Qualitative Potential Energy Surfaces. 2. Cycloadditions

The linear combination of fragment configurations (LCFC) method is used to construct potential energy surfaces for [2π2 + 2πs] and [2πs + 2πa] cycloadditions which constitute models for Huckel antiaromatic and Mobius aromatic thermal reactions as well as models of Hückel aromatic and Mobius antiaromatic photochemical reactions. The factors which control the heights of barriers and the stabilities of possible intermediates in thermal reactions are discussed. More importantly, an understanding of the key features of photochemical reactions is now made possible. In this connection, it is shown that barrier heights as well as decay efficiencies play crucial roles.