Formation of 2-imino-malononitrile and diaminomaleonitrile in nitrile rich environments: A quantum chemical study

Nitriles and nitrile chemistry are found in a variety of astronomical environments and have been utilized in many areas of astronomy and planetary science. Cyanogen (NCCN) has been observed in Titan’s atmosphere though it is not detected in comets or interstellar space. Nitriles formed in the atmosphere of Titan are expected to precipitate down onto the satellite’s surface where cosmic radiations can convert them into other molecules. In view of the significance of the HCN tetramers diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN) in reactions leading to the formation of adenine, the possibility of their formation from CN and H radicals, which are known to be abundant in the nitrile rich astronomical environments, has been explored on the basis of radical–radical and radical–molecule interaction schemes using quantum chemical methods. Global and local descriptors such as chemical potential, molecular hardness and softness, electrophilicity and condensed Fukui functions were used to understand the mechanistic aspects of chemical reaction and region-selectivity of the CN and H radical additions. The presence and nature of weak bonds in the reaction complexes and transition states formed during the reaction process have been ascertained using Bader’s quantum theory of atoms in molecules (QTAIM). The reaction leading to the formation of DAFN is found to be exothermic with a very large net energy gain but involves a potential barrier of about 4.75 kcal/mol. The present reaction scheme shall provide a simpler route to the formation of 4-amino-1H-imidazole-5-carbonitrile (AICN) and adenine. This reaction involves the formation of a neutral intermediate 2-imino-malononitrile for which no structural or spectroscopic information is available. A full vibrational analysis has, therefore, been attempted for 2-imino-malononitrile in the harmonic and anharmonic approximations and spectroscopic parameters such as rotational constants, rotation–vibration coupling constants, and centrifugal distortion constants are being reported.