Vibrational studies of reactive intermediates of aromatic amines. IV. Radical cation time-resolved resonance Raman investigation of N, N-dimethylaniline and N, N-diethylaniline derivatives

Abstract

The radical cation time-resolved resonance Raman spectra of various isotopic derivatives of N, N-dimethylaniline (DMA), N, N-diethylaniline (DEA), N, N-dimethyl-p-toluidine (4MDMA) and 3, 5, N, N-tetramethylaniline (3,5DMDMA) are reported in the 300–1800 cm−1 range. Excitation was in the weak radical cation absorption around 480 nm. Complete vibrational assignments are proposed. The band activity and the changes in frequency with respect to the neutral molecules are consistent with a quinoidal-type conformation of the framework close to planarity. Stabilization of this conformation is observed when the phenyl ring contains methyl substituents. The analysis of the Raman enhancements suggests that the quinoidal character of the radical structure is significantly lowered in the resonant excited state. An obvious analogy is found between the spectra of DMA+ ⋅ and of the biphenyl radical cation, which clearly indicates that (i) a nearly common chromophore structure characterizes these two radical cations and (ii) the distortion of this chromophore structure in the resonant excited state is comparable in both compounds, i.e., the biphenyl+ ⋅* ←biphenyl+ ⋅ and DMA+ ⋅* ←DMA+ ⋅ transitions are of similar nature. These results are consistent with structural previsions from simple molecular orbital considerations and a comprehensive interpretation of the Raman spectra is given in terms of HOMO population.