Ricardo Brito de Barros

The Infrared Spectrum of Solid L-Alanine : Influence of pH-Induced Structural Changes

The influence of the pH on the infrared spectrum of L-alanine has been analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The amino acid was precipitated from aqueous solutions and dried at 36.5 degrees C, in order to stabilize cationic L-alanine or alaninium [CH3CH(NH3(+))COOH] at pH 1, the zwitterionic form [CH3CH(NH3(+))COO(-)] at pH 6, and anionic L-alanine or alaninate [CH3CH(NH2)COO(-)] at pH 13. New insight on the specific inter and intramolecular interactions in the different forms of L-alanine was reached by a novel methodological approach: an infrared technique not used before to analyze solid amino acid samples (DRIFTS), in combination with a detailed analysis based on spectral deconvolution. The frequency ranges of interest include the carbonyl/carboxyl stretching and amine deformation modes and the OH/NH stretching modes. It was shown that intermolecular hydrogen bonds between the NH3(+) and COO(-) groups are predominant in the zwitterionic form, whereas in cationic L-alanine, H bonds between the COOH groups are responsible for the formation of dimers. In anionic L-alanine, only strong electrostatic interactions between the COO(-) groups and Na(+) ions are proposed, evidencing the relevant role of the counterion.

A RAIRS study of the methanol decomposition on oxygen precovered Ru(0001)

The effect of a low oxygen precoverage on the Ru(0001) surface towards the adsorption and decomposition of methanol was investigated by reflection-absorption infrared spectroscopy (RAIRS). At 90 K, a very low dose of methanol (0.1 L) dehydrogenates very efficiently on the Ru(0001)-(2 × 2)-O surface to methoxide (CH 3 O-), slightly tilted. For exposures above 0.5 L, a multilayer is formed, which desorbs by annealing to 130 K. At this temperature, three stages of the methoxide decomposition were identified: methoxide in a precursor configuration (lying down on the surface, as only the v as CH 3 mode could be detected), a small amount of formyl [μ 3 η 2 (C,O)-HCO] (resulting from partial dehydrogenation) and, as predominant species, on-top carbon monoxide (from total dehydrogenation). The two former stages were not detected on clean Ru(0001), where at 130 K the main species is still slightly tilted methoxide. A more extensive dehydrogenation is therefore promoted on the modified surface. Formyl apparently desorbs without decomposing into CO (ads) , above 150 K. At 190 K, for very low methanol exposures (0.05 L), the oxygen covered Ru(0001) is highly selective towards dehydrogenation, since only CO (ads) is observed, approaching saturation coverage for 2 L of methanol. For exposures between 0.1 and 2 L. a small fraction undergoes partial dehydrogenation to η 2 -formaldehyde. The surface poisoning by CO (ads) is clear when further exposure to methanol results only in the formation of methoxide and OH (ads) . Although methoxide recombination starts above 190 K, it is stable on this surface. since a considerable fraction is still present at 250 K.

The reactivity of Z-2-hexene on Ru(001) studied by RAIRS

The adsorption at low temperature and the thermal decomposition of Z-2-hexene on the clean Ru(0 0 1) surface, under ultra-high-vacuum, were studied by reflection-absorption infrared spectroscopy (RAIRS). A comparison with the reactivity of other linear hexene isomers was made. It was proposed that, at 90 K, Z-2-hexene adsorbs non-dissociatively as a di-σ complex, in a mixture of conformations. Apparently, gauche conformers with the methyl group in C6 standing vertically are favored for increasing coverage, but forming a poorly ordered layer. For an exposure of 0.6 Langmuir (L), there is evidence of a second layer, physically adsorbed, eventually resulting in the condensation of a randomly oriented multilayer above 1 L. The multilayer desorbs between 100 and 110 K. Only one decomposition mechanism was identified, either by annealing the low temperature di-σ complex to T⩾110 K or by direct adsorption at higher temperatures. It consists on the dehydrogenation of the carbons anchored to the surface, with rehybridization to ∼sp2, yielding 2-hexyne di-σ/π complex, and concomitant C–C bond breaking in the adjacent positions, yielding methylidyne (Ru3CH) and ethyne (C2H2) di-σ/π complex. The latter is stable on Ru(0 0 1) up to at least 320 K, but, in the presence of co-adsorbed hydrogen, it may hydrogenate to ethylidyne (Ru3CCH3), at 125 K. At this temperature, the third decomposition fragment (–C3H7) dehydrogenates and remains adsorbed as propylidyne (Ru3CCH2CH3). Above 170 K, decomposition of propylidyne to ethylidyne and further to methylidyne occurs, leaving methylidyne and ethyne di-σ/π complex as the only RAIRS identifiable products at 320 K.

The chemical behaviour of 3-hexene on the Ru(0001) surface: a characterisation by RAIRS

Abstract The chemical nature of Z-3-hexene/Ru(0001) bond at different temperatures was studied by reflection-absorption infrared spectroscopy (RAIRS). At 92 K and for low exposures (0.05 L) the results indicate that Z-3-hexene adsorbs non-dissociatively as a di-σ complex. By annealing to 123 K, it dehydrogenates at the Cα carbons (C3 and C4, bonded to the surface), forming a hexyne di-σ/π complex in a mixture of rotational conformers. Upon thermal activation, this complex decomposes by breaking the Cα–Cβ (i.e., C2–C3 and C4–C5) bonds, yielding adsorbed ethylidyne. At 223 K, only this species and residual di-σ/π complex are present on the surface. A different decomposition path is proposed upon direct adsorption of Z-3-hexene at 223 K, since propylidyne appears to be formed on the surface, which implies breaking the C–C double bond with dehydrogenation. Since the behaviour of Z-3-hexene is identical to that of 3-hexyne, and rather different from 1-hexene, these results suggest that the decomposition of unsaturated hydrocarbons on clean Ru(0001) is determined by the position and not by the nature (double or triple bond) of the functionality.

Decomposition of 2-hexyne on clean Ru(0 0 1) studied by RAIRS

The decomposition of 2-hexyne on clean Ru(0 0 1), under ultra-high-vacuum, was studied by reflection–absorption infrared spectroscopy (RAIRS). The spectra obtained at 100 K are compatible with the formation of a non-dissociative di-r=p complex. Evidence was obtained for two decomposition paths. By annealing the low temperature complex above 110 K, the weakened C–C bonds adjacent to the anchoratoms (C 1–C2 and C3–C4 bonds) break, eventually yielding methylidyne (RuBC–H) and ethylidyne (RuBC–CH3), at 220 K. By direct adsorption of 2-hexyne at 130 K or higher temperatures, the C–C triple bond is the first to break, ethylidyne being the major surface species identified by RAIRS. These results are compared to those obtained for 1- and 3-hexyne isomers. 2003 Elsevier Science B.V. All rights reserved.

The role of cellulose acetate as a matrix for aggregation of pseudoisocyanine iodide: absorption and emission studies

Films of pseudoisocyanine iodide in a cellulose acetate matrix were prepared by spin coating and characterized by UV/Vis absorption and fluorescence spectroscopies. The comparison with self-supported films of the same dye enabled analysing the role of the matrix in the aggregation of pseudoisocyanine iodide ([PIC]I). It was proved that cellulose acetate is a suitable support for [PIC]I J-aggregates, which form during spinning, as shown by a very sharp J-band in the absorption spectra. This indicates a perfect coherence between stacked monomers in the supported J-aggregates. It was possible to individualize the emission spectrum of [PIC]I J-aggregates in cellulose acetate, by decomposition of the steady-state fluorescence spectra of the films. The dependence on the excitation wavelength of the relative emission intensities of monomers and J-aggregates, for lambda(em) = 587 nm, lead to confirm that the latter species have an absorption maximum at approximately 500 nm in cellulose acetate. Finally, polarised absorption spectra of films obtained by the vertical spin coating technique showed that cellulose acetate allows a partial orientation of J-aggregates.

Spectroscopic Methods for Quantifying Gabapentin: Framing the Methods without Derivatization and Application to Different Pharmaceutical Formulations

This work aimed at analyzing the performance of direct spectroscopic methods for the quantification of gabapentin (GABAp), given the lack of previous studies, in comparison with the more reviewed and complex derivatization techniques, discussing their susceptibility to the pharmaceutical formulations. All of the methods analyzed showed high selectivity for this pharmaceutical analyte, with recoveries close to 100%. Absorption spectroscopy without derivatization yielded better sensitivity and lower limits of detection and quantification of gabapentin in aqueous solution (AqSol method) when compared with other solvents, such as acidic solution or ethanol/water mixture. Derivatization with sodium hypochlorite presented the highest precision, whereas derivatization with vanillin exhibited the highest accuracy. The best method for GABAp quantification in terms of highest sensitivity, lowest limits of detection, and quantification, and also with good precision and accuracy, proved to be fluorescence with derivatization by 4-chloro-7-nitrobenzofurazan. The effect of the pharmaceutical formulation (nature of excipients) was tested for the most robust and sensitive methods, with and without derivatization, on capsules of five commercial brands. Recoveries in the range of 97.9–101.5% proved that there are no matrix interfering effects. Although not presenting the best performance in all the parameters evaluated, the AqSol method, due to its simplicity, proved to be suitable for the quantification of GABAp in capsules and tables containing the molecule as the active ingredient.