Computation of through-space NMR shielding effects by aromatic ring-cation complexes: substantial synergistic effect of complexation.

The HF-GIAO method in Gaussian 03 was employed to calculate the NMR isotropic shielding values of a diatomic hydrogen probe and to predict the through-space proton NMR shielding increment surfaces above benzene complexed with ammonium, lithium, sodium, potassium, magnesium or calcium ion. The sum of the calculated isotropic shielding values for the proximal hydrogen of a diatomic hydrogen probe over benzene and those calculated at appropriate positions relative to cations were subtracted from the isotropic shielding values calculated for the complexes. The result is a shielding increment for complexation. Complexation results in a synergistic effect on NMR shielding. Enhanced shielding was observed over the pi electron cloud of benzene upon complexation with the cations, more than the sum of the separate effects of the aromatic ring and the charge. The results are interpreted in terms of polarization of the pi cloud of benzene by the cation and its consequences.