Vibrational and structural analysis of the radical cation of N,N,N′,N′-tetramethylbenzidine based on ab initio calculations and time-resolved resonance Raman spectroscopy

Abstract

New time-resolved resonance Raman spectra of the radical cation of N,N,N′,N′-tetramethylbenzidine (TMB) in solution have been probed at 752 nm, in resonance with a strong electronic transition, for several isotopic derivatives. These spectra and those obtained previously at 488 nm, in resonance with a second electronic transition, have been interpreted by means of the density functional theory (B3LYP) calculations. An unambiguous assignment of the observed vibrations has been deduced. For comparison, similar calculations have been done for the parent ground state molecule. In addition, configuration interaction singles (CIS) calculations have been used to assign the two resonant electronic excited states of the radical cation and identify the molecular orbitals involved in the corresponding transitions. On this basis, the Raman intensities of the radical cation in resonance with these two transitions have been evaluated in the Franck–Condon approximation by projecting the excited state potential energy surface gradients, determined from CASSCF calculations, on the ground state normal modes. The analysis of both the vibrational frequencies and resonance Raman intensities suggests that the TMB radical cation adopts a coplanar structure (D_2h point group).