Surface delocalization

Cremer proposed that the higher electron density inside the ring would relax the strain [9]. The surface delocalization stabilizes the lowest orbital of the ring system, which might relax the ring strain. 

Bachrach has argued against the surface delocalization by comparing the electron density at the centers of the banana bonds to that of the centers of cyclopropane 1 and phosphirane 10 (Scheme 3) [22]. Density is spread over the ring in 1, but not... 

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. 

Surface delocalization is not found for cyclobutane or larger cycloalkanes10. Furthermore, it does not appear for cyclotrisilane since in this case the overlap within the surface orbital is not sufficient to bring enough electron density into the centre of the ring71. 

Surface delocalization has been confirmed by various other authors. Coulson and Moffitt23 were the first to note that there is a plateau of relatively high electron density... 

If one takes the bond strengthening effects calculated with equations 11 and 12 (see Table 8), then surface delocalization of (j-electrons must add about 16 kcal mol 1 to the stability of 1 to lead to a CSE of 28 kcal mol 110. 

Chapter 2 of this volume2). Therefore, c-aromatic and homo-c-aromatic molecules are molecules with surface delocalization of electrons (Figure 2). 

It has also been shown that surface delocalization can adopt a preferential direction if the cyclopropyl group interacts with a -conjugated system. There are basically two directions of surface delocalization as indicated in Scheme 9 for vinylcyclopropane (20) and divinylcyclopropane 47. In homoaromatic molecules, surface delocalization is perpendicular to the 1,3-bond while in homoantiaromatic molecules it is parallel to the 1,3-bond27-85. 

SCHEME 9. Schematic presentation of surface delocalization in cyclopropane (17), vinylcyclopropane (20) and 1,2-divinylcyclopropane (47). Major axes of bond ellipticities are indicated by arrows the direction of surface delocalization in 20 and 47 is given by a bold arrow... 

In a similar way, a homoantiaromatic system formed by bond (cyclopropyl) homoconjugation can be described. There is, however, one major difference between homoaromatic and homoantiaromatic systems (observed in the case of cyclopropyl homoconjugation) that separates homoaromaticity from aromaticity. While aromaticity and antiaromaticity involve different numbers of electrons (4q + 2 or Aq), homoaromaticity and homoantiaromaticity both involve Aq + 2 electrons but differ with regard to the delocalization modes of these electrons, which are best described by the direction of surface delocalization in a three-membered ring (Scheme 15) ... 

For a homoaromatic system, surface delocalization in the cyclopropyl ring is perpendicular to the bridging bond, thus forming a Hiickel aromatic electron ensemble which is delocalized in just one part of the bi(poly)cyclic system. 

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