Computed NMR shielding of phosphorus-containing conjugated five-membered ring heterocycles 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 oriented perpendicular to the plane and moved in a square grid 2.5Å above the plane of conjugated five-membered ring heterocyclic compounds: pyrrole, furan, phosphole and thiophene and their phosphorus containing analogs. Subtraction of the calculated isotropic shielding value of diatomic hydrogen from each of these isotropic shielding values gave the shielding increment (Δσ) 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 2.5Å above the heterocyclic ring center correlated reasonably well with the other published methods of assessing aromaticity of these systems ASE (aromatic stabilization energy), Λ (exaltation of magnetic susceptibility) and NICS(1) (nucleus-independent chemical shift) values, both of which are magnetic criteria. The magnitude of the shielding increment also correlated with the number of phosphorus atoms in the ring. Greatest shielding (5.5ppm) was observed 2.5Å above 2,3,4,5-tetraphosphathiophene.