Homoconjugation, description

B. Description of Homoconjugative Interactions in Terms of Orbital Overlap... 

With these definitions, useful descriptions of non-planar rc-systems and potentially homoconjugated systems have been developed. The descriptions are particularly attractive to experimentalists because orbital overlap is accepted as a major contributing factor to bonding and it is easy to visualize. Although Haddon did not formally define homoaromaticity in his work63 one can use his threshold value of S to define homoaromaticity in the following way ... 

The description of the homotropenylium ion on the basis of the POAV analysis confirms the homoaromatic character of the cation. However, the analysis suggests in addition some consequences of homoconjugative delocalization, which are difficult to accept. These are ... 

Hehre66 69 and independently Jorgensen72,73 pointed out that the Mobius and Hiickel description of homoconjugated molecules (Figure 9) is consistent with the assumed homoaromtic and homoantiaromatic character of these compounds. However, it was also realized that in the general case such a classification might not be sufficient to describe subtle differences in orbital interactions, which determine the homo(anti)aromatic character of a molecule. 

D. Description of a Homoconjugative Bond by Bond Orders and Other Interaction Indices... 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

In the 4q case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds30,32,37 38. This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a 4q situation, an option not available to the parent 4q antiaromatic unsaturated ring systems39, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4q + 2 cases. 

A similar point has been made by Roth and coworkers who investigated potentially homoconjugated systems using a modified MM2 force field, MM2ERW. The latter contained as reference bonds not only C(sp )—C(sp ), C(sp )—C(sp ), etc. bonds needed for the description of polyenes, but also the C(sp )—C(cyclopropyl) single bond taken from vinylcyclopropane in order to adequately describe cyclopropyl-substituted compounds 17-26 listed in Table 1. With this extended set of reference bonds, Roth and coworkers were able to reproduce experimental heats of formation of polyenes and... 

B. Description of Homoconjugative interactions in Terms of Orbitai Overiap... 

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