Raman Spectroscopic Characterization of the Benzene:Acetonitrile (3:1) Cocrystal Relevant to Titan, Saturn’s Moon

Abstract

Titan, Saturn’s largest moon, hosts active organic chemistry and surface environments where small molecules are expected to crystallize as molecular minerals. Benzene and acetonitrile, both predicted to be present on Titan, were shown to form a thermodynamically stable benzene:acetonitrile (3:1) cocrystal that may occur as an evaporite phase along the margins of Titan’s hydrocarbon lakes. Here, we report a detailed morphological and Raman spectroscopic study of this cocrystal, supported by periodic density functional theory (DFT) calculations. Optical microscopy reveals that the cocrystal exhibits a distinct sheaf-like spherulitic microstructure, unlike the equiaxed grains of solid benzene and the dendritic growth of solid acetonitrile. The distinct crystal habit results from the crystal structure of the cocrystal and the peritectic mode of crystal growth. Raman spectra show characteristic frequency shifts relative to the pure components, together with additional bands arising from the reduced symmetry of the cocrystal lattice. The calculations reproduce the observed vibrational features and support their assignment, showing that, compared to the pure components, the benzene vibrational features are relatively weakly perturbed, whereas acetonitrile is more strongly influenced by the polar channel environment and linear ordering within the structure of the cocrystal. These results establish a robust spectroscopic fingerprint for the cocrystal and provide new constraints on the solid-state chemistry and possible evaporite mineralogy of Titan. Such laboratory characterizations are timely in light of the recently concluded Cassini–Huygens mission, as well as the forthcoming Dragonfly mission to Titan.