Steric hindrance is known to affect the stability, reactivity, and radical trapping ability of stable nitroxide radicals. Therefore, a quantitative evaluation and prediction model of steric hindrance is needed to select and design the optimum nitroxide radicals for specific applications. In this study, a dynamic parameter of steric hindrance (DPSH) is proposed and its characteristics are investigated. Unlike using only the equilibrium structure to evaluate the steric hindrance, DPSH is a dynamic value calculated from the theoretical activation enthalpies for two model reactions of radical addition to olefins. Using DPSH, the steric hindrance was evaluated for a total of 43 alkyl radicals, nitroxide radicals, and radicals derived from phenols, and the results were compared with those of other methods. The DPSH values for radicals can vary when the energy barrier for structural change varies, even if the equilibrium structures of the radicals have the same steric shielding. Finally, for radicals other than nitroxide radicals, the DPSH values were consistent with the predictions from their structures, which suggests that the DPSH has a wide range of applications. We expect DPSH to be used and developed in the analysis of steric factors in various reactions.

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Supported NiO nanocluster was prepared by using Ni colloid. Alkoxide stabilized Ni nanocluster supported on SiO 2 was oxidized by exposing to air at room temperature to produce NiO nanocluster. The cluster size of NiO was estimated by XAFS analysis. Supported NiO nanocluster thus prepared showed the catalytic activity for oxidative coupling of thiophenol, whereas the conventional impregnation NiO catalyst did not. Remarkable NiO cluster size dependency on activity for oxidative coupling of thiophenol was observed.

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