Pseudo Jahn–Teller origin of cis–trans and other conformational changes. The role of double bonds

Based on the pseudo Jahn–Teller effect (PJTE) theory, an approach is developed to rationalize and predict the conformations and conformational changes in molecular systems with a common pattern, a double bond. It is shown that starting with the high-symmetry geometry of the environment (in many cases D2d), the double bond descends from an e2 electronic configuration (e is a twofold degenerate MO) which produces a variety of PJT distortions, the main of which is the rotational (b1) transformation D2d → D2h accompanied by the formation of the double bond. Further PJT interactions with higher energy E-states may trigger additional distortions which in D2h symmetry are classified as in-plane (ei) cis and trans, and out-of-plane (eo) chair and boat. The realization of these conformations depends on the positions of the excited E-states and the PJTE parameter values. The two emerging PJTE problems, (3A2 + 3E1 + 3E2) ⊗ (ei + eo) and (1A1 + 1B1 + 1B2 + 1E1 + 1E2) ⊗ (b1 + ei + eo), are formulated in the matrix form and provide a general picture of the ground and excited adiabatic potential energy surfaces. Following this scheme in combination with ab initio calculations, the possible conformations and conformational transitions are analyzed for several specific systems including (in increasing complexity) N2H2, C2H4, N2(NH2)2 and N2(C6H5)2 (azobenzene). The family of molecular systems with a double bond is vast, but the importance of the PJT approach developed here is also in its general validity as it can be applied to any other systems.