Quantum Organic Photochemistry. IV. The Photoisomerization of Diimide and Azoalkanes

Potential energy surfaces for the isomerization of diimide and azomethane in the ground, 1 (n,π*), 3 (n,π*), and 3 (π,π*) states have been calculated by ab initio molecular orbital methods. Two mechanisms are considered in detail, the first involving in-plane motion of the substituent group and the second involving twisting about the N—N bond. The first mechanism is preferred for the ground and 3(π,π*) states, whereas the second mechanism is preferred for the (n,π*) states. A significant barrier to rotation is predicted for the 3 (π,π*) state. The vibrationally-relaxed (twisted) lowest triplet of diimide is predicted to lie 36 kcal mol −1 above the ground state.