Polycyclic aromatic hydrocarbons, nonplanar

Helicens are polycyclic aromatic hydrocarbons with with nonplanar helical frames formed from orf/to-fused benzene or other aromatic rings [87]. The helicens and helicene-like molecules have for a long time attracted... 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

The realization that polycyclic aromatic compounds are not necessarily planar is not a new concept. A book published in 1964 on benzenoid hydrocarbons (24) contains a chapter on nonplanar, overcrowded, aromatic hydrocarbons this chapter primarily describes high degrees of nonplanarity that are due to steric interactions of hydrogen atoms in benzenoid compounds that contain substructures related to benzo[c]phenanthrene (5) (twist angle = 31°) or to dibenzo[c,d]phenanthrene (6) (twist angle = ca. 50°) (26-28). 

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