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Black plate (236,1) further to protect the organic component from degradation by ?OH radicals, and the irradiation of an aqueous suspension of 2,4,6-triphenylpyrylium@Y results in the formation of H2O2 to an accumulated concentration of 10 3 – 10 2 M.24 The phenomenon is interesting and makes one wonder why the cation is protected to such a degree in the encapsulated state. A simple matrix isolation effect might be invoked, but the water molecules must of course enter the zeolite cages in order for the electron transfer to take place; thus the resulting ?OH radicals are in potential direct contact with the organic component. Results from molecular modelling calculations suggest that the 2,4,6-triphenylpyrylium cation is held tightly by the zeolite cavity, so that the necessary increase in dimension incurred by the overall addition of a hydroxyl anion is prevented [viz., transfer of one electron to the 2,4,6-triphenylpyrylium cation followed by combination with ?OH, is equivalent to an overall addition of OH]24. This is a useful facet, since in homogeneous solutions the degradation of the photocatalyst tends to thwart attempts to degrade pesticides by means of it. A restriction in the overall 2,4,6-triphenylpyrylium cation configuration by the zeolite explains too the unexpected observation of an ambient temperature phosphorescence29. Triphenylmethyl (trityl) carbenium cations (Ar3Cþ) are arche- typal examples of stable carbocations. This is in part because they can delocalise the positive charge over the entire molecular p-electron framework. This tends to diminish their reactivity compared to more localised carbenium cations (e.g. CH3þ), in addition to rendering the species intrinsically more stable and so more readily formed. The presence of trityl cations may be inferred from the colours imparted to various media that contain them, which range from yellow for the unsubstituted Ph3Cþ cation to orange, according to the nature of substituents that are present. The particular colour follows an additive shift in maximum absorption wavelength which can be predicted using the Woodward rules: interestingly, the well-known pigments malachite green and rose aniline are based around trityl cations30. 3. Diffusion of molecules in zeolites The diffusion of molecules within zeolites (Figure 9) is a key feature in the essential processes of adsorption and catalysis by these materials1 – 11. For this reason, there is great concern to determine fundamental details of diffusion processes in zeolites, most often using selected probe-molecules. As is clear in the 236 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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