Anharmonicity of excited-state potential surfaces: Quantum chemical analysis and resonance Raman intensities

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 different 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

Radical cation of N,N-dimethylpiperazine: Dramatic structural effects of orbital interactions through bonds

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--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