Homoconjugation definition

E. The Electron Density Based Definition of a Homoconjugative Bond.. . 375... 

A definition of homoconjugation based on the concept of bond (electron) delocalization... 

Since conjugation and homoconjugation are parallel concepts, it is logical to base a chemically relevant definition of homoconjugation on the concept of bond (electron) delocalization ... 

Homoconjugation can lead to a bond (electron) delocalization energy, which reveals an excess stability of the system when compared to suitable reference compounds. The selection of appropriate reference compounds is essential for the definition of homoconjugation. 

Cyclopropyl homoconjugation can be easily detected and described as long as one retains its topological definition. This also holds to some extent in the case of no-bond homoconjugation. However, as soon as one has to assess the chemical relevance of homoconjugation and to determine a homoconjugative bond (electron) delocalization energy, one needs, as mentioned in Section I, suitable reference compounds for comparison. 

In the following, rather than discussing the many definitions of homoconjugation or homoaromaticity that have been expressed by various experimentalists, we compare the few theoretically based attempts to derive a more general definition of homoaromaticity. 

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 ... 

This is a quantitative definition of homoaromaticity that is generally applicable and helps to specify exactly the point Rb in Figure 3, at which cyclopropyl homoconjugation starts. However, this definition is much more stringent than Winstein s definition because it excludes all those systems with 1,3-interactions that do not lead to a bond path (no-bond homoaromaticity). Hence, it describes homoaromaticity only for the case of cyclopropyl homoconjugation. For example, Kraka and Cremer have used this approach to describe cyclopropyl homoconjugation in norcaradiene (10)27 54. 

We are, of course, aware of the strict Hiickel definition linking aromaticity to the presence of 4n + 2 rc-electrons, On the other hand, we note that the radical cations discussed below have SOMOs, which have one electron less than the Hiickel requirement and which show cyclic homoconjugation, even though the parent molecules assume alternative structures. In view of the precedence established for the radical anion (156) of bicyclo[6.1.0]nonatriene [416,417], in order to emphasize the dramatic difference between the structures of these radical cations and their precursors, and for reasons of convenience, we will refer to these species as homo- or bis-homoaromatic. The paradigm of these remarkable species is found in the barbaralane system. 

Following Section III, there will be a section (Section IV) in which the basic requirements for an ab initio investigation of homoconjugated molecules are sketched. As an illustrative example, the ab initio investigation of the homotropenylium cation will be described in detail where special emphasis is laid on an assessment of those molecular properties that are a direct reflection of the homoaromatic character of the molecule. The section will close by deriving detailed definitions and requirements for homoaromaticity and homoantiaromaticity that are adjusted to the more recent developments in the field. 

---