Aromaticity in-plane

The term in-plane aromaticity has been used for molecules such as the didehy-drophenyl cation (see Section II). However, we stress that the compounds in question are homoaromatic rather than aromatic molecules, which can be directly related to a-aromaticity Therefore, the appropriate notation should be homo-o--aromaticity rather than in-plane aromaticity. The concept of c-aromaticity is very controversial. One can completely avoid this term by referring to the mode of electron delocalization as was done by Cremer While 7r-aromaticity and homo-7r-aromaticity are connected with ribbon delocalization of electrons along a conjugative cycle, molecules that have been considered to be either a-aromatic or homo-o--aromatic (in-plane aromatic) seem to prefer delocalization of electrons over a surface defined by the participating atoms (see the discussion in... 

Three characteristic ed)sorption frequencies were used in the IR studies. These are associated with the aromatic out-of-plane C-H bending (v,), the aromatic in-plane C-H... 

In a stndy on PSSA degradation in fuel cell appUcadons, it was found that peaks at 1,037cm (S=0 symmetrical stretching) and l,008cm" (aromatic in-plane vibration) diminish after a durability test, especially on the cathode side (Yu et al. 2003). Both the aromatic rings and the SOj" groups are lost after the durability test. On the basis of IR results, the hydroxyl radicals attack the polystyrene via addition to the phenyl groups (Weir 1978). 

Aromatic C-H Stretching Vibrations Aromatic In-plane C-H Deformation Vibrations Aromatic Out-of-plane C-H Deformation Vibrations and Ring Out-of-plane Vibrations in the Region 900-650 cm ... 

Some diamines carrying very bulky substituents like cardo groups can give colorless polyimides. For example, the bis-9,9-(4-aminophenyl)fluorene (FDA) or brominated and acetylenic FDA derivatives react with 6FDA giving copolymer films62 with low birefringence (low difference between in-plane and out-of-plane refraction index) (Fig. 5.8). A new cardo diamine l,l-bis[4-(4-aminophenoxy)phenyl]cyclododecane (Fig. 5.8) reacts with different aromatic dianhydrides with formation of colorless polyimides.63... 

FIGURE 6.16 ortho-Quinone methide 3 stabilization of the zwitterionic rotamer in a complex with /V-methyImorpholine /V-oxide (17). The zwitterionic, aromatic precursor 3a affords the common quinoid form of the o-QM 3 by in-plane rotation of the exocyclic methylene group. 

The above analysis illustrates why it is helpful to consider the enediyne moiety as two independent re-systems. As discussed above, the conjugated out-of-plane 7r-system of the reactant is smoothly transformed into the conjugate 7r-system of the product (e.g., the aromatic system of benzene) without an overall change in the number of bonds. We will refer to this group of electrons as out-of-plane MOs or simply as the re-system . In contrast, the two in-plane re-bonds are transformed in a more drastic way to the new cr-bond and a pair of radical centers. We will refer to this system of orbitals as in-plane re-bonds or as the a system . 

Rotation of the aromatic ring out of the enediyne plane forces the aromatic 7r-orbitals to overlap with the in-plane 7r-orbitals of the enediyne system as illustrated... 

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