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The incorporation of Crlll into the framework of high content silica zeolite is confirmed by electron spin resonance and photoacoustic spectroscopies.

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The first kinetic study is reported for archetypical substitution reactions of PMe3 and 13CO with the well defined intrazeolite system, M~(l * c o)~-N a~~Y , for which excellent isosbestic points and first order behaviour are obtained, the activation parameters indicate a highly ordered 'supramolecular' transition state consisting of activated M o (~* C O)~ and PMe3 or 13CO all anchored to the Na+ ions in the a-cage of the host lattice.

An electron paramagnetic resonance (EPR) study of crystalline chromosilicalite has been presented with the purpose of characterization of t h e distribution of Cr3+ sites in the solid. In addition, a deconvolution process is described whereby the EPR parameters can be analysed. 135.

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Quantitative temperature-dependent kinetic studies are reported of thermal reactions of Mo(CO) 6 that has been newly encapsulated in the R-cages of a Na 56 Y zeolite host. Under static vacuum, Mo(CO) 6 undergoes parallel anchoring and decarbonylation reactions, the latter yielding (OC) 3 Mo-Na 56 Y directly and the former resulting in the development of a characteristic six-band spectrum in the C-O stretching region. Kinetic and spectroscopic evidence was obtained for there being two different forms of (OC) 3 Mo-Na 56 Y. The activation parameters are unusual, with very low enthalpies and extremely negative entropies, viz. ∆H q ) 40 ( 3 kJ mol -1 and ∆S q ) -185 ( 9 J K -1 mol -1 for anchoring and decarbonylation. This contrasts with the typically high enthalpic and positive entropic factors reported for corresponding CO dissociative reactions in xylene (∆H q ) 133 ( 6 kJ mol -1 and ∆S q ) +28 ( 16 J K -1 mol -1 ) or even the quite unusual values ∆H q ) 61 ( 5 kJ mol -1 and ∆S q ) -139 ( 13 J K -1 mol -1 reported previously for 13 CO exchange in Mo(CO) 6 -Na 56 Y. It is concluded that the inner surface of Na 56 Y provides different activating environments for different reactions, behaving as multidentate anionic "zeolate" nucleophiles, with varying numbers of O 2-ions participating in highly ordered transition states. A common intermediate, with three O 2-ions participating in the anchoring and decarbonylation process, is proposed, one with two O 2-ions being proposed for the 13 CO exchange reaction. The importance of temperature-dependent time-resolved infrared spectroscopy in elucidating such intimate mechanistic details is emphasized.

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