The reaction path of cyclooctatetraene dimerization revisited

The electron density and the electron localization‐delocalization matrices (LDMs) are used to revisit the cyclooctetraene (COT) dimerization mechanism. The global minimum of a COT monomer (the tub geometry) exhibits a rare topological feature, giving rise, not to one, but to two ring critical points and a cage critical point necessary to satisfy the Poincaré‐Hopf relation. The energy profiles were used to identify the rate‐limiting step of the reaction: the penultimate transition state of a series of five transition states in total. While the monomers themselves have zero dipolar polarization, the dimer has a nonnegligible total dipole moment reaching almost 1 debye with fluctuations in strength and orientation along the reaction path. The reaction can hence, possibly, be manipulated with intense laser fields. This classic reaction has been used to elucidate whether the electron LDMs reflect structural or energetic similarity. It is found that LDMs are excellent monitors of structural/electronic similarity between different species on the reaction coordinate. A reaction can be characterized by a three‐dimensional hypermatrix whereby the matrix elements change as a function of the reaction coordinate, which can be represented as a parallelepiped of matrix elements. A study of the electron density of the system as the reaction progresses identifies and characterizes the bond paths that drive the reaction. It is hoped that this dynamic picture of the evolution of the electron density complements the usual arrow pushing reaction mechanisms used by organic chemists. A representation of the evolution of the electron localization‐delocalization matrix (LDM) of the system (dimerization of cyclooctatetraene) as a function of the reaction coordinate.