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 π-aromatic or σ-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 π-aromatic or σ-aromatic, and also regardless of the charge. Shielding increment values at 2.5 Å 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.