Ribbon delocalization

Another characteristic property of the electron density of 1 is its relatively high value at the centre e of the ring (more than 80% of that at the CC bond critical point). Density is smeared out over the ring surface and concentrated at its centre because of the occupation of the w0 -orbital (MO 8, 3a(, Figure 6), which has the character of a surface orbital . Cremer and Kraka9, n 13 have termed this phenomenon surface delocalization of electrons, to be distinguished from ribbon delocalization and volume delocalization of electrons (Figure 12)12. 

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... 

The only known tt-excessive 12-membered ring is the multiply annulated thiaannulene (30) (70JA5284). It is an unconventional molecule insofar as it incorporates an odd number of electrons (13) within an even-membered frame. As a result, its central tt-ribbon cannot be mobilized into 7r-delocalization without becoming exposed to the adversity of an electronic open shell (78AHC(23)55). The molecule s natural resistance to do so is borne out by its UV and XH NMR characteristics which are clearly implicative of a nonplanar, strictly atropic central skeleton. 

The views expressed in this brief introductory section concerning the development of delocalization in cyclic tt ribbons are certainly not new. Nonetheless, the approach followed here, i.e., the separation of the major contributing factors in terms of Eq. (1), does appear to offer certain practical advantages over some of the more conventional descriptions of the subject. 

It appears that the only known representative of this family is the heavily annulated substance depicted in 65, which was synthesized as shown.82 Being an even-membered ring and incorporating but a single -excessive heteroatom in its periphery, the twelve-membered monocyclic moiety of 65 is somewhat unorthodox in the sense that it contains an odd number of electrons, thirteen to be exact. As a result, the central 17-ribbon of 65 cannot sustain delocalization without experiencing the ill effects of a w-electronic open shell. It is hardly surprising, therefore, to find that 65 exhibits UV and 1H NMR characteristics that are indicative of a nonplanar, strictly atropic, central frame. 

Extension of the concepts of ribbon and surface delocalization of electrons to three-dimensions leads to volume delocalization and covers cases of radial aromaticity and three-dimensional (3D) aromaticity . As we will show later the most convicing example of radial aromaticity, namely the l,3-dehydro-5,7-adamantanediyl cation (Figure 2), is actually an example of homoradial aromaticity. Also, there exist several examples of homo-3D aromaticity that are normally listed under 3D-aromaticity (for an example, see Figure 2). Finally, a number of examples have been investigated that can be classified as homoheteroaromatic systems (Figure 2). It may be only a matter of time until the first molecule with homospherical aromaticity has been synthesized and investigated. 

The dominant edge can influence the conductivity of the ribbons, as seen in Fig. 12.5. The shape of the molecular orbitals affects the transport properties but the calculations show that both, armchair and zigzag ribbons feature delocalized molecular orbitals when they are passivated if the ribbon is not passivated, localized molecular orbitals are found especially along the zigzag edges. 

The electronic density on the graphene ribbons is completely delocalized on the surface, as shown with the molecular orbitals in Fig. 12.15. 

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