Isotopic Enrichment Processes

The exploration of the deuterium fractionation in the interstellar medium presents an important means to examine chemical models on the evolution of cold molecular clouds and of carbon rich circumstellar envelopes. Compared to the interstellar D/H abundances of about 10^-5, deuterium enrichments up to a factor of 10³ can be observed in interstellar molecules. So far, about 32 isotopomers have been detected ranging from mono-, di-, to triply deuterated molecules. These are, for instance, the singly deuterated species D-thioformaldehyde (HDCS), D-ethynyl (CCD), D-water (HDO), D-cyclopropenylidene (c-C₃HD), D-tricarbonhydride (C₃D), D2-ammonia (NH₂D), D-methanol (CH₂DOH), D-formaldehyde (HDCO), D-hydrogencyanide (DCN), D-iso hydrogencyanide (DNC), D-cyanoacetylene (DCCCN), D-cyanodiacetylene (DCCCCCN), D-methylacetylene (CH2DCCH), D-hydrogen sulfide (HDS), D-hydrogen (HD) together with ions H₂D⁺, DCO⁺, and DN₂⁺. D2-formaldehyde (D₂CO), D2-methanol (CHD₂OH), D2-hydrogesulfide (D₂S), D2-ammonia (ND₂H), and D2-HD₂⁺ are the only doubly deuterated species observed so far. The recent detection of the triply deuterated molecules D3-ammonia (ND₃) and D3-methanol (CD₃OH) yielded important clues on the relative importance of gas phase versus grain – surface reactions and their role in the deuterium en­richment of interstellar molecules.

In the gas phase, it is generally accepted that the essential principle leading to a deuterium enrichment involves exothermic ion-neutral reactions under thermodynamical equilibrium which have no entrance barrier. But despite elaborate ion-neutral models, the mechanism leading to a deuterium-enrichment in complex organic, hydrogen-deficient molecules like c/l-C₃H₂ and C₃H is far from being. For instance, the synthesis of c-C₃H₂ and c-C₃DH is suggested to involve an elusive C₃H₂D⁺ intermediate which fragments upon dissociative recombination to c-C₃H₂/C₃HD and C₃D/C₃H. However, the D/H ratio of 0.1 as found toward the molecular cloud TMC-1 could not be reproduced with this ion-neutral scheme; only D/H ratios between 0.004 and 0.02 were obtained in chemical reaction models simulating the deuterium fractionation in TMC-1. An alternative pathway leading to a deuterium enrichment at low temperatures could implicate rapid and barrierless neutral-neutral reactions. A recent theoretical study of the neutral-neutral reaction of carbon atoms in their C(³P_j) ground state reacting with D1-acetylene (DCCH) suggest that the formation of the linear isotopomer, l-C₃H, plus atomic deuterium is endothermic by 5.8 kJmol^-1; on the other hand, the reaction to yield l-C₃D plus atomic hydrogen was found to be exothermic by 0.3 kJmol^-1. Therefore, this process can support a deuterium enrichment in the l-C₃D isomer at low temperatures as prevailing in molecular clouds such as TMC-1.

Despite these progresses, the deuterium enrichment in more complex organic radicals like D-1,3-butadiynyl (CCCCD) as observed by Turner has not been fully understood yet. This is pretty surprising, since the corresponding 1,3-butadiynyl radical (CCCCH) in its ²Σ⁺ electronic ground state has received considerable attention due to its potential significance as a precursor to polycyclic aromatic hydrocarbons (PAHs) and possibly to fullerenes in the interstellar me­dium. However, recent crossed molecular beams experiments of dicarbon molecules (C₂) with acetylene (HCCH) suggest that the 1,3-butadiynyl radical, CCCCH (X²Σ⁺), can be formed in an exoergic and barrier-less reaction under single collision conditions via a dicarbon versus atomic hydrogen exchange pathway following indirect scattering dynamics through cyclic and acyclic C₄H₂ reaction intermediates.