Ferrocene Encapsulated in Zeolite

There is much interest in the reorientation dynamics of guest molecules in zeolite hosts. This is mainly because the mobility of the guests is of importance to the catalytic properties of these materials. Kaiser and Gubbens [30] at TU-Delft have recently applied xSR to the investigation of ferrocene encapsulated in 

The HOMO-LUMO energy gap of a material is one of the properties that will influence its chemical reactivity. Data from NMR and QENS measurements of the ring rotation dynamics of ferrocene will reflect only effects of any interaction of the ferrocene molecule with the zeolite cage up to the HOMO level of the ferrocene molecule. This is because the LUMO is empty of electrons and any interaction of this level with the zeolite cage orbitals, to a first approximation, will have no effect on the QENS measured dynamic parameters, unless electron density flows into this orbital from the zeolite host. Such a transfer of a small amount of electron density will explain the slightly higher activation energy measured by QENS for ferrocene when encapsulated in zeolite. However, the jlSR measurements are made of a molecule of ferrocene with an additional electron introduced into the LUMO. Thus any interaction of the LUMO with the 

The examples where jlSR provides Arrhenius parameters similar in magnitude to those measured by NMR and QENS are summarized in Table 7.3. The xSR measured dynamic process in all these cases is likely to be the same as that measured by the other two techniques, namely the metallocene (benzene or cyclopentadien) ring rotation. The detailed investigation of ferrocene illustrates how this can be rationalized on the basis of the occupancy of a nonbonding orbital, with respect to the dynamic process, by the unpaired electron of the radical species. These were named as the electron playing a passive role. 

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