Theoretical and pH dependent surface enhanced Raman spectroscopy study on caffeine

The surface enhanced Raman spectroscopy (SERS) spectrum of caffeine is recorded on a silver colloid at different pH values. It is discussed on the basis of the SERS “surface selection rules” in order to characterize its vibrational behavior on such a biological artificial model. To improve the previ...

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Bibliographic Details
Published in:Biopolymers 2003, Vol.72 (1), p.25-37
Main Authors: Pavel, I., Szeghalmi, A., Moigno, D., Cîntă, S., Kiefer, W.
Format: Article
Language:English
Subjects:
caffeine
Caffeine - chemistry
density functional theory calculations
Hydrogen-Ion Concentration
Models, Molecular
Molecular Conformation
Raman spectroscopy
Spectrum Analysis, Raman - methods
surface enhanced Raman spectroscopy
Vibration
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Summary:The surface enhanced Raman spectroscopy (SERS) spectrum of caffeine is recorded on a silver colloid at different pH values. It is discussed on the basis of the SERS “surface selection rules” in order to characterize its vibrational behavior on such a biological artificial model. To improve the previous assignments in the Raman spectrum and for a reliable, detailed analysis of SERS spectra, density functional theory calculations (structural parameters, harmonic vibrational wavenumbers, total electron density, and natural population analysis of the molecule) are performed for the anhydrous form of caffeine and the results are discussed. The predicted geometry and vibrational Raman spectra are in good agreement with the experimental data. The flat orientation of the mainly chemisorbed caffeine attached through the π electrons and the lone pair of nonmethylated N atoms of the imidazole ring are proposed to occur at neutral and basic pH values. At acid pH values caffeine is probably adsorbed on the Ag surface through one or both oxygen atoms, more probably through the O atom of the conjugated carbonyl group with an end‐on orientation. However, the changes in the overall SERS spectral pattern seem to indicate the electromagnetic mechanism as being the dominant one. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 72: 25–37, 2003
ISSN:0006-3525
1097-0282
DOI:10.1002/bip.10248