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Gas-phase and solid-state chemistry in low-temperature interstellar cloudsand cores leads to a D/H enhancement in interstellar ices, which is eventuallyinherited by comets, meteorites, and even planetary satellites. Hence, the D/ Hratio has been widely used as a tracer for the origins of extraterrestrialchemistry. However, the D/H ratio can also be influenced by cosmic rays, whichare ubiquitous and can penetrate even dense interstellar molecular cores. Theeffects of such high-energy radiation on deuterium fractionation have not beenstudied in a quantitative manner. In this study, we present rate constants forradiation-induced D-to-H exchange for fully deuterated small (1-2 C)hydrocarbons embedded in H2O ice at 20 K and H-to-D exchange for the protiatedforms of these molecules in D2O ice at 20 K. We observed larger rate constantsfor H-to-D exchange in the D2O ice versus D-to-H exchange in H2O ice, which wehave attributed to the greater bond strength of C-D versus C-H. We find thatthe H-to-D exchange rate constants are smaller for protiated methane thanethane, in agreement with bond energies from the literature. We are unable toobtain rate constants for the unsaturated and reactive hydrocarbons ethyleneand acetylene. Interpretation of the rate constants suggest that D/H exchangeproducts are formed in abundance alongside radiolysis products. We discuss howour quantitative and qualitative data can be used to interpret the D/ H ratiosof aliphatic compounds observed throughout space.

Radiation-induced D/H Exchange Rate Constants in Aliphatics Embedded in Water Ice