  Cyanamide (NH2CN) and its isomer, carbodiimide (HNCNH), may form adenine in
the interstellar medium (ISM) via a series of reactions. Therefore, they are
considered key prebiotic molecules in the study of the origin of life. We used
the three-phase NAUTILUS chemical code, which includes the gas, the dust
surface, and the icy mantle, to investigate the formation and destruction of
cyanamide and carbodiimide. We added over 200 new chemical reactions of the two
isomers and related species, and established a relatively complete network. We
applied cold core, hot corino/core and shock models to simulate the different
physical environments, and found that the two isomers are mainly produced by
the free radical reactions on grain surfaces. Our simulated results suggest
that cyanamide and carbodiimide molecules come from surface chemistry at early
evolutionary stages. Then they are released back to the gas phase, either by
thermal process (in hot cores, hot corinos) or shock-induced desorption (in
shock regions).We speculate that it is an inefficient route to form a tautomer
of adenine by starting from molecules cyanoacetylene (C3NH), cyanamide and
carbodiimide in ISM.
