Detailed analysis of the charge transfer complex N,N-dimethylaniline-SO2 by Raman spectroscopy and density functional theory calculations

Although the amine sulfur dioxide chemistry was well characterized in the past both experimentally and theoretically, no systematic Raman spectroscopic study describes the interaction between N,N-dimethylaniline (DMA) and sulfur dioxide (SO2). The formation of a deep red oil by the reaction of SO2 with DMA is an evidence of the charge transfer (CT) nature of the DMA-SO2 interaction. The DMA -SO2 normal Raman spectrum shows the appearance of two intense bands at 1110 and 1151 cm(-1), which are enhanced when resonance is approached. These bands are assigned to nu(s)(SO2) and nu(phi-N) vibrational modes, respectively, confirming the interaction between SO2 and the amine via the nitrogen atom. The dimethyl group steric effect favors the interaction of SO2 with the ring pi electrons, which gives rise to a pi-pi* low-energy CT electronic transition, as confirmed by time-dependent density functional theory (TDDFT) calculations. In addition, the calculated Raman DMA-SO2 spectrum at the B3LYP/6-311++g(3df,3pd) level shows good agreement with the experimental results (vibrational wavenumbers and relative intensities), allowing a complete assignment of the vibrational modes. A better understanding of the intermolecular interactions in this model system can be extremely useful in designing new materials to absorb, detect, or even quantify SO2. Copyright (C) 2009 John Wiley & Sons, Ltd.