Crystal structure of diacetylene unveiled by X-ray and neutron diffraction, Raman spectroscopy and periodic DFT.

Combining powder X-ray and neutron diffraction, the first crystallographic characterization of diacetylene (IUPAC designation 1,3-butadiyne) is presented and supplemented by Raman spectroscopy and periodic-DFT calculations. The structure is described in the space group Pnma, with a = 9.348 (2), b = 5.9890 (6), c = 5.6746 (11) Å and V = 317.69 (6) Å3 at 5 K, with four molecules in the unit cell resulting in a density of 1.0466 g cm-3. The diacetylene molecules are arranged in a layered structure dominated by C-H...π interactions, which leads to anisotropic thermal expansion behaviour. No structural phase transitions were observed across the 5-220 K temperature range at ambient pressure. The structural similarity with acetylene identifies it as a potential cocrystal component of particular importance for Titan's surface chemistry in view of the upcoming NASA Dragonfly mission.