HOSE model

The HOSE model has already been reviewed [46,47], but on the basis of its original presentation, without practical hints for further applications. We make an attempt to present the HOSE model in a way which approaches closer the way of thinking used in everyday practice of organic chemistry. Moreover, we wish to present it in such a way that will give a better insight into how the model may be applied to some new problems. Within the frame work of the HOSE model, the following assumptions are made ... 

Table III comprises parameters used in the HOSE model.

Table III. Reference bond lengths and empirical parameters for the HOSE model [44-45] ...

Some comments are needed to explain the choice of assumptions made in the HOSE model. It is also useful to take into account experience gathered after about of 15 years of its application. 

The following conclusion may be drawn from the above results the CN bond in nitrobenzene is almost cylindrical indicating a very low contribution of the n -electron component. In contrast to them, the typical aromatic CC bonds in the ring are significantly elliptical - as expected from the chemical intuition and experience. Thus the results of experimental charge density studies are in line with the much simpler treatment based on the HOSE model and precisely measured bond lengths. 

Fig. 17. Resonance structures of 2,4,6-trimethoxy-5-triazine and their weights computed from ab initio 6-31IG (d,p) geometry by use of the HOSE model.

Table 8. Empirical Constants to Bird s Modification of the HOSE Model...

The estimated stabilization energies from the HOSE model are in line with those obtained in other ways. It was shown that HOSE correlates well with the Hess and Schaad resonance energies (correlation coefficient R = 0.991 for 22 alternant TT-electron hydrocarbons R= 0.937 for 12 nonalternant hydrocarbons). In the last case, the Hess and Schaad RE values were given for nonalternant hydrocarbons whereas geometries used to calculate the HOSE values originated from substituted species. Since the substituent effect in nonalternant systems is quite considerable, these aspects will be discussed later. 

The HOSE model was originally successfully used to estimate the weights of canonical structures, Ci for the i-structure, by use of a formula... 

The application of the HOSE model permitted estimation of aromaticity in the cases of very small changes in aromatic character of the molecule in question. This can be done by assuming equal or almost equal weights of basic canonical structures as encountered in benzene or pyridine. 

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