Contributions of symmetric and asymmetric normal coordinates to the intervalence electronic absorption and resonance Raman spectra of a strongly coupled p-phenylenediamine radical cation

Resonance Raman spectroscopy, electronic absorption spectroscopy, and the time-dependent theory of spectroscopy are used to analyze the intervalence electron transfer properties of a strongly delocalized class III molecule, the tetraalkyl-p-phenylene diamine radical cation bis(3-oxo-9-azabicyclo3.3.1]non-9-yl)benzene ((k33)(2)PD(+)). This molecule is a prototypical system for strongly coupled organic intervalence electron transfer spectroscopy. Resonance Raman excitation profiles in resonance with the lowest energy absorption band are measured. The normal modes of vibration that are most strongly coupled to the intervalence transition are identified and assigned by using UB3LYP/6-31G(d) calculations. Excited state distortions are obtained, and the resonance Raman intensities and excitation profiles are calculated by using the time-dependent theory of Raman spectroscopy. The most highly distorted normal modes are all totally symmetric, but intervalence electron transfer absorption spectra are usually interpreted in terms of a model based on coupling between potential surfaces that are displaced along an asymmetric normal coordinate. This model provides a convenient physical picture for the intervalence compound, but it is inadequate for explaining the spectra. The absorption spectrum arising from only the strongly coupled surfaces consists of a single narrow band, in contrast to the broad, vibronically structured experimental spectrum. The electronic absorption spectrum of (k33)(2)PD(+) is calculated by using exactly the same potential surfaces as those used for the Raman calculations. The importance of symmetric normal coordinates, in addition to the asymmetric coordinate, is discussed. The observed vibronic structure is an example of the missing mode effect; the spacing is interpreted in terms of the time-dependent overlaps in the time domain.