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Ab initio calculations have been made on [n]helicenes and their planar, zigzag isomers, the [n]phenacenes,

The cover picture shows the molecular structure of the helically extended angular [8]phenylene. An unprecedented cobalt-catalyzed triple cyclization of an appropriate nonayne was successful in assembling the largest crystallographically characterized helical phenylene (heliphene). Its properties are intriguing; the heliphene is unusually configurationally labile and has a strongly attenuated bathochromic increment in the UV spectrum, shielding of the terminal rings as a result of spatial overlap, and alternating ring-current intensities along the angular frame. The X-ray crystallographic data detail the helical and s ± p distortive features. Most surprisingly, the remarkable flexibility of the heliphene provided an unusually low barrier (DG = (À 4.5 8C) 13.4 AE 0.4 kcal mol À1 ) for enantiomerization. The 1 H NMR spectrum was consistent with the alternation of cyclohexatrienoid and aromatic character; the terminal rings are the most diatropic and the penultimate ones the least. Further details about this chiral polycyclic benzenoid hydrocarbon are described by K. P. C. Vollhardt, et al. on p 3227 ff.

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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 ∼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.