MOLECULAR ORBITAL CALCULATIONS ON CARBONIUM IONS .2. METHYL, ETHYL, AND VINYL CATIONS . SERIES C3H7+

Semiempirical NDDO and ab initio Hartree-Fock-Roothaan SCF calculations are reported and compared for the isomeric structures of formulae C2H3+ and C2H5+. The NDDO (neglect of diatomic differential overlap) scheme is described. It is found that NDDO and all less complete schemes overestimate the stability of bridged ions, relative to the much more rigorous ab initio method. Thus NDDO favors the protonated acetylene over the vinyl cation structure by 32.0 kcal/mole, and the protonated ethylene over the ethyl cation structure by 33.2 kcal/mole. Ab initio calculations show the unbridged species to be more stable: the vinyl cation is favored by 25.1 kcal/mole, and the ethyl cation by 9.0 kcal/mole. NDDO calculations predict that an α-methyl substituent on a vinyl cation stabilizes the ion by 1.0-1.5 eV (ca. 20-35 kcal/mole) more than the methane-ethane or ethylene-propene energy differences. However, methyl substitution on protonated acetylene gives no such “ext a” stabilization. NDDO calculations show the 2-propyl cation to be 18 kcal/mole more stable than the 1-propyl cation, in good agreement with experimental values. Edge-protonated cyclopropane is calculated to be 81 kcal/mole more stable than 2-propyl cation, but the validity of this result is considered dubious on theoretical grounds. Edge-protonated cyclopropane is computed to be 137 kcal/mole more stable than the face-protonated isomer by NDDO, in good agreement with the ab initio result of 125 kcal/mole. NDDO predicts corner-protonated cyclopropane to be some 20 kcal/mole less stable than the edge-protonated isomer. © 1969, American Chemical Society. All rights reserved.