Ab initio calculation of through-space magnetic shielding of linear polycyclic aromatic hydrocarbons (acenes): extent of aromaticity.

GIAO-HF within Gaussian 03 was employed to compute the NMR isotropic shielding values of a diatomic hydrogen probe above a series of acenes (linear polycyclic aromatic hydrocarbons). Subtraction of the isotropic shielding of diatomic hydrogen by itself allowed the determination of computed through-space proton NMR shielding increment surfaces for these systems. Shielding was observed above the center of each aromatic ring, but the magnitude of calculated shielding above each ring center depends on the number of fused benzenoid rings. The computed shielding increments above each ring center were correlated to other measures of extent of aromaticity, including geometric, energetic, and magnetic measurements.