Electron Density Distribution of Urea in Co-crystals with Rigid and Flexible Dicarboxylic Acids

This is the first study on electron density distribution of urea and dicarboxylic acids co-crystals. The accurate electron density of two co-crystals of urea with oxalic acid and l-malic acid molecules is discussed, on the basis of high-resolution X-ray diffraction data collected at 80(2) K, and compared with density functional theory calculations in a crystal structure as well as in the gas phase. Rigid dicarboxylic acid favors two-dimensional planar hydrogen bond networks, whereas flexible acid promotes layers with helical arrangements of those molecules. The nature of chemical bonding is investigated using topological analysis based on quantum atoms in molecules theory, Hirshfeld analysis, noncovalent interactions approach, distributed atomic polarizabilities and electrostatic potential. In a co-crystal of oxalic acid and urea, uncommon carbonyl···carbonyl interaction is observed, representing an antiparallel motif between oxalic and urea molecules, which was so far identified only between the same molecules