AbstractsResults of semiempirical calculations (CNDO/~-FK and M1ND0/2 methods) for the a-?r complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data ( Em+ E,,,,,) concerning the classical-nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H' system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (u-complex) and nonclassical (n-complex) cation. The so called strong ?r-complex is a saddle point between two cr-complex minima and can be interpreted as transition state of 1,2-proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed. (EHF+ E,,,,,,)

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