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The avoided-level-crossing relaxation of a polarized muon spin-label was used to investigate the reorientational dynamics of muonated cyclohexadienyl radicals in NaZSM-5 with SiO 2 /Al 2 O 3 ratios of 670 and 900 and loadings of one and two benzene molecules per unit cell. It was found that on a critical time scale of about 50 ns the radical performs fast uniaxial rotation about the axis perpendicular to the molecular plane. Superimposed on this there is a two-site jump motion between two orientations that differ by an angle of 110°and 1.0 kJ mol -1 in energy. This causes increasing averaging of the orientations above 50 K. It is particularly extensive but still not isotropic in a temperature range between 200 and 450 K. Below 50 K, the uniaxial rotation slows down below the critical time scale of the experiment, and below 20 K any motion is frozen. It is suggested that such a high reorientational mobility is possible only for the radicals occupying sites at the channel intersections. Above ca. 450 K, the reversible appearance of new broad spectral features suggests that new sites with much lower mobility become thermally accessible. These may be located inside the channels.

A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt 13 H m , possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H 2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m ) 30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.

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