The UKB prescription and the heavy atom effects on the nuclear magnetic shielding of vicinal heavy atoms

Abstract

Fully relativistic calculations of NMR magnetic shielding on XYH₃ (X = C, Si, Ge and Sn; Y = Br, I), XH_n (n = 1–4) molecular systems and noble gases performed with a fully relativistic polarization propagator formalism at the RPA level of approach are presented. The rate of convergence (size of basis set and time involved) for calculations with both kinetic balance prescriptions, RKB and UKB, were investigated. Calculations with UKB makes it feasible to obtain reliable results for two or more heavy-atom-containing molecules. For such XYH₃ systems, the influence of heavy vicinal halogen atoms on σ(X) is such that heavy atom effects on heavy atoms (vicinal plus their own effects or HAVHA + HAHA effects) amount to 30.50% for X = Sn and Y = I; being the HAHA effect of the order of 25%. So the vicinal effect alone is of the order of 5.5%. The vicinal heavy atom effect on light atoms (HALA effect) is of the order of 28% for X = C and Y = I. A similar behaviour, but of opposite sign, is observed for σ(Y) for which σ^R−NR(I; X = C) (HAHA effect) is around 27% and σ^R−NR(I; X = Sn) (HAVHA + HAHA effects) is close to 21%. Its electronic origin is paramagnetic for halogen atoms but both dia- and paramagnetic for central atoms. The effect on two bond distant hydrogen atoms is such that the largest variation of σ(H) within the same family of XYH₃ molecules appears for X = Si and Y = I: around 20%. In this case σ(H; X = Sn, Y = I) = 33.45 ppm and σ(H; X = Sn, Y = H) = 27.82 ppm.