MP2-R12 CALCULATIONS ON THE RELATIVE STABILITY OF CARBOCATIONS

The MP2-R12 method (second-order Møller-Plesset perturbation theory with linear r12-dependent terms) is briefly presented, and applications to the ions CH5+, C2H3+, C2H5+, and C3H7+ are described. With the MP2-R12 method one comes rather close to the basis set limit of MP2 theory, although the MP2-R12 method does not provide strict upper bounds. The basis sets used here are a compromise between what one ought to use for perfectly reliable calculations and what one can afford. The results for CH5+ do not change much with respect to conventional calculations. Only the C2v structure becomes energetically much closer to the Cs equilibrium structure (difference only ∼0.2 kcal/mol) than in the previous most sophisticated calculations. For C2H3+ and C2H5+ the energy differences between classical and nonclassical structures are larger by ∼2 kcal/mol as compared to conventional MP2 calculations. We predict that the nonclassical ions C2H3+ and C2H5+ are more stable by 4-5 and 7-8 kcal/mol, respectively, than the classical ones. In the case of the isopropyl cation, C3H7+, it is confirmed that a "chiral" C2 structure has the lowest energy. © 1990 American Chemical Society.