Tautomerism of 9-acridinamines substituted at the exocyclic nitrogen atom in view of computational predictions and experimental findings

Abstract

Abstract Ab initio and density functional methods were used to study the tautomerisation ability of 9-acridinamine and 14 of its derivatives substituted at the exocyclic nitrogen atom and to predict their structures as well as their thermodynamic and physicochemical characteristics. The structures predicted compare quite well with those available by X-ray techniques. Electronegative substituents at the exocyclic nitrogen atom favor the formation of imino tautomers. Hence, the logarithm of the equilibrium constant of the amino → imino tautomerisation increases when the resultant negative charge of the substituent at the exocyclic nitrogen atom does so. The same tendency is noted when the medium (CH3CN, H2O) is included in the calculations. Compounds containing CH3, SiH3, NH2, PH2, OH and SH substituents can exist in a previously unconsidered imino form which we have named the “imino nitrogen atom lone pair coordinating form”. This form seems to be the most stable one when SH is the substituent. The electrostatic potential around the tautomers exhibits a dipole- or quadrupole-type distribution. Analysis of this feature reveals seven patterns of electrostatic potential distribution, which take all the possible tautomers of each compound investigated into account. The importance of the results accumulated in the design of 9-acridinamines with cognitively and practically interesting properties is indicated.