DFT calculations of 15N NMR shielding constants, chemical shifts and complexation shifts in complexes of rhodium(II) tetraformate with some nitrogenous organic ligands

Benchmark calculations of N-15 NMR shielding constants for a set of model complexes of rhodium(II) tetraformate with nine organic ligands using the Density Functional Theory (DM methods have been carried out. The calculations were performed by means of several methods: the non-relativistic, relativistic scalar ZORA, and spin-orbit ZORA approaches at the CGA-PBE/QZ4P theory level, and the GIAO NMR method using the B3PW91 functional with the 6-311++G(2d,p) basis set for C, H, N, O atoms and the Stuttgart basis set for the Rh atom. The geometry of compounds was optimised either by the same basis set as for the NMR calculations or applying the B3LYP functional with the 6-31G(2d) basis set for C, H, N, O atoms and LANL2DZ for the Rh atom. Computed N-15 NMR shielding constants sigma were compatible with experimental N-15 chemical shifts delta of complexes exhibiting similar structure and fulfil the linear equation delta = a sigma + b. The a and b parameters for all data sets have been estimated by means of linear regression analysis. In contrast to the correlation method giving "scaled" chemical shifts, the conversion of shielding constants to chemical shifts with respect to the reference shielding of CH3NO2 provided very inaccurate "raw" S values. The application of the former to the calculation of complexation shifts Delta delta (Delta delta = delta(compl) - delta(lig)) reproduced experimental values qualitatively or semi-quantitatively. The non-relativistic B3PW91/[6311++G(2d,p), Stuttgart] theory level reproduced the NMR parameters as good as the more expensive relativistic CGA-PBE//QZ4P ZORA approaches. (C) 2014 Elsevier B.V. All rights reserved.