Laser flash photolysis of fluorinated aryl azides in neutral and acidic solutions

Laser flash photolysis of perfluoro-4-biphenyl azide and N-propyl-4-azido-2 ,3,5,6-tetrafluorobenzylamide in acetonitrile, water-acetonitrile mixtures, and HCl-containing solutions was studied. The absorption spectra of primar y intermediates, singlet arylnitrenes (2a(S) and 2b(S), respectively), were recorded. The absolute rate constants of their intersystem crossing in MeC N were measured, and the corresponding Arrhenius parameters were found from the temperature dependences of the rate constants of isomerization of sing let arylnitrenes 2a(S) and 2b(S) to azirines. Protonation of singlet arylni trenes 2a(S) and 2b(S) was observed, the rate constants of their protonatio n were measured, and the transient absorption spectra of arylnitrenium ions were recorded. It was shown by quantum-chemical calculations (the hybrid B 3LYP method) that the arylnitrenium ions that formed have the singlet groun d state and the singlet-triplet gap is similar to 20 kcal mol(-1).