On The Nature of Interstellar Organic Chemistry

A theory for the origin of all organic molecules observed in regions of massive and low-mass star formation, as well as in dark molecular clouds is described. On dust grains, single atom addition reactions and stability of the intermediate radicals, mechanisms similar to those believed to form the organic component of the Murchison meteorite, lead to a very limited number of mantle compositions depending upon the degree of hydrogenation. The key step in the theory is the formation of the formyl radical by H atom addition (by quantum tunnelling) to CO. Subsequent H atom additions lead to formaldehyde and methanol, as previously suggested; C, N, and O atoms can also undergo additions to HCO. For increasing hydrogenation, the mantle types include one in which there is little contribution from formyl-initiated chemistry; one in which an acetylenic chain develops through C atom additions; and others where the acetylenic chain is increasingly hydrogenated to form aldehydes and alcohols. Following evaporation of grain mantles, such as occurs in protostellar «hot cores», these molecules can form new organics, for example, by alkyl cation transfer from alcohols. In dark clouds, different mantles lead to different gas phase organics. This scenario accounts naturally for the formation of many interstellar organics for which none presently exists, predicts observable correlations between specific interstellar molecules, indicates the presence of many new organic molecules and why several others are not observed.