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

ALEJANDRO F. MALDONADO; GUSTAVO A. AUCAR

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

ROYAL SOC CHEMISTRY

Lugar: Cambridge; Año: 2009 vol. 11 p. 5615 - 5627

1463-9076

 Fully relativistic calculations of NMR magnetic shieldings on molecular systems of the type $XH_n$ ($n=1-4$) and noble gases at RPA level of approach are presented. In order to get converged results, the performance of both Kinetic balance prescriptions, RKB and UKB were investigated. It was found that the UKB is the best option.  Relativistic effects that arise from the influence of heavy atoms on magnetic shielding of vicinal heavy atoms (HAVHA effect) in molecular systems like $XYH_3$ ($X=C, Si, Ge$ and $Sn$; $Y = Br, I$) are such that, when the central atom is not heavy (like $C$), replacing one $H$ by $Br$ or by $I$ the shielding of the central atom is modified around 28% in the last case; but when the central atom is heavier that effect becomes much less important, {it e. g.} of the order of 4% when the central atom is $Sn$ and one hydrogen is replaced by one iodine atom. On the other hand, when the central atom is modified from $C$ to $Sn$, the effect on $Br$ or $I$ is close to 18%.  In the case of halogen atoms, HAVHA effects are quite important. For molecules like $XYH_3$ $sigma(Y; Y=Br, I)$ becomes larger when $X$ is heavier. In the case of $X = C$ and $Sn$, $sigma(Br)$ change in 540 ppm or 18%.