Electronic and steric effect manifestations in the structure of 9-Azidoacridine

Abstract

The crystal structure of 9-azidoacridine (9AA) was determined by X-ray structure analysis (the compound crystallizes in the rhombic system). The crystallographically independent fragment of the structure of 9AA was found to contain two molecules. Both molecules were nonplanar, and the azido group was displaced out of the acridine nucleus plane by 34.6° (molecule A) and 28.6° (molecule B). The barriers to azido group rotations about the C-N bond were calculated by the semiempirical PM3 and nonempirical DFT B3LYP quantum-chemical methods. According to the B3LYP/6-31G* calculations, the structures with the azido group situated in the acridine nucleus plane and perpendicularly to this plane are 0.21 and 1.66 kcal/mol, respectively, higher in energy than the completely optimized structure, in which the dihedral angle between the azido group and acridine nucleus planes is 32°. The PM3 method overestimates the steric strain energy of 9AA and underestimates the energy of azido group conjugation with the acridine nucleus compared with B3LYP calculations.