Napredna pretraga

Pregled bibliografske jedinice broj: 7391

Vibrational Analysis of the Protonated Azo Group. Raman Spectra and Normal Coordinates of Protonated trans-Azobenzene and Its Isotopomers


Meić, Zlatko; Baranović, Goran; Smrečki, Vilko; Keresztury, Gabor
Vibrational Analysis of the Protonated Azo Group. Raman Spectra and Normal Coordinates of Protonated trans-Azobenzene and Its Isotopomers // XV International Conference on Raman Spectroscopy / Asher, S. A ; Stein, P. (ur.).
Chichester, New York, Brisbane, Toronto, Singapore: John Wiley & Sons, 1996. str. 234-235 (poster, međunarodna recenzija, cjeloviti rad (in extenso), znanstveni)


Naslov
Vibrational Analysis of the Protonated Azo Group. Raman Spectra and Normal Coordinates of Protonated trans-Azobenzene and Its Isotopomers

Autori
Meić, Zlatko ; Baranović, Goran ; Smrečki, Vilko ; Keresztury, Gabor

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Izvornik
XV International Conference on Raman Spectroscopy / Asher, S. A ; Stein, P. - Chichester, New York, Brisbane, Toronto, Singapore : John Wiley & Sons, 1996, 234-235

Skup
XV International Conference on Raman Spectroscopy

Mjesto i datum
Pittsburgh, PA, USA, 11-16.08.1996

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Sažetak
The C=N stretching frequency of trans-N-benzylideneaniline increases upon protonation from 1629 to 1671 cm(-1). This increase, also characteristic of the imino group in other Schiff bases, was interpreted in terms of coupling of the C=N stretching with NH bending mode and change in hybridization at the nitrogen site. It is interesting to compare vibrational behaviour of the protonated imino group with that of the protonated azo group. We have chosen here trans-azobenzene (tAB), which is particularly suitable because of the equivalence of the two nitrogen sites and the absence of possible perturbations due to a substituent. Azobenzene reacts with strong Lewis acids giving always the monoprotonated species. The difficulty to obtain good Raman spectra of trans-azobenzene using visible lasers has been overcome by the use of the near-infrared Nd-YAG laser excitation, with a Nicolet Raman System 950 FT-Raman spectrometer. The strongest band in the Raman spectrum of powdered (D_0)-tAB at 1439 cm(-1) is assigned to the N=N stretching mode. Raman spectra of trans-azobenzene ethanolic solution protonated with different acids reveal a decrease of the N=N stretching frequency by approximately 40 cm(-1), with no appreciable effect from the counter ion. It is interesting to note that Raman spectra of all protonated samples have strong bands near 1550 cm(-1), which are assigned to the NH bending mode. Three tAB isotopomers were used here to obtain Raman spectra of powdered, protonated and deuterated samples: (D_0)-tAB, (N-15)-tAB and (D_10)-tAB. In the Raman spectrum of (D_0)-tAB protonated in pure CF_3COOH, a strong broad band is found at 1547 cm(-1). In the Raman spectrum of (D_0)-tAB deuterated in the same manner, this band is missing, but a new one of a medium intensity appears at 1075 cm(-1), which could be ascribed to the ND bending mode. The most relevant Raman frequencies of unprotonated, protonated and deuterated isotopomers of tAB are given. The normal coordinate calculations showed a substantial contribution to the N=N stretching mode from the phenyl group mode 19b. Therefore the increase of the N=N stretching frequency in (D_10)-tAB is a result of a decoupling on going from undeuterated to deuterated phenyl groups. This trend is preserved also in protonated and deuterated species. In protonated (D_10)-tAB the NH bending band is shifted to the lower frequency, which also indicates a certain coupling with ring modes. Preliminary FT infrared spectra of protonated (D_0)-tAB also indicate the appearance of the weak N=N stretching and NH bending bands. C-13 NMR spectrum of the (D_0)-tAB / CF_3COOH / C_6D_6 solution reveals only one signal for the C-1,1' site (144.8 ppm), which is more shielded than in (D_0)-tAB / C_6D_6 (153.5 ppm). This implies that fast proton transfer occurs between two nitrogens of an azobenzene molecule and surroundings.

Izvorni jezik
Engleski

Znanstvena područja
Kemija



POVEZANOST RADA


Projekt / tema
00980802

Ustanove
Institut "Ruđer Bošković", Zagreb