Relativistic effects on magnetic shieldings

MALDONADO, A. F.; G.A. AUCAR; J. I. MELO

Smolenice

Conferencia; 11th International Conference on Relativistic Effects in Heavy-Element Chemistry and Physics; 2014

Slovak Academy of Sciences, and, Comenius University in Bratislava, and, Slovak University of Technology in Bratislava

Recent theoretical and computational developments of full relativistic formalisms give now the opportunity to calculate and analyse the electronic origin of magnetic molecular properties of heavy-atom containing molecules, and improve the understanding of such properties.DFT is usually the method of choice for calculation of NMR properties in medium-size heavy-atom containing molecules because it includes electron correlation at an affordable computational cost. Only in non relativistic regime it gives reliable results compared with experimental ones but it is not good enough in relativistic calculations of NMR magnetic properties. This is because the functional used were developed in order to assess the study of medium-size heavy-atom containing molecules or to analyse NR-like electronic mechanisms that may explain tendences or absolute values of magnetic shieldings. It is, therefore, of interest to assess whether DFT is a valuable tool in quantitative predictions of Sn and Pb NMR properties.In this work we present calculations of NMR shieldings of Sn and Pb atoms in XY4−nZn (X = Sn, Pb; Y , Z = H, F, Cl, Br, I) and PbH4−nIn (n = 0, 1, 2, 3, 4) family of compounds with four-component functionals. We found that results of calculations with Relativistic Polarization Propagator Appoach at Random Phase Approximation level (RelPPA-RPA) are more reliable than the DFT ones. We carried out calculations of nuclear magnetic shieldings with different functionals (KT2, KT3, PBE0, B3LYP, BLYP and BP86) and we argue on why those DFT functionals must be modified in order to obtain better results of such parameter within the relativistic regime. There is a dependence among both, electron correlation and relativistic effects that should be introduced in the functionals. They were parameterized only within a nonrelativistic regime and so they are not able to introduce properly the relativistic effects.We also applied the linear response elimination of small components (LRESC) formalism to calculate those shieldings and learn whether including only few leading relativistic correcting terms is enough for getting quantitative reproduction of the full relativistic value. It was observed that the nuclear magnetic shieldings change when the number and weight of the substituent heavy-halogen atoms varies. We analysed also each relativistic correction given by the LRESC method and splitted them in two: core-dependent and ligand-dependent, looking for the electronic mechanisms involved in the dfferent relativistic effects and in the total relativistic value. From this analysis we were able to study in more detail the electronic mechanism involved in the new relativistic effect proposed recently and named as heavy atom effect on vicinal heavy atom (HAVHA). We found that the main electronic mechanism is the Spin-orbit or σpT(1) correction, though σpS(1)p and σpS(3) corrections are also important.[1] Aucar, G. A.; Romero, R. H.; Maldonado, A. F. Int. Rev. on Phys. Chem. 29, 1 (2010).[2] Bagno, A.; Casella, G.; Saielli, G. J. Chem. Theory Comput. 2, 37 (2006).[3] Autschbach, J.; Zheng, S. Annu. Rep. NMR Spectrosc. 67, 1 (2009).[4] Komorovský, S.; Repiský, M.; Malkina, O. L.; Malkin, V. G.; Malkin Ondík, I.; Kaupp, M.J. Chem. Phys. 128, 104101 (2008).[5] Melo, J. I.; Ruiz de Azúa, M. C.; Giribet, C. G.; Aucar, G. A. and Romero, R. H. J. Chem. Phys. 118, 471 (2003).