DENSITY-FUNCTIONAL STUDIES OF CARBONIUM-IONS

The structure and bonding of all isomeric forms of the carbonium ions CH5+, C2H7+, C3H9+, n-C4H11+, i-C4H11+ and neo-C5H13+ have been studie d in depth by various quantum chemical techniques including semi-empir ical, ab initio Hartree-Fock (HF), second-order Moller-Plesset (MP2) t heory and those based upon density functional theory (DFT). Of the met hods chosen, MP2 and selected DFT functionals are shown to provide ene rgetic data comparable to those experimentally available, whilst ab in itio Hartree-Fock techniques are found to be ineffective in modelling such systems. Semi-empirical methods are found to give adequate descri ptions of the molecules, especially for the longer-chain carbonium ion s.A comparison of the binding energies shows that, although the smalle r carbonium ions are the most stable relative to their dissociation, t hey are the least likely to be formed via protonation. The relative st ability of the ions to their conjugate bases shows a trend very simila r to that witnessed in the solution phase.