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.