Relativistic and electron correlation effects on NMR J-coupling of Sn and Pb containing molecules

Abstract

We studied the influence of relativistic and electron correlation effects on NMR J-couplings in the following set of heavy-atom containing molecules: $XY_4$ and H$_3XX$H$_3$ (X = Sn, Pb; Y = H, F, Cl, Br, I). We applied two formalisms, the relativistic polarization propagator approach at random phase level of approach (RelPPA-RPA) and density functional theory (DFT) with functionals as implemented in the DIRAC code. We have chosen four functionals that have different amount of HF exchange (PBE0, B3LYP, BLYP, BP86). For those molecular systems, results of calculations with BLYP functional have the best performance as compared with available experimental data. As was previously found for magnetic shieldings in other molecular systems we are able to show here that DFT functionals must be modified in order to obtain reliable results of NMR J-coupling within the relativistic regime. We can state that there is a non-linear dependence among both, electron correlation and relativistic effects that should be introduced in the functionals. The functionals implemented in the DIRAC code are standard nonrelativistic ones which were parameterized with data taken from light-atom containing molecules. This explains why they are not able to properly introduce relativistic effects on NMR parameters, like J-coupling constant. Lastly we show that in the analysis of J-couplings for the family of compounds mentioned above, one must consider the effects of a third heavy-atom that is close to the J-coupled atoms of the same molecule, specially for $^n$J(H–H). This kind of effect is similar to the newest and so called heavy-atom effect on vicinal heavy atoms, HAVHA, proposed for the NMR-shielding constant. Such effects are among the most important relativistic effects in the family of compounds studied in this work.