Methyl-Substututed Cyclopropylcarbinyl (Alias Vinyl-Bridged Ethylene) Carbocations. Molecular Orbital Calculations and Criteria for Structure Representation-Nomenclature Decisions

Article abstract of DOI:10.1021/ja00373a045

The question of whether nortricyclylcarbinyl cations 1 have an enhanced vinyl-bridging 2-norbornyl 1b character has been probed by means of molecular orbital calculations.In order to do this, one needs data on model methyl-substituted cyclopropylcarbinyl cations, and 14 different systems, together with the parent cations, have been included in this study.Optimized geometries were evaluated by using the MNDO procedure and energies obtained by STO-3G ab initio method.Calculated heats of formation agree well with experiment and the stabilizing influence of methyl groups on the parent cyclopropylcarbinyl cation correlate well with solvolysis date.The HOMO level for most of the cations is the ?-MO, a result at variance with a simple perturbation treatment but one which can be easily rationalized.Two procedures are suggested for semiquantitatively evaluating the "mix" of the two extreme resonance structures in the actual structure, and both of these methods give similar results.Applying one of these criteria to the nortricyclylcarbinyl cation case, one finds indeed that there is an enhanced vinyl-bridging character compared to the simple cyclopropylcarbinyl model.