Nonplanar PAHs

The reduction of two types of nonplanar PAHs is discussed here. We first describe the reduction of curved PAHs that contain five-membered rings (5MR), which cause deviation from planarity. This group is represented by corannulene (2) and its derivatives, as 2 is the smallest curved subunit of buckminsterfullerene. 

The second group is the cyclophanes of pyrene, where a planar system is forced to bend by linking both ends of the pyrene with an alkane chain. This highly strained curved group represents a compromise between strain and conjugation, and the nature of these systems aromaticity is of great interest. 

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A further question which may be answered by X-ray is that of planarity. Recent papers206,207 disclose that of the thousands of possible PAHs with 10 rings, the vast majority should be nonplanar, even chiral, structures resulting from steric factors. Highly notable examples of nonplanar PAHs include helicenes, fullerenes, bowl-shaped components of fullerenes, and nanotubes.206,207 Several intriguing species have been prepared in which the rings are forced out of planarity due to intramolecular steric interaction between substituents. These interactions become important when the angles by which substituents protrude from the sp2 periphery are such that they would approach adjacent substituents closer than allowed by van der Waals radii. 

The reductive dimerization/bond-cleavage of 39 represents the first case in which a large nonplanar PAH undergoes such a process. In addition, this is the... 

SRM 869a Column Selectivity Test Mixture for Liquid Chromatography [44] is composed of three shape-constrained PAHs (phenanthro[3,4-c]phenanthrene, PhPh l,2 3,4 5,6 7,8-tetrabenzonaphthalene, TBN and benzo[a]pyrene, BaP) and is routinely employed to evaluate the shape selectivity of stationary phases. The retention differences between the nonplanar TBN and planar BaP solutes (expressed as a selectivity factor axEN/BaP = provide a numerical assessment of... 

A retention-order reversal would not occur if all three molecules underwent the same interactions with the mobile and stationary phases. Different separation mechanisms must occur in each of the two chromatograms. The dichloromethane interacts differently with each PAH, as shown by different slopes in a plot of the logarithms of relative retention versus dichloromethane concentration (Figure 5). If the molecules had the same retention mechanisms, these plots would be parallel lines. The three plots are nonparallel, and only the one for dibenzo[cd,/ra]perylene is linear. The curvature in the other two plots could indicate different interactions in each at low or high dichloromethane concentrations. This behavior could be due to the stronger solvent having increased interaction with the PAHs so that the two larger PAHs become more nonplanar. 

The degrees of planarity of the peropyrene-type PAHs appear to be determined by the balance between the loss in resonance energy when the molecules become nonplanar and the increases in energy due to decreased intramolecular steric strain and stronger solvation. The degree of planarity and the relative importance of these factors for each PAH structure can be estimated from the analytical behavior of that PAH. Conversely, these molecules can serve as excellent probes of the mechanisms of HPLC. 

Compound 45 is nonplanar, based on molecular model calculations, due to the presence of five-membered rings, and therefore offers the possibility to create bowl-shaped molecules via flash vacuum pyrolysis (FVP) [86, 89]. The C150 disc 47 is the largest disc with threefold symmetry yet made [86], whereas the giant nanographene 48 with its 222 carbon atoms is the largest PAH to be synthesized and characterized to date [88]. 

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]. 

Table 6. Comparison of selectivity of four-ring PAHs, di-substituted nitrobenzene and planar/nonplanar compounds on Sil-to-ODA2< and Sil-to-VP22 stationary phases.

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