Ab initio calculations of the ring-opened and ring-closed isomers of C2H4O+: The need for high level electron correlation techniques

Ab initio calculations, including electron correlation, have been performed with split-valence plus polarization basis sets for isomeric C2H4O+ radical cations. MP2/6-31G* calculations do not predict correct geometries for the lowest energy cation. This is attributed to spin contamination as observed in a study of the thiirane radical cation. Symmetry breaking in the closed, C-2v. cation is observed at the MP2 level. CI calculations are necessary to eliminate this effect. The lowest energy ring-opened CH2OCH2+ isomer is predicted, at the G2/QCI level, to be 26.2 kcal mol(-1) lower than the ring-closed radical cations.