Deuterium enrichment of polycyclic aromatic hydrocarbons by photochemically induced exchange with deuterium-rich cosmic ices

The polycyclic aromatic hydrocarbon (PAH) coronene (C24H12,) frozen in D2O ice in a ratio of less than 1 part in 500 rapidly exchanges its hydrogen at oms with the deuterium in the ice at interstellar temperatures and pressure s when exposed to ultraviolet radiation. Exchange occurs via three differen t chemical processes: D atom addition, D atom exchange at oxidized edge sit es, and D atom exchange at aromatic edge sites. Observed exchange rates for coronene (C24H12)-D2O and d(12)-coronene (C24D12)-H2O isotopic substitutio n experiments show that PAHs in interstellar ices could easily attain the D /H levels observed in meteorites. These results may have important conseque nces for the abundance of deuterium observed in aromatic materials in the i nterstellar medium and in meteorites. These exchange mechanisms produce deu teration in characteristic molecular locations on the PAHs that may disting uish them from previously postulated processes for D enrichment of PAHs.