Computed NMR shielding increments over benzo-analogs of unsaturated five-membered ring heterocyclic compounds as a measure of aromaticity

The GIAO-HF method in Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe moved in a square grid 2.5A above the plane of 15 benzo-fused analogs of conjugated five-membered ring heterocyclic compounds: pyrrole, furan, thiophene, and phosphole and their 2- and 3-nitrogen analogs. Subtraction of the calculated isotropic shielding value of diatomic hydrogen from each of these isotropic shielding values gave the shielding increment (Delta sigma) for each probe position. Plotting this value against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment 2.5A above the heterocyclic ring center correlated reasonably well with the only other published method of assessing aromaticity of these systems ASE (aromatic stabilization energy) and with our calculated NICS (nucleus-independent chemical shift) values, another magnetic criterion. The magnitude of the shielding increment measured over the benzene ring midpoint did not correlate well with other measures of aromaticity, however.