Density functional theory study on gas‐phase microscopic basicity of some ketenimine derivatives and their isomers/tautomers

Abstract

Microscopic basicity of some ketenimine derivatives 1-12 and their isomers/tautomers, bearing an unsaturated ring, has been studied in the present work. Isomerization process for some ketenimine derivatives and their protonated forms was also investigated by density functional theory (DFT) calculations. The protonation study in gas phase was performed by the DFT-B3LYP/6-311+G(d,p) computational method. The results show that all the proposed ketenimines (less stable than their isomers/tautomers) have proton affinity (PA) values that are more than at least 20 kJ mol À1 greater than the PA of 1,8-bis-(dimethylamino)naphthalene. The primary strategy in this study for designing new exceptionally rare ketenimine forms possessing strong basicity is to link structurally an unsaturated ring to the molecular framework and to build conjugated systems, for which the positive charge of the conjugate acids is delocalized through resonance. PA values are enhanced by placing electron-donating substituents on the unsaturated rings, stabilizing the conjugate acid. Two indices of aromaticity, the nucleus-independent chemical shift and the harmonic oscillator model of aromaticity associated with the unsaturated ring, were calculated and compared for neutral and protonated forms. Natural bond orbital (NBO)-Wiberg bond orders, highest occupied molecular orbital (HOMO), and molecular electrostatic potential investigations were also applied to explain the π-type interaction of the unsaturated ring with the ketenimine unit.