Dimerization of Methylenes by Their Least Motion, Coplanar Approach: A Multiconfiguration Self-Consistent Field Study

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Abstract
The one-dimensional path of reaction in the least motion, coplanar approach of two methylenes to form ethylene has been calculated using the nonempirical multiconfiguration self-consistent field (SCF) method in a large Gaussian basis. Orthogonality between open- and closed-shell molecular orbitals of the same symmetry is constrained by a “partitioned basis set” technique which is found to give rapid and uniform convergence to the proper SCF solution. The results obtained for the reaction path suggest that the ground state of bent (∼ 120°) methylene is the triplet. Therefore, in this study, it is the triplet states (and not closed-shell singlet states) of two appropriately oriented bent methylenes that correlate with the normal (ground) state of ethylene. For two closed-shell singlet-state methylenes the reaction path is found to be purely repulsive.