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

Abstract The GIAO-HF method within Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe at various distances (2.5 A, 3.0 A and 4.0 A) above the plane of 15 conjugated five-membered ring heterocyclic compounds: pyrrole, furan, thiophene, and phosphole and their 2- and 3-nitrogen analogs. Subtraction of the isotropic shielding value of diatomic hydrogen by itself from each of these isotropic shielding values gave the shielding increment (Δ σ ) for each probe position. Plotting Δ σ 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 at 2.5 A above the ring center (Δ σ 2.5 ) correlated reasonably well with other established methods of assessing aromaticity, including geometric (HOMA, harmonic oscillator model of aromaticity), energetic (ASE, aromatic stabilization energy), and magnetic (NICS, nucleus-independent chemical shift) criteria.