Ab initio calculations of 13C NMR chemical shielding in some N4O2, N4S2 and N6 Schiff base ligands containing piperazine moiety

Abstract

KEYWORDSThe calculation of 13 C isotropic shielding constants by means of GIAO and CSGT methods of eight Schiff base ligands containing piperazine moiety at the Hartree-Fock and B3LYP levels of theory are presented. Good linear correlations between the calculated chemical shielding at gas-phase and experimental shift values in CDCl3 solution were obtained. Density functional theory (DFT) calculations at the B3LYP/6-31G(2d,p) level of theory is used to optimize the geometry of ligands. Calculated nuclear magnetic resonance (NMR) chemical shifts 13 C are reported for the some N4O2, N4S2 and N6 Schiff base ligands containing piperazine moiety. In order to establish a convenient and consistent protocol to be employed for confirming the experimental 13 C NMR spectra of Schiff base ligands, different combinations of models and basis sets were considered. The most reliable results were obtained at B3LYP/6-311G++ (d,p) level and CSGT method which can be used to predict 13 C NMR chemical shifts with a very high accuracy for latter compounds. These results show the agreement between theoretical and experimental 13 C NMR chemical shielding of mentioned ligands. DFT GIAO CSGT Ab initio Schiff base Piperazine