Abstract: Three tautomerization reactions between benzaldoximes and their polar tautomers have been studied theoretically. All geometries of reactants, products and transition states were optimized by DFT method at B3LYP/6-31G(d) level with Gaussian 98 software package. The species with Z and E configurations have similar transition states in the three reactions. Compared with E isomers, Z isomers are favorable to the polar products. The mechanism of the first reaction involves proton migration from hydroxyl to nitrogen of oxime within single molecule through a triangle transition state . This reaction takes place difficultly at room temperature because of the high energy barrier. The second reaction is related to proton exchange within the dimers. The proton of hydroxyl group in one oxime moves to the nitrogen of another oxime of the dimer through a hexaangular ring transition state with a lower barrier. The reaction rate of this reaction is large,but the thermodynamic balances is not favorable to the products. The third path is a catalyzed reaction by methanol with a pentagonal transition states. The hydroxyl of methanol acts as a bridge of proton transfer. The reaction barrier and rate are moderate. Our results show that the tautomerization between benzadoxime and their polar tautomers can take place at room temperature. The main reaction path is the proton exchange within the dimers duo to its large reaction rate. The reaction will reach the end asthe product is consumed continuously.

Key words: Benzaldoxime, Polar tautomer, Tautomerization