| Summary: | The mechanism of the cycloaddition reaction forming a bicyclic compounds between singlet vinylidene (H2C=C:) and ethylene was investigated using the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that this reaction has one dominant channel. The presented rule of this reaction, a [2+2] cycloaddition reaction between the two reactants occurred forming a four-membered ring carbene (INT1), in which the sp lone electron of the C atom from carbene in INT1 and the π* unoccupied orbital of ethane form the sp®π* donor–acceptor effect, resulting in the formation of intermediate (INT2). Due to the further sp3 hybridization of C atom from carbene in INT1, INT2 isomerizes to the bicyclic compound (P2) via the transition state (TS2).
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