Effect of Temperature on the Formation of Radicals by Hydrogen-Atom Bombardment

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Abstract
Thymine and dihydrothymine have been exposed at different temperatures to hydrogen atoms and the number of radicals induced has been measured by ESR spectroscopy. The results demonstrate that the sample temperature during exposure strongly influences the radical formation. In the present experiments the 5‐thymyl radical is formed in both substances. In thymine a hydrogen atom is added to the ring whereas in dihydrothymine the radical is formed by a corresponding abstraction reaction. Both reactions seem to require temperatures above approximately 130°K. The yield of radicals was found to increase exponentially with the sample temperature during exposure. The apparent activation energy for the addition reaction was found to be approximately 1.2 kcal/mole whereas the abstraction reaction exhibited an activation energy of 2.5 kcal/mole.