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, C2v, 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.