瞬态吸收和共振拉曼光谱研究硝基对联苯氮宾、氮宾离子反应活性的影响

利用光解芳基叠氮化合物得到单重态氮宾,运用纳秒瞬态吸收光谱、瞬态共振拉曼光谱实验手段,辅以密度泛函理论(DFT)计算,研究了4'-硝基-4-联苯氮宾在乙腈和水溶液中的光化学反应中间体。实验结果表明,在非质子溶剂中, 4'-硝基-4-联苯氮宾发生系间窜越反应生成三线态氮宾;在质子溶剂中,单重态氮宾可被质子化产生氮宾离子。与4-联苯氮宾和氮宾离子相比,硝基对单重态氮宾系间窜越反应路径影响很小;降低了氮宾离子与水和叠氮阴离子的反应活性,却提高了其与鸟苷的反应活性。

Investigation of the Influence of Nitro-Substitution on the Reaction of Biphenyl-Nitrene and Nitrenium Ion by Transient Absorption and Resonance Raman Spectroscopic Techniques

Arylnitrenes and arylnitrenium ions are both short-lived intermediates that are highly reactive. In this work, nanosecond transient absorption and transient resonance Raman spectroscopic measurements were used to detect and identify the intermediates generated from the singlet 4'-nitro-4-biphenylnitrene after photolysis of the corresponding aryl azide in acetonitrile and aqueous solution. Combined with the density functional theory (DFT) simulation results, the structural and electronic characteristics of the above experimental intermediates were specified. The spectral results indicate that in aprotic solvents (such as acetonitrile), the singlet 4'-nitro-4-biphenylnitrene undergoes intersystem crossing (ISC) to the triplet nitrene. In contrast, in a protic solvent (such as the mixed aqueous solution used in this work), the singlet 4'-nitro-4-biphenylnitrene can be protonated to produce the nitrenium ion. Compared with its un-substituted counterpart, the nitro substitution has little influence on the ISC reaction pathway of the singlet 4-biphenylnitrene. With regard to the un-substituted nitrenium ion, the nitro group decreases its reactivity towards water and azide anion, while accelerating its reaction rate towards 2'-deoxyguanosine based on the different quench reaction rates between the nitrenium ion and azide anion/2'-deoxyguanosine. These results provide rich structural and kinetic information about related arylnitrenes and arylnitrenium ions, providing insights into their photolysis mechanism(s) through electronic and vibrational spectroscopic techniques.