In my master's thesis, I studied polymorphism with cooling and/or deuterization of acetylenedicarboxylic acid dihydrate (ADAD) by X-ray diffraction, differential thermal calorimetry (DSC) and IR spectroscopy. ADAD, along with the better known oxalic acid dihydrate, has already been described in the literature studying the effect of temperature and deuterization on hydrogen bonds. In the case of ADAD, at least one phase transition was observed at decreasing of temperature, but the crystal structure of low-temperature modifications could not be determined with the equipment of the time. The positions of nonhydrogen atoms in the structure were determined at room conditions. By X-ray diffraction and DSC analysis, I found that in the temperature range between 293 and 150 K, in the case of both protic and deuterated ADAD, there are two polymorphic crystalline forms α and β. In the case of cooling of protic ADAD α transforms to β at about 4,9 °C. The phase transition is enantiotropic and show hysteresis. Both polymorphs have a monoclinic unit cell and space group no. 14. The differences in the dimensions of their unit cell parameters and of their powder diffractogram confirm that α and β are polymorphic modifications. I performed X-ray structural analysis at 293, 263, and 150 K and labelled the appropriate ADAD structures for the protic form with α, β1, and β2, and for deuterated ADAD with αD, β1D, and β2D. In all three structures α, β1 and β2, there are two planar, centrosymmetric acid molecules and four water molecules in the unit cell. Each water molecule is a donor of one medium and one weak intermolecular hydrogen bond through both hydrogen atoms. Each water molecule is also an acceptor of a strong hydrogen bond donated by the OH group from the acid carboxylic group. Initially the structures of the two polymorphs seemed quite different, but after unit cell transformation and appropriate selection of the asymmetric unit in β, the arrangement of molecules in both polymorphs turned out to be similar. During the phase transition, there is only a change in the mutual orientation of the molecules, while the hydrogen bonds are not broken. At cooling these bonds become somewhat stronger. The same conclusions can be drawn by comparing the three structures of deuterated ADAD. During deuteration the volume of the unit cell slightly increases, due to the slightly larger volume of deuterium relative to the protium. The bond distances between nonhydrogen atoms are practically the same. The usual geometric isotopic effect can be observed, i.e. a small increase in the contact distances between the oxygen from the hydrogen bonds, but no isotopic polymorphism occurs as with oxalic acid dihydrate. The calculated isostructurality indices also confirm the isostructurality of the deuterated forms with the corresponding protic. The IR spectra of protic and deuterated ADAD recorded at room and low temperatures are in the agreement with the results of X-ray structure analysis.
|
