Theoretical study on the azido-cyclization of 3,6-diazido-1,2,4,5-tetrazine (DAT) with the dimethylsulfoxide (DMSO) solvent

The density functional theory (DFT) with B3LYP/6-311G^* level was used to study the azido-cyclization of 3,6-diazido-1,2,4,5-tetrazine (DAT) with the dimethylsulfoxide (DMSO) solvent based on the self-consistent reaction field (SCRF). The cyclization of DAT mainly occurs in the conversion azido→tetrazole, in which the molecular and electronic structure change significantly. The electrostatic potential was introduced to predict the sites where could be vulnerable to attacks by the electrophile. The thermodynamic analysis shows that all reactions are endothermic and non-spontaneous in DMSO solvent. And DMSO makes the products more stable than in gas phase, while there is a little decrease in the cyclization barrier. The barriers of the first and second cyclization are 24 and 28kcal/mol, and the most possible cyclization path may connect the ap-DAT and the ap-DAT2c. The isomeric products (ap-DAT2c and sp-DAT2c) coexist due to the extremely low barriers of azido-rotation (about 7 and 8kcal/mol). Finally, the rate constants and the Arrhenius equation were evaluated between the temperature ranges of 200 and 1000 K.