Time resolved resonance Raman spectra of anilino radical and aniline radical cation

Abstract

We report, in this paper, submicrosecond time resolved resonance Raman spectra of anilino radical and its radical cation as observed in pulse radiolytic studies of the oxidation of aniline in aqueous solution. By excitation in resonance with the broad and weak electronic transition of anilino radical at 400 nm (ε∼1250 M−1 cm−1) we have observed, for the first time, the vibrational features of this radical. The Wilson ν8a ring stretching mode at 1560 cm−1 is most strongly resonance enhanced. The ν7a CN stretching band at 1505 cm−1, which is shifted to higher frequency by 231 cm−1 with respect to aniline, is also prominent. The frequency of this latter mode indicates that the CN bond in the radical has considerable double bond character. The Raman spectrum of aniline radical cation, excited in resonance with the ∼425 nm electronic absorption (ε∼4000 M−1 cm−1), shows features which are similar to phenoxyl radical. Most of the observed frequencies of this radical in solution are in good agreement with vibrational energies determined by recent laser photoelectron spectroscopic studies in the vapor phase. The bands most strongly enhanced in the resonance Raman spectrum are, however, weak in the photoelectron spectrum. While the vibrational frequencies observed for anilino radical and its isoelectronic cation are quite similar, the resonance enhancement patterns are very different. In particular the ν14 b2 mode of anilino radical observed at 1324 cm−1 is highly resonance enhanced because of strong vibronic coupling between the 400 nm 2A2–2B1 and the higher 2B1–2B1 electronic transitions.