ESR and X-ray diffraction studies of diacyl peroxides in urea and aluminosilicate hosts
Tóm tắt
Electron spin resonance (ESR) spectroscopy was used to study the photodecomposition of long-chain diacyl peroxides trapped in channels within zeolites (silicalite and ferrierite) and urea clathrates. ESR spectra of radical pairs in single crystals of the urea clathrates of diundecanoyl peroxide (UP), lauroyl peroxide (LP) andbis(6-bromohexanoyl) peroxide (6-BrHP) show that the alkyl radicals respond to the CO2 stress field by recoiling along the channel. In each clathrate, the inter-radical distance for the most relaxed pair is approx. 9.5 Å, suggesting nearly complete relaxation of stress from the CO2s. The rotational mobility and exceptional kinetic stability of the radicals is attributed to relaxation of stress and the lack of a convenient escape route for the CO2s. X-ray diffraction indicates one-dimensional ordering of guests in 6-BrHP/urea and 3-dimensional ordering of guests in UP/urea. Solid state NMR experiments on LP/urea suggest high guest mobility under ambient conditions. When UP and 6-BrHP were intercalated into silicalite, photolysis yielded isolated radicals, but no radical pairs, even as low as 20 K.
Tài liệu tham khảo
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With UP/urea and 6-BrHP/urea radical pair decay occurs with step-wise kinetics (see reference 8). The half-lives reported here represent the slower rates, which occur at low radical pair concentrations.
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At the time of submission, the first two control experiments had not been done for 6-BrHP/ferrierite.