TETRAMETHYLENE

Ab initio MCSCF and CI computations of the potential energy surface (PES) for the tetramethylene biradical lead to a new model for the stereochemical dynamics. The model postulates that tetramethylene loses stereochemistry during its formation and decay as well as during its lifetime. For example, cis-cyclobutane-1,2-d2 can form tetramethylene with cisoid or transoid deuteration directly via the same transition state. This shared transition state model accommodates the different stereochemical properties reported experimentally in tetramethylenes generated from deuterated cyclobutane vs diazene precursors. The model describes a generalized common biradical in which the ratio of fragmentation to cyclization is independent of precursor but whose stereochemical properties may depend on the precursor. The computations detect shallow minima corresponding to gauche and trans biradicals that disappear when zero-point energy is included. Use of canonical variational transition state theory, with close attention to internal rotations, leads to the prediction that both gauche and trans conformers exist as entropy locked species at experimental temperatures. The gauche conformer is the common biradical intermediate deduced by experimentalists. On this PES the preference for allowed 2s + 2a cycloaddition is weakly maintained even though a biradical is involved. Of the seven saddle points located on the PES, at least four of them mediate more than one reaction. These reactions typically involve stereochemical changes, so that a given transition state leads to two stereochemically distinct products. This gives rise to the shared transition state model mentioned above and also to the conclusion that transition state theories cannot predict the stereochemistry of products derived from tetramethylene.