SCAVENGER ASSISTED TRAPPING OF ATOMIC-HYDROGEN IN SI8O12-CAGES

The trapping of hydrogen atoms generated by gamma-irradiation of diffe rent compounds containing the cube-shaped Si8O12-cage was investigated by means of electron spin resonance (ESR). The trapped hydrogen atoms were found to originate from the organic substituents of the cages. I t was shown that intermolecular processes are involved in trapping. Th is was done by means of a crossover experiment in which d(72)-octakis- (trimethylsiloxy)octasilsesquioxane (i.e., d(72)-Q(8)M(8)) and h(72)-Q (8)M(8) were used. Most surprising, the relative yield of trapped hydr ogen atoms is considerably increased by radical scavenging additives ( e.g., NO, O-2, and I-2) present during gamma-irradiation. In the prese nce of radical scavengers, the dose dependence of [H-tr(.)] in, for ex ample, octapropyloctasilsesquioxane becomes almost linear, whereas in the absence of any scavengers it reaches a much lower and quasistation ary level. Only for octahydridooctasilsesquioxane (HT8) and octamethyl octasilsesquioxane (MeT8) the yields of atomic hydrogen are not notice ably affected by radical scavengers. This is probably because the scav engers cannot enter the crystal lattices of HT8 and MeT8. If radical s cavengers are absent, radicals generated from the substituents can be detected at room temperature for long periods of time. Elemental iodin e facilitates the trapping of hydrogen atoms even in solutions of sils esquioxanes in cyclohexane. Moreover, there are radiation induced proc esses, which remove trapped hydrogen atoms from their traps, so the de tected concentration appears to be a net effect. Deoxygenated solution s of irradiated specimen advantageously allow the observation of a wel l-resolved Si-29-superhyper-finestructure (shfs). The comparison of th e experimental shfs pattern with the theoretically expected one proves convincingly the encapsulation of hydrogen atoms in intact Si8O12-uni ts. The values of the shf-coupling constants depend on the nature of t he substituents attached to silicon and decrease with increasing tempe rature. The thermal decay process of H-. trapped in the solid; state i s not affected by the atmosphere present and follows first-order kinet ics. This corresponds with the uniform trap sites. The activation ener gy for this process is estimated to 109.6 +/- 3.1 kJ/mol (343 less tha n or equal to T less than or equal to 387 K) in the case of H-.:Q(8)M( 8). The satellites of the hydrogen hyperfine transitions were shown to be spin-flip satellites. proposals in order to explain the observed e ffects are made.