Resonance Raman Spectra Simulation of the 4,4‘-Bipyridine Anion Radical and N-Protonated Radical

A low-cost computational methodology is utilized to calculate the resonance Raman spectra of the 4,4‘-bipyridine anion radical (44BPY-•) and N-hydro radical (44BPYH•). The resonance Raman intensities are evaluated in the Franck−Condon approximation, by projecting the excited state surface gradient (calculated with the CASSCF method) on the ground state normal modes (determined at ROHF level of theory). TD-DFT and CIS calculations are used to assign the resonant electronic states, estimate the intensity and excitation energy of the related transitions, and identify the molecular orbitals involved in these transitions. These molecular orbitals are then used to construct CASSCF active spaces as small as possible and to compute the excited-state gradients. The results appear to be very satisfactory for 44BPY-• and reasonable for 44BPYH•. They allow identifying the nature of the probed resonant electronic transitions of these transient species and provide a precise picture of their electronic configuration. A comparative analysis of these results and those reported previously for the lowest excited S1 state of 44BPY reveals a clear analogy of the electronic structures of the N-hydro radical and excited S1 state.