Quantenchemische modellierung der potentialhyperfläche des äthylkations im bereich der klassischen und nichtklassischen strukturen

Properties of the potential energy surface of protonated ethylene are calculated by semiempirical methods (CNDO/2-FK, MINDO/2, MINDO/3). These methods were chosen in respect of the agreement with new strong energy data concerning the relative stability of the nonclassical and a classical structure of the ethyl cations. They give no stationary point in that region of the hypersurfaces where until now was assumed to exist the most stable of the classical ethyl cations. The H-eclipsed classical cation is a saddle point on the CNDO/2-FK- and the MINDO/2-hypersurface, which represents the barrier of potential energy for the H-exchange in the ethyl cation; on the MINDO/3-surface a flat sea modified the character of the saddle point. Finally the results of the different quantum chemical methods (including NDDO and ab initio) are analysed concerning their agreement in calculating geometry and the distribution of electron density in the ethyl cations.