Spectres Raman de Résonance de Dérivés Substitués de l'Hydroxyazobenzène en Solution Fortement Acide

Abstract

Resonance Raman spectra of azobenzene and some hydroxyazobenzene derivatives, in strong acid solution (HCI 6 M), are analysed. All the observed bands between 900 and 1700 em-' can be assigned to vibrational modes of the aromatic rings and the azo group, and give evidence for a trans azo type structure both for azobenzene and the para substituted hydroxyazobenzene derivatives studied. Resonance Raman spectra of six substituted 2-(4'-hydroxypheny1azo)-benzoic acid derivatives in strong acid solutions are investigated. The spectral features are rather different from the above studied hydroxyazobenzene derivatives, but can be explained on the basis of an azo type structure. A strong perturbation effect on the N=N stretching mode is demonstrated using deuterium substitution. From the complexity of frequency shifts obtained on deuterium substitution, vibrational coupling between NH bending and many other vibrational modes is evident. A tentative assignment of the observed resonance Raman bands is proposed.

Structures, Vibrational Frequencies, and Normal Modes of Substituted Azo Dyes: Infrared, Raman, and Density Functional Calculations

“…The main objective for carrying out the present investigation is 3-fold: (1) An accurate force field is necessary to get vibrational mode dependent information associated with the isomerization dynamics of the substituted azo dyes, from the resonance Raman (RR) intensities. 15,16,59 (2) The normal mode descriptions for NAB, DAB and NDAB are necessary for the accurate assignment of the vibrational spectra, [26][27][28][29][30][31][32][33][34][35][36][37] and to the best of our knowledge these data are not available in the literature. (3) A critical comparison of the experimentally observed (FTIR and Raman) spectra with the theoretical (DFT) results is absolutely essential in order to test the accuracy of the theory.…”

“…In spite of the vast literature on the studies of the photoisomerization of azo dyes by means of various spectroscopic and photochemical methods including UV−vis absorption, − infrared (IR), and Raman, ,,− the structure of azo dyes are not well established. This is possibly due to the fact that majority of the IR and Raman studies on azo dyes have focused their attention on a qualitative understanding of the vibrational structure and band assignments without paying much attention on the complete normal coordinate analysis, until recently. ,− In fact, an accurate normal mode analysis is essential in order to understand mode dependent structural distortions involved during the isomerization process.…”

Density Functional Calculations of Structures, Vibrational Frequencies, and Normal Modes of trans- and cis-Azobenzene

“…To elucidate the structural properties of the azo compounds, various spectroscopic and photochemical methods including UV−vis absorption, − femtosecond time-resolved absorption, infrared (IR), , Raman, ,− and NMR have been extensively used. In particular, from resonance Raman (RR) intensities, ultrafast dynamical information of the isomerization process can also be obtained .…”

“…14 Accurate normal mode analysis for TAB and CAB is essential to understand the mode-dependent structural distortions involved during the isomerization phenomenon. However, the analysis of vibrational data [37][38][39][40][41][42][43][44][45][46][47][48]50,51 for TAB has been qualitative both in terms of assignments as well as the description of normal modes. In a recent report, Armstrong et al 51 have presented the normal mode analysis for TAB with the optimized geometry resulting in two imaginary vibrational frequencies (indicating a transition state structure) and some of the vibrational assignments not matching the experimental data.…”

“…Interestingly, DFT calculations have also been successfully used only for the computation of vibrational frequencies for polyatomic systems such as fluorene 58 (consisting of 63 normal modes), carbazole 59 (60 normal modes), 2,3dimethoxy-1,4-benzoquinone 60 (54 normal modes), etc., but accurate mode descriptions in terms of potential energy distributions (PEDs) for each mode have not been addressed. The main objectives for carrying out the present investigation are as follows: (a) to obtain an accurate force field, which is necessary to get vibrational mode dependent isomerization dynamical information of TAB from RR intensities, 14,74 (b) to obtain PEDs and exact normal mode descriptions, which are not available in the literature for CAB and TAB but are necessary for the accurate assignment of the vibrational spectra of azo dyes, [37][38][39][40][41][42][43][44][45][46][47][48]50,51 (c) to clarify the deviations in the calculated vibrational frequencies for various isotopes of TAB, with respect to the experimental data, for the main vibrations such as NdN and C-N in comparison to results obtained using the MP2 method, 51 (d) to evaluate the use of DFT methods in predicting accurate normal mode descriptions of polyatomic molecules in addition to vibrational frequencies, and (e) to compare the calculated IR intensities with those obtained from experiments. 35 A comparative study of RHF, hybrid Hartree-Fock/density functional (HF/DF), and DFT methods have been carried out for TAB and CAB (including their isotopic analogues), and the results are critically compared with the available experimental frequencies 25,35 and the previously published MP2 calculations.…”