“…The availability of absorption spectral data for radical cations and carbocations of the olefin in question frequently helps to distinguish between the possibilities. For example, literature data on the absorption spectra for radical cations of vinyl anisole and indene were used to establish that these species were not responsible for the color produced when the precursor olefins were included in zeolites. , On the other hand, the origin of the bright color that appears when diphenylethylene is included within zeolites or adsorbed on silica−alumina surfaces has been the subject of controversy for over three decades. − The green color that develops upon inclusion of diphenylethylene within CaY and HY is the same as that produced by treatment of diphenylethylene with strong Brønsted or Lewis acids . It has been established that the green color is a combination of yellow (λ max 432 nm) and blue (λ max 615 nm) colors resulting from two independent species.…”
Section: Introductionmentioning
confidence: 99%
“…While the 430 nm species has been conclusively identified as the methyl diphenyl carbonium ion by comparison with solution spectra, the origin of the latter is still debated. A number of suggestions have been made to explain the color produced upon addition of diphenylethylene to zeolites and silica/alumina (Scheme ). − We address in this article the question of the nature of the species responsible for the blue color (615 nm absorption) produced when diphenylethylene is included within activated divalent cation exchanged zeolites. 1 …”
Inclusion of 1,1-diarylethylenes within activated CaY results in the formation of colored samples that are stable for prolonged periods. For example, diphenylethylene generates a green color within CaY. Two reactive intermediates have been shown to be responsible for this color. The species with the short wavelength (λ max 410 nm) absorption has been established to be the diphenylmethyl cation. The identity of the species with the long wavelength ((λ max 620 nm) absorption has been debated for three decades. We establish that the diarylethylene radical cation is not responsible for the long wavelength absorption. Laser flash photolysis studies show that a number of diarylethylene radical cations have a characteristic absorption in the 400 nm region in both solution and in NaY zeolite. The absence of this absorption for diarylethylene-CaY samples unequivocally eliminates the possibility that these radical cations are responsible for color formation. This is substantiated by the fact that a number of 2-methyl-substituted 1,1-diarylethylenes generate only the short wavelength cation absorption upon inclusion in CaY; radical cation formation would be expected to be equally facile for the methyl-substituted derivatives. Our results suggest that an allylic dimer cation is the most likely source of the long wavelength ((λ max 620 nm) absorption within CaY.
“…The availability of absorption spectral data for radical cations and carbocations of the olefin in question frequently helps to distinguish between the possibilities. For example, literature data on the absorption spectra for radical cations of vinyl anisole and indene were used to establish that these species were not responsible for the color produced when the precursor olefins were included in zeolites. , On the other hand, the origin of the bright color that appears when diphenylethylene is included within zeolites or adsorbed on silica−alumina surfaces has been the subject of controversy for over three decades. − The green color that develops upon inclusion of diphenylethylene within CaY and HY is the same as that produced by treatment of diphenylethylene with strong Brønsted or Lewis acids . It has been established that the green color is a combination of yellow (λ max 432 nm) and blue (λ max 615 nm) colors resulting from two independent species.…”
Section: Introductionmentioning
confidence: 99%
“…While the 430 nm species has been conclusively identified as the methyl diphenyl carbonium ion by comparison with solution spectra, the origin of the latter is still debated. A number of suggestions have been made to explain the color produced upon addition of diphenylethylene to zeolites and silica/alumina (Scheme ). − We address in this article the question of the nature of the species responsible for the blue color (615 nm absorption) produced when diphenylethylene is included within activated divalent cation exchanged zeolites. 1 …”
Inclusion of 1,1-diarylethylenes within activated CaY results in the formation of colored samples that are stable for prolonged periods. For example, diphenylethylene generates a green color within CaY. Two reactive intermediates have been shown to be responsible for this color. The species with the short wavelength (λ max 410 nm) absorption has been established to be the diphenylmethyl cation. The identity of the species with the long wavelength ((λ max 620 nm) absorption has been debated for three decades. We establish that the diarylethylene radical cation is not responsible for the long wavelength absorption. Laser flash photolysis studies show that a number of diarylethylene radical cations have a characteristic absorption in the 400 nm region in both solution and in NaY zeolite. The absence of this absorption for diarylethylene-CaY samples unequivocally eliminates the possibility that these radical cations are responsible for color formation. This is substantiated by the fact that a number of 2-methyl-substituted 1,1-diarylethylenes generate only the short wavelength cation absorption upon inclusion in CaY; radical cation formation would be expected to be equally facile for the methyl-substituted derivatives. Our results suggest that an allylic dimer cation is the most likely source of the long wavelength ((λ max 620 nm) absorption within CaY.
“…However, in this contribution we contend that strained sixmembered surface siloxanes can also react by forming surface radicals and serve as synthetically useful radical initiators. Our hypothesis that relatively stable surface radicals can be generated is based on various electron paramagnetic resonance (EPR) investigations published over the last four decades involving silica-alumina and probe molecules (Rooney & Pink 1961, 1962Hall 1962).…”
Section: Silsesquioxane Analogues Of Strained Surface Siloxanesmentioning
We discuss herein selected examples of metal complexes of polyhedral oligosilsesquioxanes (POSSs) as models of single-site heterogeneous surface catalysts. The utility of these compounds as such models is illustrated when employed as analogues of single-site titanium species supported on a silica surface. Deep insights into structure-functionality relationships can be gained. In particular, it was possible to probe the relationship between accessibility of the reactive centre and turnover frequencies in a manner that is impossible for a purely heterogeneous catalyst. We also report that the partially dehydroxylated SiO 2 surface alone is an effective radical polymerization initiation catalyst. This surface reactivity is modelled by the solution reactions between the olefin substrate and two POSSs, the completely condensed triganol prism, Si 6 Cy 6 O 9 (a6b0, Cy = Cyclohexyl, C 6 H 11 ), and the incompletely condensed partial cube, Si 7 Cy 7 O 9 (OH) 3 (a7b3). The former, with six-membered Si 3 O 3 rings, is a catalyst. The latter, without this feature, is not. Similar reactivity discrimination is observed in the gas phase reactions of these POSSs with the olefin substrate, examined using atmospheric pressure chemical ionization-and collision-induced decomposition spectroscopies. Silsesquioxane a6b0, containing Si 3 O 3 rings, reacts with the olefin, forming grafted olefin monomers and dimers, while this reactivity is not observed with silsesquioxane a7b3.
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