Applications of a simple molecular wavefunction. Part 3.—Ethyl and propyl carbonium ions

Abstract

Calculations have been performed on bridged and classical isomers of the ethyl and propyl cations, using the FSGO model, and with extensive parameter searches. Geometries are in reasonable agreement with those obtained by Lathan et al. and by Radom et al., using an STO–3G basis. The FSGO results indicate that classical structures are more stable than bridged. The simple FSGO calculations were extended by the use of independently floating double gaussian functions in the bridged systems. For the ethyl ion, the classical structure is still found to be the most stable one. The double gaussian calculations on the propyl ion have been related semi-empirically to the FSGO results, and a potential surface has been constructed for the C₃H⁺₇ system. The secondary C₃H⁺₇ ion is found to have the most stable structure, in agreement with experiment. The results indicate that it is possible for the primary ion to lie at a comparatively shallow potential minimum. A distorted edge-protonated cyclopropane structure, with a definitely non-classical geometry, is predicted to lie 23–27 kJ mol^–1 in energy above the secondary ion. This coincides with a possible interpretation of an ion cyclotron resonance (i.c.r.) experiment in which a protonated cyclopropane species reacted with methanol; ΔH_f of c-C₃H⁺₇ is estimated to be ca. 29 kJ mol^–1 higher than that for the secondary isomer. Other results indicate that the complex between c-C₃H⁺₅ and H₂ is bound by 12 kJ mol^–1, which makes this type of structure a good candidate for a highly reactive C₃H⁺₇ species. Methyl migration is found to be an unfavourable process in the gas phase.