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 studied in depth by various quantum chemical techniques including semi-empirical, ab initio Hartree-Fock (HF), second-order Moller-Plesset (MP2) theory and those based upon density functional theory (DFT). Of the methods chosen, MP2 and selected DFT functionals are shown to provide energetic data comparable to those experimentally available, whilst ab initio Hartree-Fock techniques are found to be ineffective in modelling such systems. Semi-empirical methods are found to give adequate descriptions of the molecules, especially for the longer-chain carbonium ions. A comparison of the binding energies shows that, although the smaller carbonium ions are the most stable relative to their dissociation, they are the least likely to be formed via protonation. The relative stability of the ions to their conjugate bases shows a trend very similar to that witnessed in the solution phase.