C4H4 radical cation isomers: Generation, structure, stability, and isomerization reactions

Abstract

The B3LYP, M06-2X and MP2 methods are used to study the generation, relative stability, structure, charge and spin distributions, along with isomerization reactions, of C4H4+ radical cations in gas phase. The highly endothermic generation of C4H4+ species by ionization of neutral C4H4 precursors is for 7 cases. Stability orders for 10 C4H4+ isomers as derived from the three methods correlate well with one another, where methylenecyclopropene radical cation is the global minimum. Available computational data also agrees well with these orders, and experimental data fairly well. The charge and spin distributions describe one isomer as distonic (with charge and spin separated). Concerted isomerization reactions for the methylenecyclopropene, butatriene, vinylacetylene and methyleneallene radical cations and 2 hypothetical higher energy species proceed via well-defined transition states described with regard to their relative positions along the reaction coordinate in accordance with Hammond’s postulate. These one-step isomerizations are combined to yield five multi-step conversions of various C4H4+ species to the lowest energy methylenecyclopropene isomer. The application of transition state theory and computation of rate coefficients leads to the estimation of activation energies for the concerted isomerizations.