Dynamics of singlet biradicals. Application of ab initio variational transition state theory to the fragmentation of tetramethylene

Abstract

The fragmentation of the anti conformation ( 1) of the singlet tetramethylene biradical was studied by canonical variational transition state theory using ab initio wavefunctions. The minimum energy path for fragmentation was calculated using a 6-31G basis set and a 4-electron-4-orbital (4-in-4) CASSCF wavefunction. Energies were corrected by a truncated singles-and-doubles multi-reference CI based on all references of a 4-in-4 CASSCF using a 6-31G* basis set. The fragmentation has no calculated potential energy barrier, but does have a small free energy barrier due to the decrease in entropy associated with a tightening of the terminal CH 2 rotations as fragmentation proceeds. The fragmentation rate is predicted to decrease slightly with increasing temperature. The calculations are consistent with, but do not demand, the hypothesis that 1 is a mechanistically insignificant intermediate, and that it fragments as soon as it is formed.