Ab initio molecular orbital calculations on the interconversion of allene and propyne cation radicals and the mechanism for hydrogen loss from C3H4+·

Ab initio molecular orbital calculations (6-31G*/MNDO) of the C₃H₄^+· potential energy surface (electronic ground state) reveal the following features. The molecular ions of allene (1) and propyne (2) are separated by substantial potential energy barriers which preclude easy interconversion. The minimal energy requirement path for the 1 ⇌ 2 isomerization proceeds via two successive 1,2-hydrogen migrations and involves the as yet unknown stable, linear C₃H₄^+· ion 5. Isomerization via a direct 1,3-hydrogen migration is, if it is involved at all, energetically less favoured. The ion 5 also serves as the central intermediate for ring closure to the cation radical of cyclopropene (4), which itself is the actual precursor for loss of H^· thus generating the cyclopropenylium ion (3). The complete geometric data of the various C₃H₄^+· isomers and the transition states connecting them are reported.