Raman and infrared spectra of some aromatic amine–sulphur dioxide molecular complexes

The Raman and infrared spectra of the molecular complexes formed by sulphur dioxide and aniline, N ‐methylaniline (NMA), N , N ‐dimethylaniline (DMA), N, N ‐diethylaniline (DEA), p ‐phenylenediamine (PPD) and N, N, N′,N′ ‐tetramethyl‐ p ‐phenylenediamine (TPD) were obtained. With NMA, DMA and DEA only minor changes were observed in the vibrational frequencies of the amines and the SO 2 vibrational shifts show that the complexes are considerably weaker than the well known trimethylamine‐sulphur dioxide complex. On the other hand, with PPD and TPD there are significant variations in the spectra of the amines on complexation and the shifts of the SO 2 vibrational frequencies show that the interaction is comparable in strength to that existing in the trimethylamine‐sulphur dioxide complex. In the aniline‐sulphur dioxide complex the spectral data reveal strong intermolecular hydrogen bonding interactions. The pre‐resonance enhancement observed for the DMA and TPD complexes confirms the participation of the SO 2 moiety in the chromophoric group and adds evidence for the change‐transfer nature of the interaction.