An exploration of the three phenylacetylene polymorphs and discovery of a benzene-phenylacetylene co-crystal by vibrational spectroscopy and powder diffraction.

Phenylacetylene is an organic species thought to be present in the atmosphere of Titan that has a complex crystallisation behaviour under cryogenic conditions, possessing three known polymorphs formed under varying pressure conditions. To better understand the relationship between temperature and the stability of the three phenylacetylene polymorphs and allow the co-crystallisation of phenylacetylene with benzene to be explored, density functional theory calculations, infrared and Raman spectroscopy, and X-ray diffraction were performed. Supported by molecular symmetry analysis, it was found that under Titan conditions all three polymorphs may occur and that a co-crystal between benzene and phenylacetylene may form by freezing of a 1 : 1 liquid mixture of the coformers. Structural elucidation was consistent with a monoclinic Z = 2,2 unit cell for the co-crystal, which presents as a new cryomineral relevant to Titan.