THE C4H5-ENERGY SURFACE - STRUCTURE, RELATIVE ENERGIES, AND ENTHALPIES OF FORMATION OF ISOMERS OF C4H5+( POTENTIAL)

The potential energy surface for C4H5+ as calculated by ab initio mole cular orbital theory is reported at two levels of theory, HF/6-31G(d,p ) and MP2(full)/G-311G(d,p). Fourteen minima have been located at HF/6 -31G(d,p), but inclusion of electron correlation reduced this number t o nine. The methylcyclopropenyl cation, 1, is the global minimum, and the 2-cyclobutenyl (2), alpha-vinylvinyl (3), gamma-methylpropargyl (4 ), alpha-methylpropargyl (5), and 1-cyclobutenyl (6) cations are 9.1, 19.9, 25.3, 27.5, and 26.9 kcal/mol, respectively, above 1. Eleven tra nsition structures, permitting interconversion between the nine minima , are reported. Enthalpies of formation (in kcal/mol) calculated at MP 4SDTQ/6-311++G(2df,p) are 231.4 for 1, 241.7 for 2, 246.9 for 3, 255.7 for 5, 259.9 for 6, and 264.3 for 7.