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Melo J.I.,CONICET | Maldonado A.F.,Northeastern University of Argentina | Maldonado A.F.,Institute of Modelling and Innovation on Technology | Aucar G.A.,Northeastern University of Argentina | Aucar G.A.,Institute of Modelling and Innovation on Technology
Journal of Chemical Physics | Year: 2012

Nuclear magnetic shieldings of both carbon and hydrogen atoms of haluro methyl molecules are highly influenced by the substitution of one or more hydrogen by halogen heavy atoms. We applied the linear response elimination of small components, LRESC, formalism to calculate such shieldings and learn whether including only few terms is enough for getting quantitative reproduction of the total shieldings or not. First, we discuss the contribution of all leading relativistic corrections to σ(C), in CHX2I molecular models with X = H, F, and Cl, and show that spin-orbit (SO) effects are the main ones. After adding the SO effects to the non-relativistic (NR) results, we obtain ∼ 97 (93) of the total LRESC values for σ(C) (σ(H)). The magnitude of SO terms increases when the halogen atom becomes heavier. In this case, such contributions to σ(C) can be extrapolated as a function of Z, the halogen atomic number. Furthermore, when paramagnetic spin-orbit (PSO) contributions are also considered, we obtain results that are within 1 of the total LRESC value. Then we study in detail the main electronic mechanisms involved to contribute C and H shieldings on CHnX4 - n (n = 1, 3), and CHXYZ (X, Y, Z = F, Cl, Br, I) model compounds. The pattern of σ(C) for all series of compounds follows a normal halogen dependence (NHD), though with different rate of increase. A special family of compounds is that of CHF2X for which σnr(C) follows an inverse halogen dependence though the total shielding have a NHD due to the SO contributions. For the series CH3X (X = F, Cl, Br and I), we found that σSO ∼ ZX2.53. Another important finding of this work is the logarithmic dependence of σSO(C) with the substituent atomic number: ln σSO(C) = AX aX Z Y for both family of compounds CH2XY and CHX2Y. We also performed four-component calculations using the spin-free Hamiltonian to obtain SO contributions within a four-component framework. © 2012 American Institute of Physics. Source


Maldonado A.F.,Northeastern University of Argentina | Maldonado A.F.,Institute of Modelling and Innovation on Technology | Aucar G.A.,Northeastern University of Argentina | Aucar G.A.,Institute of Modelling and Innovation on Technology | Melo J.I.,University of Buenos Aires
Journal of Molecular Modeling | Year: 2014

The nuclear magnetic shieldings of Si, Ge, and Sn in MH4−nYn (M = Si, Ge, Sn; Y = F, Cl, Br, I and n = 1–4) molecular systems are highly influenced by the substitution of one or more hydrogens by heavy-halogen atoms. We applied the linear response elimination of small components (LRESC) formalism to calculate those shieldings and learn whether including only a few of the leading relativistic correction terms is sufficient to be able to quantitatively reproduce the full relativistic value. It was observed that the nuclear magnetic shieldings change as the number of heavy halogen substituents and their weights vary, and the pattern of σ(M) generally does not exhibit the normal halogen dependence (NHD) behavior that can be seen in similar molecular systems containing carbon atoms. We also analyzed each relativistic correction afforded by the LRESC method and split them in two: core-dependent and ligand-dependent contributions; we then looked for the electronic mechanisms involved in the different relativistic effects and in the total relativistic value. Based on this analysis, we were able to study the electronic mechanism involved in a recently proposed relativistic effect, the “heavy atom effect on vicinal heavy atom” (HAVHA), in more detail. We found that the main electronic mechanism is the spin–orbit or σp T(3) correction, although other corrections such as σp S(1) and σp S(3) are also important. Finally, we analyzed proton magnetic shieldings and found that, for molecules containing Sn as the central atom, σ(H) decreases as the number of heavy halogen substituents (of the same type: either F, Cl, or Br) increases, albeit at different rates for different halogens. σ(H) only increase as the number of halogen substituents increases if the halogen is iodine. © 2014, Springer-Verlag Berlin Heidelberg. Source

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