Intramolecular Nonbonding Interactions and Geared Phenyl Twisting in Para‐Disubstituted 1,4‐Diphenylazines: Electron Correlation Effects on Molecular Conformations and Enantiomerization

ABSTRACT The results are reported of potential energy surface (PES) analyses of six symmetrical azines Y p − Ph − RC=N − N=CR − Ph − Y p , namely, the benzaldehyde azines 1H , 2H , and 8H with R = H and the acetophenone azines 1M , 2M , and 8M with R = Me. The Y substituents Cl ( 1 ), Br ( 2 ), and Me ( 8 ) were studied because sets of polymorphs I ( C 2 ‐symmetry, azine twist τ ≈ 135 ± 10°, disrotatory phenyl twists ϕ i ≠ 0°) and II ( C i ‐symmetry, τ = 180°, conrotatory ϕ i ≠ 0°) were crystallized for these three azines. The azine‐Me groups in ( E , E )‐acetophenone azines cause steric strain with the N lone pairs, and this strain is reduced by small rotations about the Az‐CH 3 bonds and geared phenyl twisting may lead to C 2 ‐ or C i ‐structures. The in‐depth exploration of the ( τ , ϕ 1 , ϕ 2 ) conformational space on the MP2(full)/6‐311G* PES of 1M shows that the enantiomerization of C 2 ‐ 1M to C 2 ‐ 1M ’ involves, first, one Ph twist inversion to a C i ‐like structure, the subsequent inversion of the azine twist in, and the, second, Ph twist inversion on the path to C 2 ‐ 1M ’. The essential characteristics of this enantiomerization mechanism apply in general to disubstituted acetophenone azine.