Relation between pi-electron localization/delocalization and H-bond strength in derivatives of omicron-hydroxy-schiff bases

Detailed investigations of electronic effects taking place within the molecular system of o-hydroxy Schiff bases have been performed. The analysis of geometry, local and global aromaticity, selected AIM-based parameters, and finally, pi-electron currents induced in the systems under consideration have been performed on the basis of quantum chemical calculations at the B3LYP/6-311+G** level of theory. The relation between localization/delocalization of pi-electrons within the whole system has been described. It has been shown that the character of the bond which is common to the phenylic ring and the quasi-ring formed as a result of H-bond formation has a crucial impact on the strength of H-bonding. The strongest H-bonds can be observed for the systems in which the sequence of formally single and double bonds within the H-bridged quasi-ring enable a pi-electronic coupling. These observations indicate that pi-electron effects play a fundamental role in the stabilization of the hydrogen bridge within omicron-hydroxy Schiff bases. Analysis of pi-ring currents induced by a magnetic field perpendicular to the molecular plane of selected analyzed systems confirms these conclusions.