Spectroscopic arguments for a new crystal phase of glycolaldehyde

In an attempt to elucidate the nature of glycolaldehyde dimer solid phases further, low‐temperature Raman spectroscopy was employed and phonon spectra were recorded in the temperature in range 10–300 K. It seems probable that the α phase of glycolaldehyde dimer belongs to the P21/n space group with Z = 2, the same crystal class in which 1,4‐dioxane and DL‐glyceraldehyde crystallize. Studying the temperature dependence of the low‐wavenumber Raman bands of glycolaldehyde dimer α phase, one observes an increase in the number of phonons from six to 13 when the sample is cooled below 80 K. We suggest that a phase transformation into a new phase γ takes place, with the number of molecules per unit cell probably equal to Z = 4. Normal coordinate analysis was performed for glycolaldehyde dimer with two equatorial and with two axial hydroxyl groups, in order to predict the number of internal modes falling in the phonon region. If there were any centrosymmetric dimers with two axial hydroxyl groups present, than a band around 150 cm−1 would have been expected in the Raman spectrum, which was not observed. This is by far the strongest argument in favour of the hypothesis that the α phase consists of dimers with two equatorially placed hydroxyl groups.