Polycyclic nonplanar

Clathrate. 2. Any molecular entity that is nonplanar and often polycyclic that encloses a cavity. 3. Photosensitive precursors that generate chemically reactive species upon irradiation with a laser or flash-lamp. See Photore-... 

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. 

Planar and nonplanar polychlorinated biphenyls (Fig. 5.2) differ in the type of induction they will cause. Thus, 3, 3, 4, 4, 5, 5 -hexachlorobiphenyl is a planar molecule, which is an inducer of the polycyclic hydrocarbon type. 2, 2, 4, 4, 6, 6 -Hexachlorobiphenyl is a nonplanar molecule due to the steric hindrance between the chlorine atoms in positions 2 and 6 and is a phenobarbital type of inducer. The variety and type of inducing agents are shown in Table 5.21. Some compounds may indeed be mixed types of inducers, and thus mixtures of planar and nonplanar polychlorinated biphenyls are found to act as inducers of both the polycyclic and phenobarbital type. 

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). 

Structures determined by modern X-ray techniques show that significant nonplanarities exist in benzenoid compounds of this type. This finding infers that nonplanar polycyclic aromatic compounds are quite common. Structures 7-9, which have twist angles ranging from 30° to 42°, are examples... 

AU of the PAH and super acenes are planar molecules. However, incorporation of a seven-membered ling results in nonplanar (curved) molecules that are of interest in the context of curved % surfaces and nonplanar aromatic compounds. Cycloaddition/dehydrogenation strategy shown in Scheme 16.42 results in the formation of seven-membered ring fused curved polycyclic aromatic molecule [46]. 

The special stability and reactivity associated with cyclic delocalization is not unique to benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes can be aromatic, but only if they contain (An + 2) tt electrons (n = 0, 1, 2, 3,. . . ). In contrast, An tt circuits may be destabilized by conjugation, or are antiaromatic. This pattern is known as Hiickel s rule. Nonplanar systems in which cyclic overlap is disrupted sufficiently to impart alkene-like properties are classified as nonaromatic. Let us look at some members of this series, starting with 1,3-cyclobutadiene. 

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... 

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