Homoconjugative interaction

Homoconjugational interactions and nonbonding intramolecular contacts of the four ji-systems are responsible for the observed distribution of bond angles at the central atom, which are also significantly inequivalent [105.1(1)° to 111.8(1)°]. 

Thermochemical and X-ray studies on the simplest 7r-trishomobenzene, cisxisxis-1,4,7-cyclononatriene [8], indicate that this molecule is not homoaromatic (Roth et al., 1964). However, photoelectron spectroscopy clearly demonstrates that there are considerable homoconjugative interactions within [8] (Bischof et al., 1970 Martin and Mayer, 1983). These seemingly contradictory conclusions can be reconciled when it is realized that, using the PES data, the delocalization energy in [8] is estimated to be only about 5% of that in benzene. The manifestations of such a small delocalization energy would be very difficult to detect experimentally by means of NMR spectroscopy. X-ray crystallography, or thermochemistry. 

In the acetylenic analogue of [111], 1,5,9-cyclodecatriyne [119] there is certainly a strong homoconjugative interaction between the acetylenes (as evidenced by the UV and photoelectron spectra). However, the molecule is... 

It is therefore improbable that homoaromaticity in neutral radicals will be of any importance. However, as already discussed, the driving force for delocalization of charge is high, and hence homoconjugative interactions in charged radicals are significant (Roth, 1987). 

The relative importance of through-bond (hyperconjugative) and through-space (homoconjugative) interactions between the heteroatom and the double bond in 2,5-dihydro-furan, -thiophene and -pyrrole has been the subject of a CNDO/S study (76ZN(A)215). This analysis concluded that the proportion of through-space interaction increased from 19% in the dihydrofuran and 20% for dihydrothiophene to 83% for the dihydropyrrole (cf. Section 2.3.3.9). 

A successive polarization of the exocyclic double bond, as monitored by the carbon shifts in the 7-methylene derivatives of norbornane, norbornene and norbornadiene (Table 4.12), is attributed to homoconjugative interaction of it bonds, pushing it electrons towards the exo methylene carbon [239]. 

In order to clarify the role of cyclopropyl homoconjugation within the concept of homo-conjugation, it is appropriate to classify the various types of homoconjugative interactions according to the following seven critera ... 

Considerable homoconjugative interactions could also be retained if X in a homocon-jugative system is a heteroatomic group such as NH, O, SiH2, PH, S, etc. Indeed, it is possible to encounter homoconjugative interactions with almost any heteroatomic group... 

Recent investigations by Szabo and Cremer35 have shown that Si can also participate in homoconjugation and homoaromaticity. This may also be true for atoms such as Ge or Sn. There are many more possibilities of homoconjugative interactions than have been considered thus far in the literature. 

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

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. 

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