Relativistic DFT Calculation of 119Sn Chemical Shifts and Coupling Constants in Tin Compounds

The nuclear shielding and spin-spin coupling constants of 119Sn in stannane,
tetramethylstannane, methyltin halides Me4-nSnXn (X ) Cl, Br, I; n ) 1-3), tin halides, and
some stannyl cations have been investigated computationally by DFT methods and Slater allelectron
basis sets, including relativistic effects by means of the zeroth order regular
approximation (ZORA) method up to spin-orbit coupling. Calculated 119Sn chemical shifts
generally correlate well with experimental values, except when several heavy halogen atoms,
especially iodine, are bound to tin. In such cases, calculated chemical shifts are almost constant
at the scalar (spin-free) ZORA level; only at the spin-orbit level is a good correlation, which
holds for all compounds examined, attained. A remarkable "heavy-atom effect", analogous to
that observed for analogous alkyl halides, is evident. The chemical shift of the putative stannyl
cation (SnH3
+) has also been examined, and it is concluded that the spectrum of the species
obtained in superacids is inconsistent with a simple SnH3
+ structure; strong coordination to
even weak nucleophiles such as FSO3H leads to a very satisfactory agreement. On the contrary,
the calculated 119Sn chemical shift of the trimesitylstannyl cation is in very good agreement with
the experimental value. Coupling constants between 119Sn and halogen nuclei are also well modeled
in general (taking into account the large uncertainties in the experimental values);
relativistic spin-orbit effects are again quite evident. Couplings to 13C and 1H also fall, on the
average, on the same correlation line, but individual values show a significant deviation from
the expected unit slope.