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.