摘要：

The potential energy surface involving the cyclopropylcarbinyl and cyclobutyl cations is investigated with ab initio molecular orbital calculations. The relative energies of many proposed structures are given at the MP4SDTQ/6-31G*// MP2(FULL)/6-31G* level of theory and compared with experimental observations of the stable ion in solution. The puckered cyclobutyl cation (symmetrical bicyclobutonium ion) is slightly less stable than the bisected cyclopropylcarbinyl cation, but both are energy minima. Roberts' unsymmetrical bicyclobutonium ion is the apparent transition state that connects these species. The chemical shifts of the stable ion solution at various temperatures are found to be in close agreement with those calculated with the IGLO approach. The pathway of interconversion between the cyclopropylcarbinyl and bicyclobutyl ions is studied by using electron correlated optimizations away from the symmetric stationary points. The 3-fold degenerate rearrangement barrier is very low, but possible "tricyclobutonium ions" with 3-fold symmetry are not involved. Mechanisms that interchange the protons within the ionic system are discussed and compared with experimental limits. Hydrogen exchanges via a planar cyclobutyl cation and via 1,2-H shifts both appear to be possible but 1,3-H shifts require much higher activation.

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