Structural and physicochemical features, as well as abilityto tautomerisation of acridin-9-amines substituted at the exocyclic nitrogen atom

The results of MP2 geometry optimisation for 9-(methoxyamino)acridine (1), 9-hydrazinoacridine (2), N-(2-chloroethyl)acridin-9-amine (3) and N-(5-methylpyridin-2-yl)acridin-9-amine (4) were compared with X-ray structure investigations done on 1. The structural properties of the amino tautomer of 3 and the imino tautomer of 1 extracted from X-ray data were well predicted by the computational method applied. Dipole moments, electrostatic features of the substituents at the exocyclic N atom (mean Mulliken charges) and equilibrium constants for the amino→imino tautomerisation were also predicted reasonably well by this method. This information was used to analyse the relations between the ability to tautomerise and the properties of substituents or features of the medium. It was confirmed that electron-attracting and weakly electron-donating substituents at the exocyclic N atom cause imino tautomers to be more stable, while the presence of electron-donating ones means that amino forms prevail. Equilibrium constants account well for the tautomeric equilibria in which 4 participates, and are reflected in the NMR investigations that we carried out earlier. Each tautomeric form of a given acridin-9-amine exhibits a unique distribution of electrostatic potential around its molecules, which is an important feature worth taking into account in investigations of interactions of these compounds with other molecules and their recognition by other molecules.