engleski

Raman Spectra of Protonated and Deuterated Azobenzene

Azo compounds have a wide application in different fields such as: chemistry of dyes, acid-base indicators, polymers, biomolecules and so on. Protonation of the azo group is particularly interesting since it occurs very often during chemical reactions. We have chosen trans-azobenzene (tAB) as a model compound because of the equivalence of the two nitrogens and the absence of possible perturbations due to a substituent. The difficulty to obtain good Raman spectra of protonated and deuterated trans-azobenzene using visible lasers has been overcome by the use of the near-infrared Nd-YAG laser excitation with a FT-Raman spectrometer. We recorded Raman spectra of (D_0)-, (D_10)- and (N-15)-trans-azobenzene isotopomers as powders and solutions in CF_3COOH and CF_3COOD. An effort was also made to measure Raman spectra of protonated trans-azobenzenes in different acids (HCl, HClO_4, H_2SO_4, CF_3COOH) using ethanol as a solvent. There was no appreciable effect from the counter ion on any signal in the recorded spectra. The normal coordinate calculations have shown a substantial contribution of the phenyl group mode 19b to the N=N stretching (1439 cm(-1) in (D_0)-tAB) mode in the neutral species. This interaction is further complicated with the new coupling between N=N stretching (1401 cm(-1) in (D_0)-tAB) and N-H bending (1547 cm(-1) in (D_0)-tAB) modes in the protonated species. The assignation of the most significant vibrational modes of protonated and deuterated trans-azobenzene isotopomers, as well as a comparison with the protonated and deuterated isoelectronic trans-N-benzylideneaniline, will be presented.

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