A mechanistic study of the ring-opening reaction of singlet oxirane

Abstract

The C-C ring opening reaction of the oxirane molecule on the singlet potential energy surface has been investigated within the context of ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration SCF (TCSCF), and configuration interaction with single and double excitations (CISD) levels of theory are employed in conjunction with two basis sets, double-[zeta] plus polarization (DZP) and triple-[zeta] plus double polarization (TZ2P). For the SCF and TCSCF methods, the TZ2P basis set supplemented with higher angular momentum functions, TZ(2df,-2pd), is also used. Ten stationary points are located on the singlet reaction surface and unambiguously characterized by analyzing the energy second derivatives (Hessian). Three minima are located on the surface corresponding to the closed-shell ring structure (as reactant) and two closed-shell open structures (as products) of C[sub 2] and C[sub s] symmetry. These minima are shown to be connected via four genuine transition states along the corresponding reaction coordinates. Planar open oxirane with C[sub 2v] symmetry (edge-to-edge, E-E isomer) is characterized as neither a minimum nor a transition state but as a higher order stationary point with Hessian index 2. The electrocyclic ring opening of the C-C bond in oxirane is shown to occur preferably through a conrotatory double methylene rotationmore » pathway leading to the C[sub 2] open structure with an activation energy of 52.0 kcal/mol with the zero-point vibrational energy (ZPVE) correction. 57 refs., 7 figs., 3 tabs.« less