PARAPHENYLENEDIAMINE RADICAL CATION STRUCTURE STUDIED BY RESONANCE RAMAN AND MOLECULAR-ORBITAL METHODS

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460-480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the deltaNH2 scissor mode at 1658 cm-1 and the Wilson modes nu8a (CC stretch) at 1644 cm-1, nu7a (CN stretch) at 1423 cm-1, and nu9a (CH bend) at 1184 cm-1. Also observed are nu1 (ring breathe) at 840 cm-1 and nu6a (CCC bend) at 467 cm-1. With excitation in the 340-410 nm region the nu1 band becomes relatively stronger and an additional band at 1524 cm-1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration nu8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to deltaNH2 and nu8a (CC stretch) modes in the 1600-1700 cm-1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 angstrom are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u <-- 2B2g transition, the very weak 360 nm absorption to 2A(u) <-- 2B2g and the very strong approximately 325 nm absorption to a higher 2B3u <-- 2B2g. Calculations with the simple relation I(k) is-proportional-to (V(k)')2/omega(k), which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410-340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2A(u) state via the b3g symmetry nu8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.