Computation of through-space NMR shielding effects by small-ring aromatic and antiaromatic hydrocarbons

The GIAO-HF method in Gaussian 03 was employed to calculate the isotropic NMR shielding values of a diatomic hydrogen probe above simple small-ring aromatic and antiaromatic hydrocarbons, including neutral and ionic examples. Subtraction of the isotropic shielding of diatomic hydrogen by itself allowed the prediction of through-space proton NMR shielding increment surfaces for these systems. Substantial shielding was observed above the center of aromatic rings, regardless of whether the ring was pi-aromatic or sigma-aromatic, and also regardless of the charge. In sharp contrast, deshielding was observed above the center of antiaromatic rings, regardless of whether the ring was pi-aromatic or sigma-aromatic, and also regardless of the charge. Shielding increment values at 2.5 angstrom above the ring centers were compared to NICS values at the same position. The shielding effects predicted by using diatomic hydrogen as a computational probe are diagnostic of whether a structure possesses aromaticity or antiaromaticity.