Molecular organization in self-assembled binary porphyrin nanotubes revealed by resonance Raman spectroscopy.

Porphyrin nanotubes were formed by the ionic self-assembly of tetrakis(4-sulfonatophenyl) porphyrin diacid (H(4)TPPS(4)(2-)) and Sn(IV) tetra(4-pyridyl) porphyrin (Sn(OH(-))(X)TPyP(4+/5+) [X = OH(-) or H(2)O]) at pH 2.0. As reported previously, the tubes are hollow as revealed by transmission electron microscopy, approximately 60 nm in diameter, and can be up to several micrometres long. The absorption spectrum of the porphyrin nanotubes presents monomer-like Soret bands, as well as two additional red-shifted bands characteristic of porphyrin J-aggregates (offset face-to-face stacks). To elucidate the origin of the J-aggregate bands and the internal interactions of the porphyrins, the resonance Raman spectra have been obtained for the porphyrin nanotubes with excitations near resonance with the Soret J-aggregate band and the monomer-like bands. The resonance Raman data reveal that the Sn porphyrins are not electronically coupled to the J-aggregates within the tubes, which are formed exclusively by H(4)TPPS(4)(2-). This suggests that the internal structure of the nanotubes has H(4)TPPS(4)(2-) in aggregates that are similar to the widely studied H(4)TPPS(4)(2-) self-aggregates and that are segregated from the Sn porphyrins. Possible internal structures of the nanotubes and mechanisms for their formation are discussed.

Analysis of the vibrational spectra of new OH-containing E-4-arylmethylene-3-isochromanones and 3-arylcoumarins.

A combined experimental and theoretical approach is presented to structural characterization of fairly large, newly synthesized organic molecules in order to enhance the effectiveness of their instrumental analysis by vibrational spectroscopy. The method consists of measurement of FT-IR and Raman spectra of the reaction products and subsequent ab initio or DFT quantum mechanical calculations (prediction) of the vibrational spectra for any anticipated structural varieties of the synthesized molecules. Comparison of the measured and computed frequencies as well as the observed and simulated spectra is performed to resolve any uncertainties in identifying the reaction products. Vibrational frequency and normal mode calculations based on scaled quantum mechanical (SQM) force fields performed at the DFT/B3LYP/6-31G* level of theory are demonstrated to provide a wealth of information that have been used in this work to ascertain the molecular structure, probable conformation and H-bond properties of three new isochromanone or coumarin derivatives, namely: 3-([2'-hydroxymethyl]-phenyl)-coumarin (1), E-4-(3'-hydroxyphenylmethylene)-3-isochromanone (2), and 2-[(2'-hydroxymethyl)phenyl]-3H-naphto[2,1-b]pyran-3-one (3).

Towards the solution of the eluent elimination problem in high-performance liquid chromatography-infrared spectroscopy measurements by chemometric methods.

The high-performance liquid chromatography-infrared spectroscopy (HPLC-IR) technique utilizing on-line flow through cell (FTC) detection has an inherent practical problem: strong absorption bands of the eluent may mask valuable analytical regions of the IR spectrum. The experimentalists' answer to this challenge is physical elimination of the chromatographic eluent before spectroscopic detection, which however results in off-line measurement of spectra. In the present work, the capabilities of some chemometric algorithms using iteratively applied multi-way methods such as parallel factor analysis (PARAFAC) and PARAFAC2, developed with the aim of overcoming the problems of eluent elimination are examined and evaluated. Test calculations done on simulated liquid chromatographic infrared (LC-IR) data cubes have shown that although PARAFAC2 performs much better than the simple PARAFAC method, it does not give correct decompositions, just like multivariate curve resolution with alternative least squares (MCR-ALS) and related bilinear data based methods. In search for a better solution, a method named objective subtraction of solvent spectrum with iterative use of PARAFAC and PARAFAC2 (OSSS-IU-PARAFAC and OSSS-IU-PARAFAC2) has been developed. Calculations performed with the corresponding Matlab program developed by the authors and run with the appropriate functions in PLS_Toolbox yielded very promising results in evaluations of both simulated and real HPLC-IR data sets, after necessary data pretreatments.

Applicability of the SQM force field method to the vibrational spectra of sodium acetate.

The applicability of the scaled quantum mechanical force field (SQM FF) method to the prediction of the vibrational spectra of a charged molecule has been studied by the example of the acetate ion (CH3CO2-) in sodium acetate for which an efficient empirical valence force field (SVFF) based on observed IR spectra of six isotopomers of sodium acetate is available in the literature. Standard SQM FF calculations done on a free acetate ion at the B3LYP/6-31G level failed to give an acceptable estimation of even the most characteristic features of the observed spectra, which can be exemplified by the gross overestimation of the frequency separation of the nu(a)CO2- and nu(s)CO2- vibrations. In search for a better description, SQM calculations were done for three simple structural models of sodium acetate, testing different QM methods. The results indicate that in addition to taking into account the dielectric field effect of the surrounding medium, incorporation of a Na+ counterion is necessary to achieve a realistic simulation of the IR and Raman spectra. Satisfactory results were obtained with a bidentate Na-acetate complex by the SQM method coupled with a continuum model at the B3LYP/6-31+G level, whereas the use of the Onsager-type spherical cavity model and the polarizable continuum model (PCM) were found preferable over SCI-PCM.

Analysis of vibrational spectra of some new E- and Z-4-arylidene-3-isochromanones; Part 2. Isomers and conformers of the 2'-pyrrolyl and 2'-nitrophenyl derivatives.

The infrared and Raman spectra of resolved E and Z isomers of some 4-arylidene-3-isochromanone derivatives were analyzed with the aim of pointing out the differences in vibrational behavior of their coexisting stable conformers. Quantum mechanical (QM) density functional (DFT/B3LYP/6-31*) and normal coordinate calculations were carried out to establish the equilibrium structures and to facilitate the interpretation of the vibrational spectra of the 2'-pyrrolyl and 2'-nitrophenyl derivatives. The frequencies and intensities calculated according to the scaled quantum mechanical (SQM) force field method were used to simulate the IR and Raman spectra and compare them to the measured ones. The results demonstrate that the adopted methodology is capable of treating these fairly large polycyclic molecules. The resulting spectral simulations and detailed vibrational description can be highly useful in clarifying spectral differences brought about by cis-trans isomerism and indicating the extent of spectral changes due to further conformational changes of the aryl substituent attached to the olefinic C=C bond.

Normal Raman and surface enhanced Raman spectroscopic experiments with thin layer chromatography spots of essential amino acids using different laser excitation sources.

A comparative study of the feasibility and efficiency of Raman spectroscopic detection of thin layer chromatography (TLC) spots of some weak Raman scatterers (essential amino acids, namely, glycine and L-forms of alanine, serine, valine, proline, hydroxyproline, and phenylalanine) was carried out using four different visible and near-infrared (NIR) laser radiations with wavelengths of 532, 633, 785, and 1064 nm. Three types of commercial TLC plates were tested and the possibility of inducing surface enhanced Raman scattering (SERS) by means of Ag-sol was also investigated. The spectra obtained from spotted analytes adsorbed on TLC plates were of very different quality strongly depending on the excitation wavelength, the wetness of the samples, and the compounds examined. The best results were obtained with the simple silica TLC plate, and it has been established that the longest wavelength (lowest energy) NIR excitation of a Nd:YAG laser is definitely more suitable for generating normal Raman scattering of analyte spots than any of the visible radiations. Concerning SERS with application of Ag-sol to the TLC spots, 1-3 orders of magnitude enhancement was observed with wet samples, the greatest with the 532 nm radiation and gradually smaller with the longer wavelength excitations. It is shown, however, that due to severe adsorption-induced spectral distortions and increased sensitivity to microscopic inhomogeneity of the sample, none of the SERS spectra obtained with the dispersive Raman microscope operating in the visible region were superior to the best NIR normal FT-Raman spectra, as far as sample identification is concerned.

FT-IR, FT-Raman and FT-SERS spectra of 4-aminosalicylic acid sodium salt dihydrate.

FT-IR, FT-Raman and FT-SERS spectra of 4-aminosalicylic acid sodium salt dihydrate (4ASAS) have been recorded and analysed. The vibrational bands due to NH2, OH, carboxyl group, and the benzene ring are identified. The CX ipb(17a, 17b), CC ipb(6,18a) and CH ipb (3,14a, 14b) bands are more enhanced in SERS. Broadening of the inplane carboxyl bend indicates interaction with the silver surface. Further the vC=O, v(C-O)c and v(C-O)h are intense in the SERS spectrum. The rocking and wagging modes of NH2 also show up in SERS. The molecule (O, N donor ligand) is thought to adsorb through the carboxyl oxygen atom with the benzene ring in a 'perpendicular side on orientation' with respect to the silver surface.