Why N₂O₂ is cis while (CHO)2 is trans: MO and VBstudies

The nonempirical valence‐bond method as well as the molecular orbital method are applied to investigate the structures of N 2 O 2 and (CHO) 2 . Results show that in either molecule the σ‐component tends toward a trans conformation, whereas, the π‐component tends toward a cis conformation. Moreover, even if there are some long‐range bonding overlaps in the cis structure, the bonding energy cannot counteract the increased Coulombic repulsive energy compared with the trans structure. In other words, for a cis conformation, there is a competition between the Coulombic destabilization and the π electronic stabilization. The latter is more preferable to a cis conformation of N 2 O 2 , while the Coulombic repulsive interaction should be responsible for the trans conformation of (CHO) 2 . The nonempirical vB calculations show that the π delocalization energy in N 2 O 2 is negligible, while the value in (CHO) 2 is about 4.5 kcal/mol.