Spatial packing of diols and esters with imidazoquinazoline ring—quantum-mechanical modeling

This work explains the spatial arrangement of two derivatives of 1-phenyl-2H,6H-imidazo[1,5-c]quinazoline-3,5-dione (PIQ), i.e., diester 2,6-bis(ethoxycarbonylmethyl)-1-phenylimidazo[1,5-c]quinazoline-3,5-dione (BEPIQ) and 2,6-bis(2-hydroxyethyl)-1-phenylimidazo[1,5-c]quinazoline-3,5-dione (BHEPIQ). Studies of a single crystal by X-ray crystallography showed a different way of arranging the diol and diester molecules in the crystals. Quantum-mechanical modeling showed the overlapping of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of BEPIQ enantiomeric molecules in relation to each other in the “head to tail” position. The formation of charge-transfer complexes stabilizes the system and is responsible for the packing density of the BEPIQ molecules in the crystal. The BEPIQ enantiomeric pairs are positioned at an angle of 67° to each other forming the herringbone in the crystal lattice. In addition, the BEPIQ crystal has the ability to form inclusion compounds. Based on the calculations, it was found that the overlapping of molecular orbitals of BHEPIQ molecules is not possible. It results from a different distribution of electron density in imidazoquinazoline ring as compared to the molecule of the BEPIQ. The formation of intermolecular hydrogen bonds is responsible for the spatial arrangement of the BHEPIQ molecules. The BHEPIQ molecules are arranged in a “head to head” position, and in the crystal, they form stackings.