Encapsulated
atomic hydrogen in silsesquioxane cages is a promising
candidate for applications in emerging technologies like spin-based
quantum computing, magnetic field sensing, and atomic clock devices.
Previous studies on different polyhedral octasilsesquioxanes (POSS)
of the type Si8O12R8 have shown that
key parameters for quantum computing like electron spin relaxation
times T
1 and T
M depend strongly on the type of peripheral organic substituents.
Herein we examine for the first time the effect of deuterium isotopic
substitution on the spin relaxation properties of H@h
72Q8M8, the derivative with R =
OSi(CH3)3, by applying pulsed electron paramagnetic
resonance (EPR) methods on its deuterated analogues H@d
72Q8M8 and D@d
72Q8M8. For the latter species we measure
a phase memory time of 60 μs at 190 K, the largest obtained
so far for this family of molecular spins. We show that substitution
of peripheral hydrogen atoms with deuterium reveals high-temperature
relaxation mechanisms that were previously hidden by proton nuclear
spin diffusion. Unusually short T
M values
observed for all deuterated species even at liquid helium temperatures
are discussed in terms of tunneling reorientation of methyl groups.