RESUMEN
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 pi 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.
Asunto(s)
Cafeína/química , Espectrometría Raman/métodos , Concentración de Iones de Hidrógeno , Modelos Moleculares , Conformación Molecular , VibraciónRESUMEN
FT-Raman of solid rivanol (2-ethoxy-6,9-diaminoacridinium lactate C(15)H(16)N(3)O-C(3)H(5)O(3) . H(2)O) and surface-enhanced Raman scattering (SERS) on silver surface of rivanol solution at pH 5.5 have been obtained and compared. The assignment of vibrational modes has been made for the monocation specie of rivanol. SERS spectrum shows a physisorption of rivanol on the silver surface.
RESUMEN
Se realizo un estudio de la frecuencia de las parasitosis intestinales en colonias comprendidas en dos delegaciones del area sur del Distrito Federal por medio de examenes coproparasitoscopicos cualitativos con el metodo de Faust. Se obtuvieron frecuencia de protozoosis y helmintiasis inse senalan en cuadros. Se hacen comentarios de los resultados obtenidos en esta encuesta epidemiologica.encuesta epidemiologica