An Mo-theoretical characterization of organic reaction mechanisms—IV : Thermal (2+2) reactions

Various types of thermal (2+2) reactions are mechanistically differentiated in terms of the phase continuity plus the triplet stability of the restricted Hartree-Fock molecular orbitals relevant to the entire reaction systems. The former criterion allows to discriminate between the symmetry-allowed and -forbidden reactions while the latter permits distinction between biradical and nonradical reactions. It follows that the two-step cycloaddition involving a zwitterionic intermediate, for instance, should be characterized as a symmetry-forbidden nonradical process. The mechanistic features theoretically inferred for several known examples of reactions are all found to be compatible with observation.