Radiation-induced D-to-H Exchange in Ices Containing Ethane or Benzene: Reactions and Rate Constants

The rate constants for radiation-induced deuterium to hydrogen (D-to-H) exchange were recently measured for CD4 in H2O and C2D6 in H2O at 20 K. In the current study, rate constants for radiation-induced D-to-H exchange were obtained for C6D6 in H2O at 20, 50, and 100 K. The rate constants for D-to-H exchange for an organic molecule in water were found to be an order of magnitude greater for benzene than ethane. Additionally, D-to-H exchange for benzene was found to be far more favorable than the production of any other radiolysis product, a result that contrasts with our previous findings for methane and ethane. Finally, rate constants were obtained for C2D6 in H2O at 50 K. For all ice mixtures, increasing the temperature of the experiment yielded a larger rate constant. There is a notable difference in the magnitude of the response of the rate constant for D-to-H exchange as a function of temperature for C2D6 versus C6D6 containing ices. The results suggest that radiation will have an asymmetric effect on D-to-H exchange between different types of organic compounds and water ice matrices. These results suggest that the radiation history of an extraterrestrial sample will influence the D/H ratios of its organic compounds.