OXIRENE - TO BE OR NOT TO BE

The C-2-symmetry structure of oxirene has been examined using ab initi o molecular orbital calculations with large basis sets and a variety o f methods of including electron correlation. Different qualitative con clusions regarding the nature of oxirene are reached, depending on the choice of basis set and method of electron correlation incorporation. With certain combinations of basis set and theoretical method, the sy mmetric oxirene structure is found to be a saddle point on the potenti al energy surface, while for other combinations oxirene is a local min imum. Inclusion of triple excitations in the correlation treatment has a large effect, tending to make the curvature of the surface correspo nding to a ring-opening distortion of the C-2v-symmetry structure less positive (or more negative). This is counterbalanced by a basis set e ffect, with inclusion of f functions making the curvature more positiv e. At our highest level of theory, CCSD(T) with basis sets of triple-z eta quality and including multiple d shells and an f shell on C and O and multiple p shells and a d shell on H, oxirene is a genuine minimum under harmonic vibrational analysis, with a ring-opening frequency of 139-163 cm(-1)