The isomerization and rearrangement of the radical cations of the title compounds were studied using semiempirical (AM1) and ab initio (HF/6-31G*) molecular orbital calculations. The energetics for these interconversions were further defined using ab initio methods which include electron correlation (MP2/6-31G*//HF/6-31G*). Several energetically favorable interconversions were identified; however, it is clear that even though some of these reactions are exothermic, geometrical constraints will inhibit some from taking place. The most exothermic reactions were the ion-molecule reactions combining neutral methylenecyclopropane (1) with its radical cation (6 or 7) to give a dimer radical cation (8, 9, 10, 14, 17a, or 25). These dimerization reactions were considered in more detail, beginning with different initial orientations of the reactants, 4 Angstrom apart, without imposing any symmetry constraints. Bonding occurred in five cases, but the anticipated cyclic dimer(s) were not found. Four of the dimers (31, 34, 35, and 37) were conformers : 31 and 34 are gauche; 35 and 37 are anti. In the gas phase the gauche conformers are of lower energy than the anti conformers by 0.3 kcal/mol. These dimeric radical cations are expected to arise in gas phase experiments (mass spectrometry) and matrix isolation studies.