Abstract The structure of SO3 was optimized and compared by the ab initio methods of MP2, QCISD, CCSD and density function theory (DFT) method of B3LYP under the basis set of 6-311++G(3df，3pd), and the results showed the optimization geometry parameters were optimal through QCISD/6-311++G(3df，3pd) method. Based on this method, the hybrid orbitals, hybrid modes and delocalization π bonds of SO3 were analyzed through the natural bond orbital (NBO) theory. The results reveal that S adopts sp2 equivalent hybridization and each sp2 orbital is resided by one pair electrons and remaining an unfilled 3pz orbital. O atoms adopt sp3 nonequivalent hybridization. In fact, the σ bonds between S and O atoms are S→O coordination bonds. The coupling of three 2pz lone pair electrons in O atoms with empty 3pz orbital of S atom forms the π64 delocalization π bond in SO3 molecule. The Mayer bond order is 1.77 between S and O atoms, which further verifies the obvious double bond property of S=O bonds.
