Encapsulation of atomic hydrogen into silsesquioxane cages and ESR of encapsulated hydrogen atoms

Hydrogen atoms are encapsulated in octasilsesquioxane, R(SiO_1.5)₈, by irradiation with γ-rays at room temperature. In deca- and dodecasilsesquioxane, hydrogen atoms are encapsulated by irradiation at 77 K. The thermal decay of the encapsulated hydrogen is well described by a single-exponential function. The excitation energies of the decay are 110–117, 50.4, and 55.6 kJ/mol for the hydrogen atoms in Q₈M₈, Q₁₀M₁₀, and Q₁₂M₁₂, respectively. Theg-values and hyperfine splitting (hfs) constants of the hydrogen atoms in R(SiO_1.5)₈ are dependent on the substituents in the corners of the polyhedra, R. Theg-values are independent of the temperatures, while the hfs constants increase as the temperature decreases. Theg-values of the hydrogen atoms in a large encapsulating cavity decrease with increasing cavity size and approach 2.0023. The temperature dependence of the hfs constants for the hydrogen atoms in Q₁₀M₁₀ or Q₁₂M₁₂ suggests that the hydrogen atoms are localized within the encapsulating cages at low temperatures.