Carbonyl, quenching effect

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

Only one thiophene has been reported to undergo (2 + 2)-cyclo-addition with carbonyl compounds, viz., 2,5-dimethylthiophene.210 In this reaction, too, only one type of isomer (180) was obtained, in 50-60% yield. N-Benzoyl-l//-pyrrole reacted with two molecules of ketone to give cycloadduct 181.211 Similarly, JV-acyl-1//-indoles reacted with certain ketones to produce adducts 182.212 The lack of reactivity of other heterocycles, such as 1//-pyrroles, oxazoles, and isoxazoles, has been attributed to a quenching effect on the excited ketone of the non-bonded electrons on the heteroatom.212... 

Utilization of resonance effects can facilitate unenhanced Raman measurement of surfaces and make the technique more versatile. For instance, a fluorescein derivative and another dye were used as resonantly Raman scattering labels for hydroxyl and carbonyl groups on glassy carbon surfaces. The labels were covalently bonded to the surface, their fluorescence was quenched by the carbon surface, and their resonance Raman spectra could be observed at surface coverages of approximately 1%. These labels enabled assess to changes in surface coverage by C-OH and C=0 with acidic or alkaline pretreatment [4.293]. 

It was predicted, a priori, that isocomene could be formed simply by treating 4 with methyllithium, followed by exposure of the resultant tertiary carbinol to acid.2b However, many attempts to effect the addition of a variety of nucleophilic methyl derivatives to ketone 4 were unsuccessful. It was revealed by deuterium quenching experiments that ketone 4 undergoes ready enolization in the presence of nucleophilic methyl derivatives. Despite its hindered nature, however, the ketone carbonyl in 4 reacts with methylenetri-... 

Sc(OTf)3 is stable in water, and effectively activates carbonyl and related compounds as a Lewis acid in water. This is remarkable, because most Lewis acids react immediately with water rather than the substrates, and are decomposed or deactivated. It has already been found that lanthanide trifiates Ln(OTf)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium trifiate Y(OTf)3 are stable in water, and can act as Lewis-acid catalysts in aqueous media.46-48 They are used catalytically in many reactions and can often be recovered and reused, because they are stable under the usual water-quenching conditions. 

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

Irradiation of olefins and dienes in the presence of oxygen and various sensitizers produces hydroperoxides and cyclic peroxides. Effective sensitizers include not only high-energy carbonyl triplets, but also low-energy organic dyes such as methylene blue, rose bengal, chlorophyl, and riboflavin. The same species that quench the phosphorescence of these complex molecules also quench their photosensitizing ability. 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

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