Conformational analysis of malonic acid and its derivatives: ab initio, CNDO/2 and empirical calculations

Lack of the 'long-distance' dynamical co-operative interactions due to low symmetry of hydrogen-bonded malonic acid aggregates in molecular crystals

The paper explains the relationship between the energy of hydrogen bonds and the distance between associated carboxyl groups of malonic acid (MA) molecules by means of infrared spectroscopic studies of crystals of its four isotopic varieties [CH₂(COOH)₂, h₄-MA; CH₂(COOD)₂, d₂^c-MA; CD₂(COOH)₂, d₂^m-MA; CD₂(COOD)₂, d₄-MA]. The effects associated with impact on the isotopic dilution and changes in the temperature of spectrum registration on the fine structures of the ν_O-H and ν_O-D bands were analyzed. MA molecular crystals are characterized by a tendency to spontaneous H/D isotopic exchange both within centrosymmetric hydrogen bond cycles and methylene groups. The mono- and polycrystalline spectra obtained in the infrared range of isotopically neat and isotopically diluted by deuterons do not indicate the occurrence of anomalous temperature evolution in the course of lowering their registration temperature to 77 K. Theoretical calculations did not give clear confirmation of the nature of the phenomena analyzed.

Powder X-ray Diffraction as an Emerging Method to Structurally Characterize Organic Solids

The current level of laboratory instrumentation and computational resources allows X-ray powder diffraction to be implemented into the toolbox of organic chemists, providing a means for rapid (i.e., within a day) structural characterization of organic solids, without the need for single crystals. We illustrate such use of powder diffraction using two case studies of molecular cocrystals of trifluoroacetic acid and malonic acid, involving theobromine, a model active pharmaceutical ingredient. We also report on a previously unobserved conformation of malonic acid in the solid state.