Theoretical study on mechanism of decomposition reaction of 1,2,4-triazole derivatives

The internal connection between the electronic structure and decomposition mechanism for ANTA and AHNTA is investigated by DFT method. The molecular electrostatic potential shows that hydrogen atoms of the amino group are easily transferred to the nitro group. The excitation process of <i>S</i>₀→<i>T</i>₁ can be regarded as the local π→π∗ excitation. For the substituent dissociation, amino group is easily separated from the ring. For the ring cleavage, the C–N bond will be weakened by substituents on the C and N atoms. Especially at the excited state, the C–N bond with a nitro group on the C atom is the easiest to break. The charged state is beneficial to CONO isomerisation, and the H transfer is promoted at the excited state. For ANTA, the reaction rate of CONO isomerisation is faster than ring cleavage at the ground or the positively charged state. At the negatively charged state, the reaction rate of H transfer is the fastest. For AHNTA, at the ground state or the charged state, the reaction rate of CONO isomerisation is the fastest, compared to ones of the H transfer and ring cleavage reactions. At the excited state, the order is <i>k</i>_{(H transfer)}&gt;<i>k</i>_{(ring cleavage}₎for ANTA and AHNTA.

本研究采用密度泛函理论（Density Functional Theory, DFT）方法，探究了ANTA与AHNTA的电子结构与其分解机理之间的内在关联。分子静电势分析结果表明，氨基上的氢原子易于向硝基发生转移。S₀→T₁的激发过程可被视为局域π→π*激发。就取代基解离过程而言，氨基易于从环结构上脱离。对于环断裂过程，碳、氮原子上的取代基会削弱C-N键的键能。尤其在激发态下，碳原子连有硝基的C-N键最易发生断裂。带电态有利于CONO异构化反应，而激发态则会促进氢转移过程。对于ANTA而言，在基态或带正电的状态下，其CONO异构化反应的速率快于环断裂反应；在带负电的状态下，氢转移反应的速率最快。对于AHNTA，在基态或带电状态下，相较于氢转移与环断裂反应，其CONO异构化反应的速率最快；在激发态下，ANTA与AHNTA的反应速率排序均为：氢转移反应速率(k_(H转移)) > 环断裂反应速率(k_(环断裂))。