The
charge density distribution in a novel cocrystal (1) complex
of 1,3-dimethylxanthine (theophylline) and propanedioic acid (malonic
acid) has been determined. The molecules crystallize in the triclinic,
centrosymmetric space group P1̅, with four
independent molecules (Z = 4) in the asymmetric unit
(two molecules each of theophylline and malonic acid). Theophylline
has a notably high hygroscopic nature, and numerous cocrystals have
shown a significant improvement in stability to humidity. A charge
density study of the novel polymorph has identified interesting theoretical
results correlating the stability enhancement of theophylline via cocrystallization. Topological analysis of the electron
density highlighted key differences (up to 17.8) in Laplacian (∇2ρ) between the experimental (EXP) and single-point (SP)
models, mainly around intermolecular-bonded carbonyls. Further investigation via molecular electrostatic potential maps reaffirmed that
the charge redistribution enhanced intramolecular hydrogen bonding,
predominantly for N(2′) and N(2) (61.2 and 61.8 kJ mol–1, respectively). An overall weaker lattice energy
of the triclinic form (−126.1 kJ mol–1) compared
to that of the monoclinic form (−133.8 kJ mol–1) suggests a lower energy threshold to overcome to initiate dissociation.
Future work via physical testing of the novel cocrystal
in both dissolution and solubility will further solidify the correlation
between theoretical and experimental results.