Homoaromaticity definition

The experimental assessment of homoaromaticity is often based on working definitions of homoaromaticity that are influenced by the context of the experimental measurements and available reference data. Such a working definition may be incompatible with other definitions, be limited to a small set of related compounds and frequently be rather vague with regard to a general understanding of homoaromaticity. However, it can be useful to... 

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 definition covers both bond and no-bond homoaromaticity with a clear distinction between the possibility of insignificant through-space interactions with r < 0.2. 

It is interesting to apply this definition, which is a clear improvement over Winstein s original definition of homoaromaticity, to a particular case, namely the homotropenylium cation. 

The homotropenylium cation is the prima facie example of homoaromaticity, and therefore any useful definition of homoaromaticity has to cover this example. Haddon has calculated [at the HF/6-31G(d) level of theory using 5 d functions] the PES of the homotropenylium cation as a function of the 1,7 interaction distance by optimizing the geometry of the molecule for fixed values of R (1,7)M. The results of his POAV analysis are summarized in Figure 8. 

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. 

In general, there is no strong evidence to support homoaromatic formulations of the structures of any of these systems. There are indications from PE spectroscopy of some degree of interaction between the unsaturated fragments of these molecules. However, as we have pointed out, PE spectroscopy as a technique has limited value in probing homoaromaticity. Magnetic evidence has either not been examined in detail in most systems or, where chemical shifts have been examined, is not definitive. Thermochemical... 

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. 

The problem of the reference compound is inherent to most chemical concepts. By definition a suitable reference compound is a compound that possesses the same properties as the target compound with the exception of the electronic and structural features to be investigated. In most cases, such a compound cannot be found since changes in the (electronic) structure automatically lead to changes in all properties and hinder meaningful comparison. This is the reason why many chemical concepts are discussed at a qualitative rather than a quantitative level. In fact, as has been forcefully described by Binsch, attempts to quantify a concept very often lead to the collapse of the whole concept. This potential collapse-by-quantification problem exists for the concepts of homoconjugation and homoaromaticity just as it does for the concept of aromaticity. 

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