Aromaticity Hess-Schaad model

If other models of aromaticity are taken, which are also based on Hiickel jt energies but use more elaborate reference structures, opposite results were obtained, with benzene being more aromatic than Ceo-The Hess—Schaad scheme, in which the reference structure is given in terms of six independent parameters, gives a HSRE/e which is about one-half that of benzene (Table 4). A scheme developed by the Zagreb group affords values comparable to those obtained with the Hess—Schaad model. ... 

We have applied the Hess—Schaad version of the Dewar reference structure to the systems listed in Table 2. Theoretical investigations of the REPE index were also published. Changes in the energy of the o electrons in a series of compounds were found to vary linearly with though in opposite direction to ji energies. This provides some explanation of why jr-elec-tron-only models of conjugated systems can be as successful as they are. Reference structures were investigated."" " and comparisons were made with other recent theoretical aromaticity indices. ... 

A recent Hflckel calculation based on the proper choice of models correctly accounts for the observed differences in w-electronic makeup between lff-azonine (26a) and oxonin (24a) which were calculated to possess resonance energies of 0.139 and 0.0010, respectively [B. A. Hess, L- J. Schaad, and C. W. Holyoke, Tetrahedron 28, 3657 (1972)]. In terms of simple Hiickel theory the differences between 26a and 24a were attributed chiefly to the lower LVMO energy of the latter (A. G. Anastassiou, in Topics in Nonbenzenoid Aromatic Chemistry (T. Nozoe, R. Breslow, K. Hafner, S. Ito, and I. Murata, eds.), Vol. I, pp. 1-27. Hirokawa Publ., Tokyo, 1973). 

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