Possible Gas-Phase Synthesis of Neutral Malononitrile (C₃H₂N₂) and Isocyanoacetonitrile (NCCH₂NC) under the Upper Atmospheric Conditions of Titan

Malononitrile (C3H2N2) is a highly reactive compound that plays an important role in the synthesis of heterocyclic compounds and may have contributed to the formation of nucleobases and nucleosides in a pre-RNA world. Despite its significance in prebiotic chemistry and potential implications in astrobiology, its possible synthesis has not yet been documented. To date, the search for malononitrile in outer space has been unsuccessful, but the atmospheric conditions and abundance of carbonitrile compounds detected in Titan's atmosphere suggest that this celestial body could be a conducive environment for its formation and existence. The purpose of this publication is to examine the feasibility of the gas-phase synthesis of malononitrile under the physical conditions present in the upper atmosphere of Titan, using chemical species detected or predicted in that atmosphere as reactants. We propose 62 initial gas-phase reaction mechanisms for synthesizing C3H2N2. Molecular energies are computed at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory, and thermodynamic function values are calculated using the Maxwell-Boltzmann quantum theory. A benchmarking study of 14 DFT methods is conducted to identify the best geometry response that matched the reference (CCSD(T)-F12) under both room and Titan's upper atmosphere pressure and temperature profiles. To identify the most kinetically favorable reaction mechanisms for malononitrile, VTST theory and RRKM simulations are performed across a range of temperatures (80-200 K) and pressures (9.87 x 10(-10) to 2.0 atm). The most favorable reaction for the production of malononitrile is R8 ((HCCN)-H-1 + (HNC)-H-1) with a rate coefficient of 1.08 x 10(-11) cm(3) molecule(-1) s(-1), at 145 K and 9.87 x 10(-10) atm and a modified Arrhenius expression of k = 1.65 x 10(-11) +/- 0.005(T/300)(0.005) exp((-64.46/T)) cm(3) molecule(-1) s(-1). Chemical models predict that singlet (HCCN)-H-1 should exist in the upper atmosphere of Titan, while HNC has already been detected there. Some of the proposed reaction mechanisms lead to isocyanoacetonitrile (NCCH2NC) as a product. The most probable formation mechanism for it is R2 (C2N2 + (CH2)-C-1) with a rate coefficient of 1.20 x 10(-10) cm(3) molecule(-1) s(-1) for the described pressure and temperature profiles with a modified Arrhenius expression of k = 4.48 x 10(-15) +/- 0.017(T/300)(1.862) exp(-(-42.44/T)) cm(3) molecule(-1) s(-1). The Cassini mission has directly identified both reactants, C2H2 and HCN, providing clear evidence of their presence. Our results further suggest that isocyanoacetonitrile may be roughly 1 order of magnitude more abundant than malononitrile assuming sufficient abundances of (CH2)-C-1.