A re-examination of the S→S1 excitation spectrum of dimethylaniline

Abstract

A new assignment for the S 0 →S 1 transition of N,N-dimethylaniline ͑DMA͒ and related derivatives is presented. The low frequency bands and long Franck-Condon envelope observed in DMA-h 6 and DMA-d 6 are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C-N bond. This new assignment is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transitions observed in the excitation spectra of other alkyl anilines. The assignment was confirmed by simulations of the excitation spectra of DMA-h 6 and DMA-d 6 , with parameters of the calculated potential energy surface determined to be V 3 ϭ148.0Ϯ0.5 cm Ϫ1 , V ϩ ϭϪ31.6Ϯ0.5 cm Ϫ1 , V Ϫ ϭ8.5Ϯ0.5 cm Ϫ1 , and V 6 ϭϪ15Ϯ0.5 cm Ϫ1 . By Franck-Condon analysis, it was determined that the weak origin transition is due to the shifting of the S 1 torsion minimum by 40°along the gearing coordinate relative to the corresponding minimum in the ground state.

Fluorescence excitation spectra of jet-cooled 4-(diisopropylamino)benzonitrile and related compounds

“…The weak 0±0-transition was found at E 0 0 32246:5 cm À1 , in excellent agreement with the value given by Salgado et al [11]. As in dimethylaniline, the low frequency part (0±250 cm À1 ) of the LIF excitation spectrum is dominated by transitions arising from torsion of the methyl rotors of the dimethylamino substituent [18]. Though there still may remain some uncertainty regarding the detailed assignment and the coupling with the amino inversion mode [16,19,20], following the arguments put forward in the case of dimethylaniline [18] it seems very reasonable to assume that the weak 0±0-transition observed for DMABN is caused by a substantial change of the equilibrium torsion angle of the methyl rotors between the ground and the locally excited state.…”

“…As in dimethylaniline, the low frequency part (0±250 cm À1 ) of the LIF excitation spectrum is dominated by transitions arising from torsion of the methyl rotors of the dimethylamino substituent [18]. Though there still may remain some uncertainty regarding the detailed assignment and the coupling with the amino inversion mode [16,19,20], following the arguments put forward in the case of dimethylaniline [18] it seems very reasonable to assume that the weak 0±0-transition observed for DMABN is caused by a substantial change of the equilibrium torsion angle of the methyl rotors between the ground and the locally excited state. The low frequency pattern then repeats itself in combination transitions with 365, 498, and 783 cm À1 that are assigned to (benzenelike notation [21]) modes 6a (in-plane ring deformation), 12 (in-plane ring stretch), and 1 (in-plane ring breathing), respectively [19].…”

Resonance Raman Spectroscopic and Density Functional Theory Study of Benzoin Diethyl Phosphate

“…These torsional vibrations are typically observed at frequencies lower than 200 cm -1 . 61 Because both of the resonance Raman spectra obtained here (Figures 4 and 5) show no recognizable features below 300 cm -1 , we conclude that the modes responsible for the vibrational coupling are not detected here but could be detected by scanning the Raman excitation through the lowest three absorption bands. Although we intended to do so, these resonance Raman experiments were hindered by very prompt and strong fluores- cence and absorption from the photoproduct (as described before), and an alternative method is needed to address this point in the future.…”

“…It is known that torsional vibrations are normally in the low-frequency range (∼100-300 cm -1 ). 61 In our resonance Raman spectra, the absence of any obvious feature at frequencies below 500 cm -1 indicates that such a mode does not appear to contribute significantly to either the ππ* (S 3 ) or nπ* (S 1 ) transition. The cyclization, though rapid, is therefore not induced directly by photoexcitation of the three low-lying transitions.…”

Excitation energies and ionization potentials at high electric fields for molecules relevant for electrically insulating liquids

“…The calculated and experimental IP are both 7.12 eV at zero electric field. 47 The first excitation around 4.0 eV, compared to the experimental lowest excitation energy of 4.08 eV, 48 is from the lone pair of nitrogen atom (n) to the first p à b1 orbital of benzene ring. The second excitation with the energy about 5.0 eV is from n to the second p à b2 orbital of benzene ring.…”