Theoretical and experimental resonance Raman study of the fluorene radical cation

Raman spectra of the fluorene radical cation (FR + ) have been recorded for four isotopic derivatives in the 300–1800 cm −1 spectral range, in resonance with two different electronic transitions at 370 and 625 nm. Quantum chemical investigations, at the B3LYP/6-31G* level of theory, of the ground state vibrational modes of FR + provided a reliable assignment of the resonance Raman active (totally symmetric) modes and emphasized the complexity of the experimental vibrational features with the presence of a great number of harmonic and combination modes sometimes superimposed to fundamental frequencies. The predicted structure of FR + was validated on the basis of a good agreement between the observed and calculated vibrational data. Simulating the resonance Raman intensities in the Hartree–Fock formalism (ROHF and CIS) allowed us identifying the resonant states involved in both the investigated electronic transitions.