Structure and Stability of the Carbocations C2H3+ and C2H4X+ X=H, F, CI, and CH3. Ab Initio Investigation Including Electron Correlation and a Comparison with MINDO/3 Results

Ab initio calculations both at the SCF level and with inclusion of electron correlation in the framework of the IEPA-PNO, PNO-CI, and CEPA-PNO methods were performed for the title molecules. The geometries were taken from previous ab initio calculations. Electron correlation turns out to be crucial for a satisfactory description of relative stabilities. For C2H3+ and C2H5+ we compute, with the CEPA method, the nonclassical structures to be more stable than the classical ones by 4.0 and 6.4 kcal/mol, respectively. CH2FCH2+ (nonclassical) is only slightly more stable than CH2FCH2+ (classical) (Δ E(CEPA) = 1.4 kcal/mol) whereas for the analogous chlorine compounds Δ E(CEPA)=30.0 kcal/mol. The most stable C3H7+ isomer is the isopropyl cation. We find edge-protonated cyclopropane more stable than corner-protonated cyclopropane by 5 kcal/mol. For the pure hydrocarbon systems MINDO/3 results are in good agreement with the CEPA values. In the case of the haloethyl cations the MINDO/3 method exaggerates by far the stability of the nonclassical structure. In the MINDO/3 calculations all structures were characterized by the matrix of force constants.