Hydrogen abstraction by thermal hydrogen atoms at 10–30 K: Electron spin resonance evidence

The paper describes the direct and conclusive evidence that thermal H atoms abstract a hydrogen atom from the CH bond at cryogenic temperatures (10–30 K). It has been observed by ESR that thermal H atoms are trapped in CH 4 containing 0.5 mol% of C 2 H 6 when X-irradiated at 4.2 K and that upon warming to 10–20 K the trapped H atoms are freed and react selectively with C 2 H 6 forming C 2 H 5 radicals. The difference in the CH bond dissociation energies between CH 4 and C 2 H 6 must be responsible for this selectivity. The reaction of thermal H atoms at cryogenic temperatures is further confirmed by the 4.2 K photolysis of HI(0.15 mol%) in xenon matrices containing C 2 H 6 (0.75%). The H atoms trapped immediately after the photolysis are freed at 20–30 K and react with C 2 H 6 forming C 2 H 5 radicals. The tunnel effect must play an important role in the reactions at cryogenic temperatures.