Perimeter bonds

A strategic bond must be an exendo bond within a primary (i.e. non-peripheral, or non-perimeter) ring of 4-7 members and exo to a primary ring larger than 3-membered. 

The perimeter of the (L,0) tubule is composed of L parallel hexagon building blocks bonded side by side, with the bonded side parallel to the tubule axis. Its length is equal to LI. 

Fig. 1. Unrolled representation of the tubule (5,3). The QM distance is equal to the perimeter of the tubule. 0M = a,/3ff +jVf- LjM), where a is the C-C bond length.

Fig. 13. Model of the growth of a nanotubule bonded to the catalyst surface, (a) Growth of a straight (5,5) nanotubule on a catalyst particle, with perimeter I5ak (b) growth of a straight (9,0) nanotubule on a catalyst particle whose perimeter is 18ak (k is a constant and the grey ellipsoids of (a) and (b) represent catalyst particles, the perimeters of which are equal to 5ak and 18a/t, respectively) (c) (5,5)-(9,0) knee, the two sides should grow optimally on catalyst particles having perimeters differing by ca. 20%.

Whereas in porphyrins, chlorins and bacteriochlorins the 1871-aromatic perimeter includes two nitrogens excluding their lone pairs, the.1871 perimeter of the isobacteriochlorin includes three nitrogens, one with its electron lone pair as part of the aromatic perimeter. The electron pair of the /J,/J -C —C double bond of ring D of isobacteriochlorin is not involved in the cyclic conjugation path. 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

The synthesis of twistane involving an intermediate of type D was reported in 1976 by Hamon and Young [2]. This synthetic approach constitutes a violation of Corey s rule number 4 (which refers to "perimeter" and "core bonds") and involves a bicyclo[3.3.1]nonane precursor in which the relative configuration of the C(6) side-chain is crucial if cyclisation is to occur. The cost of such a "violation" was relatively high and a great number of drawbacks are found in the original article. 

The bonding in the parent benzophospholide features an extended r-de-localisation around the ring perimeter which is expressed by dissipation of the excess charge on a larger number of canonical formulae such as IIA-IIE (and their mirror images of which all but IIB were omitted). Phosphonio-substituents in 1- and 3-positions increase the weight of the symmetry equivalent canonical structures IIB, IIB. As before, this implies a partial ti-bond localisation and a reduction of the energy of the isodesmic reaction (2) in Fig. 4 [16]. 

First number refers to litiiium atoms in-plane, second to out-of-plane atoms relative to the C3 perimeter. First number [Pg.62]

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