WHY N2O2 IS CIS WHILE (CHO)(2) IS TRANS - MO AND VB STUDIES

The nonempirical valence-bond method as well as the molecular orbital method are applied to investigate the structures of N2O2 and (CHO)(2). Results show that in either molecule the sigma-component tends toward a trans conformation, whereas, the pi-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 pi electronic stabilization. The latter is more preferable to a cis conformation of N2O2, while the Coulombic repulsive interaction should be responsible for the trans conformation of (CHO)(2). The nonempirical VB calculations show that the pi delocalization energy in N2O2 is negligible, while the value in (CHO)(2) is about 4.5 kcal/mol. (C) 1995 John Wiley and Sons, Inc.