Relativistic effects on the nuclear magnetic shielding of CHn X4−n and CHXYZ (X, Y, Z = H, F, Cl, Br, I)

JUAN I. MELO; ALEJANDRO F. MALDONADO; G. A. AUCAR

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Lugar: Chicago; Año: 2012 vol. 137 p. 214319 - 214329

0021-9606

The nuclear magnetic shieldings of both, the central carbon atom and the hydrogens ofhaluro methyl molecules are highly influenced by the substitution of one or more hydrogenby halogen heavy atoms. For such molecules only the effect of Fluorine is mostly inductive.From Cl atoms and down the pattern of that shieldings become similar when the number ofheavy atoms is increased. We applied the linear-response elimination of small components,LRESC formalism, to calculate such shieldings. First we discuss the contribution of allleading relativistic corrections to σ(C), for CHX2 I with X = H, F and Cl. We found thatthe spin-orbit (SO) effects are the main one. When including the SO effects to the non-relativistic (NR) results, one obtain ∼ 97 % (93 %) for σ(C) (σ(H)) of the total LRESCvalues. They show that there is a HALA effect on the shielding of both, C and H atoms.The SO contribution increases its value when the halogen atom becomes heavier and thepattern of contributions for σ(C) can be extrapolated as a funcion of the atomic numberZ. When including paramagnetic spin-orbit (PSO) contributions we obtain results thatare within the 1 % of the total LRESC. We analyzed the pattern of both, the total andthe main electronic mechanisms that contribute to the shieldings of C and H atoms forCHn X4−n (n = 1, 3), and CHXY Z (X, Y, Z = F, Cl, Br, I) model compounds. Thewhole pattern of σ(C) for all series of compounds follows a normal halogen dependence(NHD) though with different rate of increase. An special case is that of CHF2 X for whichσ nr (C) follows an inverse halogen dependence but the total shielding have a NHD dueto the SO(FC) contributions. For the serie CH3 X (X = F, Cl, Br and I) we found thatσ SO ∼ Z2.53 . On the other hand, we performed four-component calculations using theXspin-free Hamiltonian. They can be related with the spin-orbit contributions plus anothercorrections obtained from LRESC method.