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Theoretical and pH dependent surface enhanced Raman spectroscopy study on caffeine
Author(s) -
Pavel I.,
Szeghalmi A.,
Moigno D.,
Cîntă S.,
Kiefer W.
Publication year - 2002
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.10248
Subject(s) - chemistry , raman spectroscopy , lone pair , surface enhanced raman spectroscopy , density functional theory , spectroscopy , molecule , computational chemistry , atom (system on chip) , analytical chemistry (journal) , raman scattering , organic chemistry , physics , quantum mechanics , computer science , optics , embedded system
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