A REEXAMINATION OF THE S-0-]S-1 EXCITATION SPECTRUM OF DIMETHYLANILINE

A new assignment for the S-0 --> S-1 transition of N,N-dimethylaniline (DMA) and related derivatives is presented. The low frequency bands a nd long Franck-Condon envelope observed in DMA-h(6) and DMA-d(6) are a ssigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C-N bond. This new assignme nt is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transition s observed in the excitation spectra of other alkyl anilines. The assi gnment was confirmed by simulations of the excitation spectra of DMA-h (6) and DMA-d(6), with parameters of the calculated potential energy s urface determined to be V-3 = 148.0 +/- 0.5 cm(-1), V-+ = -31.6 +/- 0. 5 cm(-1) V-- = 8.5 +/- 0.5 cm(-1), and V-6 = -15 +/- 0.5 cm(-1). By Fr anck-Condon analysis, it was determined that the weak origin transitio n is due to the shifting of the S-1 torsion minimum by 40 degrees alon g the gearing coordinate relative to the corresponding minimum in the ground state. (C) 1995 American Institute of Physics.