Topological Analysis of Charge Density Distribution in Concomitant Polymorphs of 3-Acetylcoumarin, A Case of Packing Polymorphism

Detailed investigation of the charge density distribution in concomitant polymorphs of 3-acetylcoumarin in terms of
experimental and theoretical densities shows significant differences in the intermolecular features when analyzed based on the
topological properties via the quantum theory of atoms in molecules. The two forms, triclinic and monoclinic (Form A and Form
B), pack in the crystal lattice via weak C−H···O and C−H···π interactions. Form A results in a head-to-head molecular stack,
while Form B generates a head-to-tail stack. Form A crystallizes in P1̄ (Z‘ = 2) and Form B crystallizes in P21/n (Z‘ = 1). The
electron density maps of the polymorphs demonstrate the differences in the nature of the charge density distribution in general. The
charges derived from experimental and theoretical analysis show significant differences with respect to the polymorphic forms. The
molecular dipole moments differ significantly for the two forms. The lattice energies evaluated at the HF and DFT (B3LYP) methods
with 6-31G** basis set for the two forms clearly suggest that Form A is the thermodynamically stable form as compared to Form
B. Mapping of electrostatic potential over the molecular surface shows dominant variations in the electronegative region, which
bring out the differences between the two forms.