Potential Homoaromatics

The electronic properties of potential homoaromatics are effectively probed by means of photoelectron spectroscopy (PES). In this technique electrons are ejected from the valence level of the molecules under... 

As stated at the beginning of this Introduction, (homo)aromaticity refers to a special (thermodynamic) stability relative to some hypothetical reference state. It is therefore most attractive to use a thermochemical discriminator for the designation of homoaromaticity. However, such thermochemical methods suffer the same disadvantages when applied to homoaromaticity as they do in the case of aromaticity (see for example Garratt, 1986 Storer and Houk, 1992). There have been several recent studies using the heats of hydrogenation of potential homoaromatics in an attempt to classify these species (vide infra). Due, in the main, to the hypothetical nature of the localized model reference states there is some debate regarding these results (see Dewar and Holder, 1989 Storer and Houk, 1992). 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

One of the most used approaches for predicting homoaromaticity has been the perturbational molecular orbital (PMO) theory of Dewar (1969) as developed by Haddon (1975). This method is based on perturbations in the Hiickel MO theory based on reducing the resonance integral (/3) of one bond. This bond represents the homoaromatic linkage. The main advantage of this method is its simplicity. PMO theory predicted many potential homoaromatic species and gave rise to several experimental investigations. 

With the increasing possibilities of doing semi-empirical or even small basis set ab initio calculations on homoconjugated compounds, PMO theory was used both to predict and to rationalize the results of quantum chemical calculations on potentially homoaromatic... 

In the seventies and eighties, ab initio calculations on potentially homoaromatic molecules were preferentially carried out with the Hartree-Fock (HF) method using minimal or double-zeta (DZ) basis sets. However, neither HF nor small basis sets are appropriate to describe a homoaromatic system. In the case of cyclopropyl homoconjugation, the use of a DZ + P basis set is mandatory since polarization (P) functions are needed to describe the bond arrangements of a three-membered ring. 

Schleyer was the first to fully realize the sensitivity of calculated NMR chemical shifts with regard to molecular geometry and use this for ab im n o/IGLO/NMR-based structural determinations in many cases"6. Cremer and coworkers realized the usefulness of this approach for the determination of geometries of potentially homoaromatic compounds not amenable to experiment49-58. 

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. 

As typical of many other attempts to describe homoconjugative interactions with the help of bond orders, we mention here recent investigations of Williams, Kurtz and Farley ". These authors used various semi-empirical methods (MNDO, AMI, MINDO-CI, AMI-Cl) to study cycloheptatriene, l,6-methano[10]annulene, elassovalene and some other potentially homoaromatic compounds. For the 1,6 interactions in cycloheptatriene and l,6-methano[10]annulene, small bond orders <0.1 were calculated suggesting the absence of homoconjugative interactions although homoaromatic character is generally accepted in the case of the l,6-methano[10]annulene. The authors concluded from this that bond orders seem to be of no use as possible discriminators of homoconjugative interactions ". ... 

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