Mechanism and Dynamic Correlation Effects in Cycloaddition Reactions of SingletDifluorocarbene to Alkenes and Disilene

摘要

Mechanisms of the cycloaddition reactions of singlet difluorocarbene (CF_2) to alkenes and disilene werestudied using CASSCF, MR-MP2, CR-CC(2,3), and UB3LYP methods in combination with basis sets up to6-311++G(3d,p). The CASSCF(4,4) energies suggest that the cycloadditions all follow the stepwisemechanism. However, energies calculated using the MR-MP2(4,4) and CR-CC(2,3) methods in combinationwith the 6-311G(d) or larger basis sets consistently show that the reactions follow a concerted mechanism.The stepwise mechanisms predicted at the CASSCF level are "artificial" because of their neglect of dynamicelectron correlation effects. The importance of dynamic electron correlation in determining the mechanisticnature of the reactions is explained through knowledge of the reacting system's geometries and charges alongthe reaction path.