueous system, 5j -cholestane-3, 5-diol (723 54%) is also formed. With deuterium oxide instead of water, cholesterol gave the 6< -monodeuterio-3, 5i9-diol, showing the reaction to... 

Oxidation of the amino acid moieties in irradiated aqueous systems by reaction with OH is well established for fluid systems, but it is not likely to be encountered in frozen systems. Being a strong oxidant, the OH reacts by electron transfer. It also adds readily to double bonds and abstracts H from C—H, N—H, and S—H bonds, but with lower reaction rate constants. A compendium of rate constants for aqueous solution has been published (52) and a few representative values for amino acids are shown in Table I. As discussed by Simic (53), the predominant sites for reaction in amino acids and peptides can be inferred from these values, which indicate that the ring groups are favored, while abstraction from the peptide backbone is less likely. Hydroxylation of the phenylalanine ring also occurs as was found for the prototype reaction with benzene (54). Formation of phenoxyl radical following OH addition to tyrosine should be similar to the mechanism established for phenol (55) in which elimination of water occurs as is shown in reaction 12 ... 

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