The existence and stability of singlet tetramethylene biradicals: an ab initio MCSCF/MP2 study

The formation and evolution of singlet tetramethylene biradicals that occur in the dimerization of ethylene to cyclobutane has been investigated using ab initio MCSCF and MCSCF/MP2 methods with double-zeta and 6-31G* basis sets. Two minima, corresponding to a trans and gauche structure, have been located. In contrast with earlier studies, when the contribution of dynamic correlation is computed at the MP2 level, the fragmentation barriers associated with these two minima do not disappear. However when the zero-point vibrational energy corrections are taken into account, the region of the surface corresponding to the closure to cyclobutane becomes extremely flat and the gauche minimum disappears. These results seem to indicate that the tetramethylene biradical exists as a single species with a trans geometry. The MP2 results yield an activation barrier of 44.70 kcal/mol with the 6-31G* basis and 43.21 kcal/mol at the double-zeta level; the experimental value is 43.8 kcal/mol. © 1992.