Time resolved absorption and resonance Raman spectroscopic studies of the oxidation of benzidine in aqueous solution

We report a time resolved resonance Raman study of transient radicals produced in the pulse radiolytic oxidation of benzidine in aqueous solution. The intense and structured transient absorption in the 400–470 nm region, observed at microsecond times in the acidic medium, is attributed to the benzidine radical cation. The Raman spectrum, observed by excitation in resonance with this absorption, exhibits eight prominent bands which are assigned to planar phenyl vibrations. The ring breathing mode (v₁) at 844 cm^-1 is most highly resonance enhanced, indicating an overall expansion of the ring CC bonds in the excited state. The interring CC bond, with partial double bond character, is characterized by an intense (v₁₃) Raman band at 1335 cm^-1. The frequency of the in-phase v_7a CN stretching vibration is 1540 cm^-1. These frequencies and the presence of weak bands attributable to non-planar phenyl vibrations indicate the radical to be slightly non-planar. The pK_a for the proton loss from the radical cation is 10.87, four units higher than for the aniline radical cation. At high pH the observed transient has a broad and structureless absorption at ∽ 380 nm. It is identified from its resonance Raman features as the 4(4′aminophenyl)anilino radical formed by proton loss from the radical cation. The interring CC bond is characterized by a Raman band at 1292 cm^-1, indicating it to be a single bond. The structure of this neutral radical is highly nonplanar, with little conjugation between the two ring systems so that electronic excitation is primarily confined to the anilino moiety. The acidic and basic forms of the radical react rapidly in second order processes to produce products which absorb strongly at, respectively, 360 and 410 nm.