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 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.

Fourier-transform Raman and infrared spectroscopic analysis of 2-nitro-tetraphenylporphyrin and metallo-2-nitro-tetraphenylporphyrins

The Fourier-transform Raman (FT-Raman) and infrared (FT-IR) spectra of 2-nitro-tetraphenylporphyrin (2-NO(2)-TPP), nickel-2-nitro-tetraphenylporphyrin (Ni-2-NO(2)-TPP), zinc-2-nitro-tetraphenylporphyrin (Zn-2-NO(2)-TPP) and copper-2-nitro-tetraphenylporphyrin (Cu-2-NO(2)-TPP) were acquired for the first time and carefully assigned and discussed. The effects of a beta-NO(2) group and the influence of the central metal on the molecular symmetry and vibrational spectra of the porphyrin macrocycle were also examined. The bands at 1323-1339, 1516-1526 and 961-971 cm(-1) were attributed to the symmetric and asymmetric stretching vibration of the NO(2) group and to the stretching vibration of the C(beta)-N bond, respectively, which connects the NO(2) group with the beta-carbon of the porphyrin macrocycle. These bands can act as a marker to distinguish beta-NO(2) TPPs from other beta-substituent TPPs. Cu-2-NO(2)-TPP has a C(4nu) molecular symmetry, which is different from those of Ni-2-NO(2)-TPP and Zn-2-NO(2)-TPP, i.e. D(4h).

Silylene-bridged dinuclear iron complexes [Cp(OC)2Fe]2SiX2 (X = H, F, Cl, Br, I). Synthesis, molecular structure, vibrational spectroscopy, and theoretical studies

The mu2-silylene-bridged iron complexes [Cp(OC)(2)Fe](2)SiX(2) (X = F (2), Br (4), I (5)) have been prepared from the mu2-SiH(2) functional precursor [Cp(OC)(2)Fe](2)SiH(2) (1) by hydrogen/halogen exchange, using HBF(4), CBr(4), and CH(2)I(2), respectively. The fluoro- and bromo-substituted derivatives 2 and 4 are converted upon UV irradiation to the carbonyl- and dihalosilylene-bridged dinuclear complexes [Cp(OC)Fe](2) (mu2-CO)(mu2-SiX(2)) (X = F (6), Br (7)) via CO elimination. All new compounds have been characterized spectroscopically, and, in addition, the molecular structure of 2, 4, and the previously reported chloro derivative [Cp(OC)(2)Fe](2)SiCl(2) (3) has been determined by single-crystal X-ray diffraction methods. For 1-5, the Fourier transform infrared and Raman spectra have been recorded and discussed, together with density functional theory calculations, which support the experimental results of the structural and vibrational analysis. The computed geometries, harmonic vibrational wavenumbers, and their corresponding Raman scattering activities are in good agreement with the experimental data. A significant dependence of the CO and Fe-Si stretching modes on the X substituents of the mu2-silylene bridge has been observed and discussed.

Fourier-transform Raman and infrared spectroscopic analysis of dipyrrinones and mesobilirubins

The Fourier-transform Raman (FT-Raman), infrared (FT-IR), and UV-visible absorption spectra of four dipyrrinones and two mesobilirubins have been investigated in the solid state and in CH2Cl2 solutions. A detailed spectral analysis, assignment and discussion of these spectra are presented. The bands at 1735-1738, 1691-1707 and 1359-1377 cm(-1) which were assigned to the stretching vibrations of the C-O-C and C-O-H and symmetric deformation of C-H bonds, respectively, can act as a marker to distinguish the compounds of this class. The striking differences between the spectra of the compounds suggest that mesobilirubin XIIIalpha is tending to adopt as ridge-tile conformation, rather than linear conformation.

Fourier-transform Raman and infrared spectroscopic analysis of novel biliverdin compounds

The vibrational spectroscopy of novel biliverdin compounds were studied by Fourier-transform Raman (FT-Raman) and infrared (FT-IR) spectroscopy. The effects of type, length and position of substituents at C(8) and C(12) or C(1) and C(19) of tetrapyrroles on FT-Raman and FT-IR spectra of these compounds, are discussed. The marker bands are developed in order to distinguish between etiobiliverdin and mesobiliverdin.