A method for determing Huckel-Hubbard (H-H) parameters was proposed, and the H-H theory is extended to analyze [2,3] sigmatropic rearrangements including Witting rearrangement, sulfur ylide rearrangement and allylic sulfoxide rearrangement. Taking the concerted mechanism into account, the ground state potential energy surface was constructed as well as the low-lying excited state ones for two different reaction pathways which correspond to Huckel mode and Mobius mode resp. The stereoselectivities of the thermal and photochem. processes as well as the activation energies were given and discussed in detail.e

Key words: PHOTOCHEMICAL REACTION, REARRANGEMENT REACTION, STEREOSELECTIVITY, ACTIVATION ENERGY, TRANSITION STATE, SULFOXIDE P, HOT REACTION