Motions of molecules in condensed systems. XIII. Polarized infrared spectrum of single crystals of diacetylene

The polarized infrared spectra of single crystals of diacetylene, as well as those of the liquid and the vapor, are presented and analyzed. Modes forbidden activity in the vapor are observed in the spectra of the condensed phases, as are combinations of molecular fundamentals with lattice modes. The crystal and liquid spectra exhibit frequency shifts and band broadenings that are characteristic of hydrogen bonding. The crystal system is found to be either orthorhombic or monoclinic with two or more molecules in the primitive unit cell. The molecules are essentially coplanar and this plane is the only symmetry element that may be present at the site. An orthorhombic structure, of Space Group P22₁2₁ with four molecules on sites of no symmetry, is proposed for the crystal.