Resonance Raman spectra of 2‐(2‐pyridylazo)‐5‐diethylaminophenol and its complex at various pH values

Abstract

The structure of 2-(2-pyridylazo)-5diethylaminophenol (PADAP) and its complex at various pH values was studied by resonance Raman spectroscopy. The spectral features are sensitive to the pH and the solvent, which can be explained in terms of azo-hydrazone tautomers. Two types of quinonoid structures are found under neutral conditions: a hydrazone structure in chloroform solution and a pyridone-like structure with an N-H bond in the pyridine ring in aqueous solution. The quinoid structure is found at both pH 0.1 and 2.3. The relationship between the structure and the absorption spectrum under neutral conditions is discussed on the basis of CNDO/2 calculations.

Fluorescence and Raman Study of pH Effect on the Adsorption Orientations of Methyl Red on Silver Colloids

“…As mentioned above, MR is a weakly basic azo dye and is a common pH indicator in the range 4.4-6.0, corresponding to color change from yellow in basic or neutral solution to red in acidic solution, with protons attached to the a-/b-azo nitrogen (N) atom and/or dimethyl amino N atom. 26 From pH ¼ 6 to pH ¼ 2, the UV-Vis absorption intensities of MR increased, with its position red-shifted from 444 nm to 516 nm (o-MR), 450 nm to 505 nm (m-MR), and 463 nm to 503 nm (p-MR) ( Table I and Fig. 2). In acidic solutions, the protonated azo form, with absorption around 500 nm, has a quinonoid resonance structure, which is responsible for the red shift from about 450 nm absorption of neutral MR. 27 From the electronic spectra of azobenzene derivatives, the absorption bands at 444 nm (o-MR), 450 nm (m-MR), and 463 nm (p-MR) should be assigned to the contribution of hydrazone structure to the resonance 5,7,9 at pH ¼ 6.…”

“…Methyl red is a weakly basic azo dye and is a common pH indicator in the range 4.4-6.0, corresponding to color change from yellow in basic or neutral solution to red in acidic solution, with protons attached to the a-/b-azo nitrogen (N) atom and/or dimethyl amino N atom. 26 In acidic solutions, the protonated azo form of MR has the resonance structure of a quinonoid. 27 The MR anion deprotonated in basic solution may be represented as a resonance hybrid between the azo and the hydrazone structures.…”

Azo-Imine Resonance in Palladium(II)−Pyridylazo Complex Adsorbed at Liquid−Liquid Interfaces Studied by Centrifugal Liquid Membrane-Resonance Raman Microprobe Spectroscopy

“…Parts a−d of Figure show the resonance Raman spectra of PdLCl in acetone, 1-BuOH, 1-OcOH, and chloroform, respectively, and the observed Raman shifts are listed in Table . Band assignments were made by comparison with those reported for pyridylazo complexes − and the expected Raman spectral change with the change of the dielectric constant of the solvent. Molecular orbital calculation by PM3/MOPAC showed that the dipole moment of ZnLCl(H 2 O) 2 complex increased with the increase of the dielectric constant of the medium.…”

Structure of some arylazophenols and arylazonaphthols in the solid state: A resonance Raman study

“…22 In addition, the ring modes in the region 1600-1400 cm-' are enhanced. 22 In addition, the ring modes in the region 1600-1400 cm-' are enhanced.…”