Abstract: Cheletropic addition of sulfur dioxide to 2,4-hexadiene has been calculated by density functional theory(DFT) at the level of B3LYP/6-311+G**, the result of IRC calculation shows that the cheletropic addition is a concerted reaction. In reaction, each reactant gives its electrons in HOMO to the LUMO of other reactant simultaneously, which is different from the conditional mechanism of 4+2 cycloaddition. Before reaction, the difference of energy level between HOMO of sulfur dioxide and LUMO of 2,4-hexadiene is largish(8.4 eV), but in the course of reaction, the antibonding LUMO of 2,4-hexadiene turns into a bonding orbital and decreases its energy, so the electrons in HOMO of sulfur dioxide can flow into this orbital easily. The net result of the reaction is that sulfur dioxide transfers 0.23e negative charge to 2,4-hexadiene.

Key words: Sulfur dioxide, 2,4-hexadiene, Cheletropic addition, Mechanism, Orbital interaction