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Photochemical changes

The authoritative documents on plutonium 0 >2) do not include photo-chemical reactions of plutonium in aqueous systems. The first papers in Western world literature on studies that were dedicated to aqueous plutonium photochemistry appeared in 1976 (3, 4 ), even though photochemical changes in oxidation states were indicated as early as 1952 (5,, ]) ... [Pg.263]

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. [Pg.265]

Many such reactions may indeed be carried out preparatively under photochemical conditions, though, for reasons that cannot be gone into here (the detailed mechanism of photochemical changes), they are often not concerted but proceed via biradical intermediates. One photochemical (2n + 2n) cycloaddition that does, however, proceed via a concerted process is the example we have already referred to ... [Pg.349]

Photochemical changes in both II and the acetoxy derivative have been monitored in fluid solution and incorporated in a polymer film. Fig. 5 shows the spectral changes accompanying photochemical transformation of the acetoxy derivative. Then changes may be interpreted in terms of scheme 3, which proposes a photochemical 1,3 acyl shift to form "in situ" an ultraviolet stabilizer chromophore which also has a carbonyl functionality. [Pg.36]

We experience heat, visible light, UV and radio waves by the way they interact with our thermometers, our eyes, skin and our radio sets respectively. This is a tremendously important concept. Photons of infrared light are experienced as heat. The photons that cause photochemical changes in the retina at the back of the eye are termed visible . These photochemical reactions in the eye generate electrical signals which the brain encodes to allow the reconstruction of the image in our mind this is why we see a scene only with visible light - indeed this is why we call it visible . [Pg.430]

The photon must have sufficient energy to cause the photochemical changes in a single molecule, ion, atom, etc. [Pg.433]

A shortcoming is the instability against external conditions, in particular temperature. To induce a phase transition with a minimum amount of photochemical change, the molecular aggregate system... [Pg.210]

Although CMC is not a sharp phase transition and the degree of amplification is not phenomenal, a clear non-linear change in surface tension against the amount of photochemical change is observed as shown in Figure 2. [Pg.212]

The use of photoreactive liquid crystal systems in imaging devices is not unprecedented. As early as 1971, Sackmann showed for the first time the photochemical change of pitch in cholesteric liquid crystals (H). Since then several techniques for using liquid... [Pg.217]

The Grotthuss-Draper law states that only light which is absorbed by a chemical entity can bring about photochemical change. [Pg.5]

We saw in the last section that because of the rapid nature of vibrational relaxation and internal conversion between excited states an electronically-excited molecule will usually relax to the lowest vibrational level of the lowest excited singlet state. It is from the Si(v = 0) state that any subsequent photophysical or photochemical changes will generally occur (Kasha s rule). [Pg.53]

Outline the important features of the concerted photoreactions of C=C compounds, including the excited state involved, the stereochemistry and the factors which determine the direction of photochemical change. [Pg.145]

From photolysis of methylene blue by ruby-laser giant pulses, Danzinger et al. found that a 0.5 Joule, 30 nsec laser pulse causes almost total conversion of the original molecules into transients, but that the photochemical change is completely reversible. The lifetimes of three transients have been measured as 2, 30 and 140 jusec resp. at a 5.5 x 10 M dye solution. [Pg.38]

Let us consider molecular switches based on intramolecular electronic transition. Generally, transfer of energy or an electron within a molecule proceeds in femtoseconds. The aim is to produce molecular electronic devices that respond equally rapidly. Molecular switches that employ optically controlled, reversible electron-transfer reactions sometimes bring both speed and photostability advantages over molecular switches which are usually based on photochemical changes in their molecular structure. Important examples are the molecnlar switches depicted in Scheme 8.3 (Debreczeny et al. 1996). [Pg.405]

The so-called first law of photochemistry stating that only the radiation absorbed by a molecular entity or substance is effective in producing a photochemical change. [Pg.325]

Setlow et al.83 have studied the photolysis of poly dl poly dC (polydeoxyinosinic acid polydeoxycytidylic acid, see Glossary) and poly dA dT. The photochemical changes were estimated by following absorbance changes, by chromatographic separation of acid hydrolysates, and by chromatographic separation of products from enzymatic hydrolysates. [Pg.258]

This is illustrated by the fluorescence spectrum of acenaphthene in Figure 8. In the absence of photochemical change = k% = 0) the relative quantum... [Pg.182]

Perhaps the most convincing evidence for internal conversion is provided by the observations of Hammond et al.sl who find that the quenching of molecular fluorescence by conjugated dienes is not accompanied by either an increase in yield of molecular triplet states (kfa = 0), by any detectable photochemical change (kg = 0), or by the appearance of a characteristic exciplex band (k% = 0). Since the observed quenching constants, KQ, vary by several orders of magnitude it may be assumed that reversible photoassociation is operative in these systems (Section III.C) in which case with (cf. Eq. 33)... [Pg.206]

Actinometers are chemical systems or physical devices that determine the number of photons in a beam either integrally or per unit time. In a chemical actinometer, the photochemical change can be directly related to the number of photons absorbed, while the physical device gives a reading correlated to the number of photons detected. ... [Pg.221]

The photochemical change of 2,3,5-triphenyltetrazolium chloride (CXXX) to triphenylformazan (CXXXI) and 2,3-(2,2 -biphenylene) 5-phenyltetrazolium chloride (CXXXII) is an example of a photooxidation-photoreduction process.84 106... [Pg.99]

Benzo-2//-pyrans and their thia analogs, e.g., heterocycles not treated in this chapter, are photochemically labile and form reversible photochromic systems with open-chain and colored photoisomers. This behavior was observed for 2-benzyl-2,4,6-triphenyl-2//-species 467a,b (Y = H)399,409 and naturally occurring 2//-pyran 92a452a but only at about 77K. At room temperature, these as well as other 2//-pyrans are photochemically stable. Only 2,2,4,6-tetramethyl-2//-pyran (176) was photochemically changed to its exocyclic double bond isomer 308a.404 An irreversible photolysis of 2-azido-2//-thiopyran derivative 212b (R = H) has been discussed in reference 254. [Pg.290]

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. [Pg.614]

Some general considerations about Photochemical Changes. The Union of Hydrogen and Chlorine. [Pg.72]


See other pages where Photochemical changes is mentioned: [Pg.195]    [Pg.5]    [Pg.4]    [Pg.474]    [Pg.99]    [Pg.209]    [Pg.210]    [Pg.107]    [Pg.186]    [Pg.140]    [Pg.283]    [Pg.153]    [Pg.308]    [Pg.418]    [Pg.3]    [Pg.329]    [Pg.457]    [Pg.732]    [Pg.1]    [Pg.3]    [Pg.24]    [Pg.30]    [Pg.148]   
See also in sourсe #XX -- [ Pg.35 , Pg.72 ]




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