Investigation of the influence of nitro-substitution on the reaction of biphenyl-nitrene and nitrenium ion by transient absorption and resonance raman spectroscopic techniques

Abstract

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.