Resonance Raman spectroscopy and DFT calculations of the protonation of 4-(2-pyridylazo)- N, N-dimethylaniline in solution and adsorbed on oxide surfaces

The protonation of 4-(2-pyridylazo)- N, N-dimethylaniline (PYAD) in aqueous solution and its adsorption on oxide surfaces has been studied by resonance Raman (RR) spectroscopy. The gas phase structures of neutral, protonated and diprotonated forms of PYAD were modelled by SCF-DFT calculations at the...

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Bibliographic Details
Published in:Chemical physics 2006-03, Vol.322 (3), p.445-458
Main Authors: Dines, Trevor J., MacGregor, Louise D., Rochester, Colin H.
Format: Article
Language:English
Subjects:
DFT calculations
Resonance Raman
Surface acidity
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Summary:The protonation of 4-(2-pyridylazo)- N, N-dimethylaniline (PYAD) in aqueous solution and its adsorption on oxide surfaces has been studied by resonance Raman (RR) spectroscopy. The gas phase structures of neutral, protonated and diprotonated forms of PYAD were modelled by SCF-DFT calculations at the B3-LYP/DZ level, enabling determination of the simulated vibrational spectra of these species, together with vibrational assignments, and providing confirmation that protonation occurs initially at the pyridyl nitrogen atom. Electronic absorption spectra were interpreted using time-dependent DFT calculations. Adsorption of PYAD on SiO 2 or Al 2O 3 surfaces is mainly via the neutral species, hydrogen bonded to surface OH groups, although a small proportion of adsorbed molecules are protonated. By contrast, adsorption on SiO 2–Al 2O 3 results in complete protonation, indicating the presence of Brønsted acidic sites with p K a values ⩽ 4.5, whereas adsorption on H-mordenite results in diprotonation, indicating the presence of Brønsted acidic sites with p K a values ⩽ 2.
ISSN:0301-0104
DOI:10.1016/j.chemphys.2005.09.017