Elucidation on the Treatment of <i>π</i> Bond Order of CnHn (n = 4 and 8) Systems Using Hückel Molecular Orbital Method

Abstract

The C4H4 and C8H8 monocyclic polyene systems are treated automatically as the conjugated delocalized triplet molecules by Hückel molecular orbital (HMO) method, but this is inconsistent with their actual ground state structures associated with singlet state. In this work, both the singlet and triplet molecular structures of the fourmembered ring and the eight-membered ring were optimized by density functional theory (DFT), and the more accurate π bond orders were calculated. Based on the DFT calculated results, we suggested a solution that can correctly compute the π bond orders in the C4H4 and C8H8 monocyclic polyene systems by using the HMO method.