Electron spin relaxation properties of atomic hydrogen encapsulated in octavinyl POSS cages

Abstract

The electron spin relaxation times of encapsulated atomic hydrogen in the vinyl derivative of silsesquioxane (R₈Si₈O₁₂) cages (R = CHCH₂) are studied in detail by pulse electron paramagnetic resonance (EPR) methods in the temperature range between 10 and 300 K. The temperature dependence of the spin–lattice relaxation time, T₁, shows similar behaviour with previously studied derivatives that typically involve Raman and thermally activated processes. The room-temperature phase-memory time T_M = 9 μs is comparable to those reported for different alkyl derivatives and exhibits a characteristic temperature dependence with a considerable reduction below 200 K as a result of dynamic effects like methyl group rotation. However, this reduction is modest for the vinyl derivative since the minimum observed T_M = 5 μs is much longer than the value of 1 μs reported for methyl-containing derivatives like R = C₂H₅, C₃H₇ (n-propyl), or OSi(CH₃)₂H. This discrepancy is attributed to the different rotation dynamics of the vinyl group, as evidenced by the determined activation energy and rotation frequency.