Spiro versus Planar Transition Structures in the Epoxidation of Simple Alkenes. A Reassessment of the Level of Theory Required

Abstract

High level ab initio and CASSCF calculations on the transition structure for the peroxyformic acid epoxidation of ethylene have been carried out to distinguish between a spiro versus a planar orientation of the peracid. The optimized spiro CASSCF (12,12)/6-31+G(d,p) transition structure (Figure 1a) is a first-order saddle point that is 4.0 kcal/mol lower in energy than the corresponding planar TS after correction for dynamic correlation [CASSCF(MP2)]. The planar TS is 11.5 kcal/mol higher in energy than an unsymmetrical spiro TS. A RSPT2 (6,6)//CAS(12,12)/6-31G(d) correction also favors the spiro TS by 5.3 kcal/mol. Single-point calculations on the spiro and planar CASSCF (12,12)/6-31+G(d,p) transition structures at the UB3LYP, UQCSD(T)and UBD(T) levels favor the spiro symmetrical TS by 9.0, 7.9, and 5.4 kcal/mol [/6-31+G(d,p)]. The objective choice of the active space is demonstrated to be critical to the transition structure obtained.