Direct Observation of 4-Phenoxyphenylnitrenium Ion: A Transient Absorption and Transient Resonance Raman Study.

Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with t = 346 ps or protonation in mixed aqueous solution with t = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.