X-Ray, Quantum Mechanics and Density Functional Methods in the Examination of Structure and Tautomerism of N-Methyl-Substituted Acridin-9-amine Derivatives

Abstract

X-Ray diffraction has shown that N,N-dimethylacridin-9-amine (4) and N ,10-dimethylacridin-9-imine (5) both crystallize in the monoclinic space group P21/c (No. 14) with four molecules in the unit cell. The dimethylamino group in (4) is twisted through an angle of 58·6° relative to a nearly planar acridine moiety. On the other hand, the central ring in (5) is folded along the C(9) · · · N(10) axis through an angle of 26·3° and the exocyclic nitrogen atom with the methyl group attached to it is directed away from the concave side of the acridine nucleus. Theoretical ab initio Hartree–Fock (HF) and semiempirical (MNDO, AM1, PM3) quantum mechanics, as well as density functional (DFT) methods predicted that the N,N-dimethylacridin-9-amine molecule is planar within the acridine moiety and exhibits Cs 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 stable than those arising from the imino form. The negative LUMO and HOMO energies, both predicted at the semiempirical level of theory and at the HF level in the latter case, imply that the relevant states are electronically stable. The comparable thermodynamic stabilities of both types of derivatives, as well as the fact that they can be synthesized, undoubtedly speak in favour of the existence of tautomeric phenomena in acridin-9-amine