Chemical shifts of the [N]phenylenes and related compounds

Proton chemical shifts and nucleus-independent chemical shifts (NICS) have been computed for [N]phenylenes and related compounds using the GIAO/HF method. Proton chemical shifts calculated in the 6-31G* basis agree with measured values, where available, within similar to 0.3 ppm. NICS values were calculated at the HF/3-21G level using B3LYP/6-31G* geometries. Both proton chemical shifts and NICS values show attenuation of aromatic and antiaromatic character in the six- and four-membered rings, respectively, in a manner dependent upon the positions of these rings in the molecule. These effects are greater in angular than in linear phenylenes. The central ring of a branched phenylene is found to be only slightly aromatic. The protons of the cyclic phenylenes resonate at slightly higher field than their angular counterparts. Both NICS and proton shifts of antikekulene suggest only slight additional antiaromatic character.