X-RAY, QUANTUM-MECHANICS AND DENSITY-FUNCTIONAL METHODS IN THE EXAMINATION OF STRUCTURE AND TAUTOMERISM OF N-METHYL-SUBSTITUTED ACRIDIN-9-AMINE DERIVATIVES

X-Ray diffraction has shown that N, N-dimethylacridin-9-amine (4) and N,10-dimethylacridin-9-imine (5) both crystallize in the monoclinic sp ace group P2(1)/c (No. 14) with four molecules in the unit cell. The d imethylamino group in (4) is twisted through an angle of 58.6 degrees relative to a nearly planar acridine moiety. On the other hand, the ce ntral ring in (5) is folded along the C(9)... N(10) axis through an an gle of 26.3 degrees and the exocyclic nitrogen atom with the methyl gr oup attached to it is directed away from the concave side of the acrid ine nucleus. Theoretical ab initio Hartree-Fock (HF) and semiempirical (MNDO, AM1, PM3) quantum mechanics, as well as density functional (DF T) methods predicted that the N,N-dimethylacridin-9-amine molecule is planar within the acridine moiety and exhibits C, symmetry, while the other four derivatives originating from the amino or imino tautomeric forms of acridin-9-amine do not have any symmetry elements. Molecules retaining the amino constitution are thermodynamically somewhat more s table than those arising from the imino form. The negative LUMO and HO MO energies, both predicted at the semiempirical level of theory and a t the HF level in the latter case, imply that the relevant states are electronically stable. The comparable thermodynamic stabilities of bot h types of derivatives, as well as the fact that they can be synthesiz ed, undoubtedly speak in favour of the existence of tautomeric phenome na in acridin-9-amine.