The radical cation of N,N-dimethylpiperazine (DMP) has been studied using time-resolved optical absorption and resonance Raman spectroscopy. Different quantum-chemical methods were used to calculate the molecular structures and vibrational force fields in the ground state of the radical cation and in the resonant excited state. An excellent agreement between theoretical and experimental vibrational frequencies as well as resonance Raman intensities could be achieved. It is concluded that through-σ-bond interaction between the formal lone pair on one amino nitrogen and the odd electron on the other is strong enough to lead to a symmetric charge-delocalized molecular structure of the DMP radical cation, with a chair-type geometry.

Electronic absorption and resonance Raman spectra of the radical cation of bithiophene are reported. The bithiophene radical cation was produced by γ-radiolysis in a glassy matrix at 77 K, and the Raman spectrum excited in resonance with the two absorption bands at 425 and 590 nm. The electronic states relevant to the observed electronic transitions were identified and characterized by CASSCF calculations. The optical absorption and resonance Raman spectra were calculated by wave packet propagation methods using the ab initio calculated molecular parameters. The calculated spectra agree well with the experimental ones. The importance of carrying out full wave packet propagation calculations is underlined by the fact that in one case the simple Savin formula gave a completely wrong prediction of the resonance Raman spectrum.

The experimental absorption and resonance Raman spectra of the radical cation of N,N-dimethylpiperazine are interpreted on the basis of ab initio density functional calculations and wavepacket propagation techniques. In particular, properties of the excited electronic state active in the resonance transition are discussed. It is shown that the excited-state potential energy surface of the radical cation is strongly anharmonic. The observed resonance Raman spectra and their interpretation using di †erent approaches are discussed in relation to this anharmonicity. It is concluded that resonance Raman spectroscopy, in combination with quantum chemical calculations, is a valuable tool for obtaining information on possible anharmonicity of the excited-state potential energy surface.

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The time correlator formalism is used to develop the expression for nth order (overtone) resonance Raman scattering (RRS) to include both Raman frequency shifts upon electronic excitation as well as non-Condon vibronic coupling. In particular the compact operator formalism recently introduced by Hizhnyakov and Tehver [J. Raman Spectrosc. 19, 383 (1988)] to obtain several RRS correlators (including overtone scattering with frequency shift, but in the Condon approximation) is used to extend the theory. At the same time a formal advantage is achieved by the limited introduction of the Born–Oppenheimer approximation. Also transform relationships including non-Condon effects are given that link the Raman excitation profile of nth order scattering to the absorption spectrum. Finally, it is emphasized how all three vibronic parameters—potential energy surface displacement, Raman mode frequency changes, and the linear non-Condon coupling parameter—can be quantitatively determined without the need for absolute Raman cross-section measurements. The relative scattering intensity of the fundamental and three (or more) overtones suffices to fix the three. By way of application, the vibronic parameters are determined from published single wavelength overtone RRS in six molecules.