Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)(2)C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G** and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (C-alpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. C-13 NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. C-13 NMR chemical shifts of the related tert-butyl cation ((CH3)(3)C+ ) and protonated tert-butyl dication ((CH3)(2)CCH42+) were also computed at the same level to compare and explore the effect of the additional charge in dications.