The Beginning of HCN Polymerization: Iminoacetonitrile Formation and Its Implications in Astrochemical Environments

Abstract

Hydrogen cyanide (HCN) is known to react with complex organic materials and is a key reagent in the formation of various prebiotic building blocks, including amino acids and nucleobases. Here, we explore the possible first step in several such processes, the dimerization of HCN into iminoacetonitrile. Our study combines steered ab initio molecular dynamics and quantum chemistry to evaluate the kinetics and thermodynamics of base-catalyzed dimerization of HCN in the liquid state. Simulations predict a formation mechanism of iminoacetonitrile that is consistent with experimentally observed time scales for HCN polymerization, suggesting that HCN dimerization may be the rate-determining step in the assembly of more complex reaction products. The predicted kinetics permits for iminoacetonitrile formation in a host of astrochemical environments, including on the early Earth, on periodically heated subsurfaces of comets, and following heating events on colder bodies, such as Saturn's moon Titan.

Figure 1

Selection of compounds proposed to form in HCN reaction mixtures. This work explores how HCN 1 can react to form iminoacetonitrile 2. Iminoacetonitrile is a suspected key intermediate in the formation of more complex molecules, such as the HCN tetramer diaminomaleonitrile 3, biologically relevant molecules such as adenine 4¹, and various polymers (e.g., 5, 6).^, Iminoacetonitrile can also be referred to as C-cyanomethanimine.

Figure 2

Schematic representation of the base-catalyzed iminoacetonitrile formation. Nucleophilic attack by a cyanide anion is followed by proton transfer.^,

Figure 3

Beginning of HCN polymerization. Formation of E-iminoacetonitrile as predicted by a metadynamics simulation of liquid HCN in the presence of a cyanide anion. Panel a. The identified mechanism is concerted and involves carbon–carbon bond formation occurring simultaneously with a proton transfer between two HCN molecules. Distances are provided in Ångström. Panel b. The free-energy landscape of iminoacetonitrile formation in path-collective variable space. I represents the position of the reactant complex, TS represents the position of the transition state, and II represents the product complex. The white regions correspond to areas of the path-collective variable space that were not sufficiently sampled during the subsequent umbrella sampling simulations.

Figure 4

Predicted and experimental time scales of HCN polymerization. Iminoacetonitrile formation (black solid line) estimated using the Eyring equation and an assumed pseudo-first-order reaction rate. Dashed lines are upper and lower bounds resulting from statistical uncertainty in our simulations at 278 K. Assuming pseudo-zero-order rate kinetics would result in a time scale of the same order of magnitude in pure HCN but would be less accurate for aqueous solutions of HCN. Reported experimental time scales are shown for neat HCN polymerization (squares) and in aqueous solution (triangles) and are indicated as ^aLevy et al.,^bSanchez et al.,^cMamajanov and Herzfeld,^dHe et al., and ^eMas et al. Colored vertical lines indicate the temperature in our simulations (orange), representative measures of cometary surfaces in the inner solar system (blue and red), and the eutectic freezing temperature of HCN and water mixtures (green).