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The Assignment of Several Key Vibrations in Raman Spectra of Protonated and Deuterated trans-Azobenzene Isotopomers

trans-Azobenzene (tAB) molecule was chosen as a model system for investigation of the process of azo group protonation, a very important process common for dyes, acid-base indicators, etc. Specifically deuterium- and (N-15)-labelled isotopomers of trans-azobenzene have been synthesized, where (D_10)-, (N-15)- and ((N-15)_2)-tAB are relevant for this study. All isotopomers were protonated by trifloroacetic acid (CF_3COOH), which proved to be a good solvent for protonated molecules, too. Deuteration was made by deuterium labelled trifluoroacetic acid (CF_3COOD). Raman spectra of both protonated and deuterated species were measured using FT-Raman spectrometer equipped with the near-infrared Nd-YAG laser (excitation line at 1064 nm). Ab initio calculations were performed using the GAUSSIAN 94 program. Structure optimization and vibrational analysis of tAB isotopomers and their protonated and deuterated forms were made with the 6-31G(d) basis set. The final structure is a minimum on a potential surface, which is confirmed by vibrational analysis without imaginary frequencies. The assignment of several key vibrations obtained experimentally and with the help of ab initio calculations will be presented and discussed.

calculated assignment; Raman spectra; protonation;azobenzene

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