Glycolaldehyde dimer in the stable crystal phase has axial OH groups: Raman, infrared and X-ray data analysis

Although a small molecule and a simple 1,4-dioxane derivative, 2,5-dihydroxy-1,4-dioxane or glycolaldehyde dimer has a crystal structure that has eluded researchers so far, chief reason lying presumably in substance polymorphism. Here we report for the first time on the stable crystal structure of the glycolaldehyde dimer characterized by the X-ray powder diffraction method at room temperature. It crystallizes in the monoclinic system, space group P21/c, with unit cell parameters a = 5.9473(1) Å, b = 8.3997(1) Å, c = 5.61953(8) Å and β = 114.8814(9)°. The glycolaldehyde dimer molecules are the trans-isomers with the electronegative hydroxyl groups in axial positions and molecules are arranged in hydrogen bonded layers parallel to (1 0 0). Each layer is stabilized by intermolecular medium strong O–H⋯O hydrogen bonds. The 1,4-dioxane ring of the molecule adopts a chair conformation. and infrared spectra were re-examined and assigned with the help of ab initio calculation followed by a normal modes analysis modes most sensitive to crystal packing were identified as the C–OH deformations which were assigned to the IR and Raman bands at 1239 and 1237 cm−1, respectively; the two O–C–O deformation modes, both associated with the Raman and IR bands at 561 and 535 cm−1 and, the two OH torsion modes assigned to the IR bands at 630 and 604 cm−1.