Mixed Hydrocarbon and Cyanide Ice Compositions for Titan's Atmospheric Aerosols: A Ternary-Phase Co-crystal Predicted by Density Functional Theory

Abstract

A benzene/acetylene/hydrogen cyanide co-crystal has been predicted using a periodic density functional theory approach based on the empirical structure of the 1:1 benzene and acetylene co-crystal. This example of a stable ternary-phase system - a three-component co-crystal comprising small neutral molecules - finds relevance as a possible Titan aerosol composition formed by the condensation of abundant volatile photoproducts in the lower stratosphere. Calculated thermochemical data confirm the 2C6H6:C2H2:HCN co-crystal as a viable laboratory target, with free and cohesive energies competitive with those of binary-phase ices. Harmonic vibrational frequencies computed for the periodic system indicate that the co-crystal can be identified using low-frequency far-infrared or Raman spectroscopy, where distinctive intermolecular lattice signatures are predicted to lie. The geometry of the individual components within the unit cell appears optimal to promote ring-expansion chemistry upon ultraviolet or fast particle irradiation of the molecular co-crystal surface. Such co-crystal systems are unexplored in laboratory simulations of astrophysical ices and may have important implications for the solid-state formation of complex organic molecules in Titan's atmosphere.