Jean-Claude Merlin

Conformational and spectroscopic investigation of 3-hydroxyflavone–aluminium chelates

3-Hydroxyflavone (3HF), which is the simplest molecule of the flavonol class, possesses chelating properties towards Al(III). Spectrophotometric methods have shown that the 3HF molecule forms an Al(3HF)2 complex in pure methanol. The structure of this complex, obtained by quantum semi-empirical AM1 method, indicated that complexed 3HF adopts a pyronium form. Structural and electronic modifications induced by chelation are illustrated by the important frequency shifts observed between free and complexed 3HF FT-Raman spectra and by the chemical shifts variations in the 13C NMR spectra of the two species. Complexes with the same stoichiometry were formed when AcO- or MeO- are present in the medium. However, in acidic medium the chelate composition is Al2(3HF).

Microscopic and imaging Raman scattering study of PbS and its photo‐oxidation products

Spatially resolved molecular information on galena (PbS) crystals was obtained by microspectrometric Raman mapping through a high numerical aperture objective. Different experimental conditions were used: excitation wavelength, excitation power, crystal size and temperature. Spatially guided analysis was obtained from the spectra recorded in the point-by-point microprobe mapping mode with 1 µm spacing between the points. Multivariate data processing yielded the typical spectra of PbS and its degradation products. The Raman features of PbS were found to be weak under non-damaging excitation conditions. The laser-damaging effect generates complete and irreversible oxidation of PbS to α-PbO for particles of size of the order of micrometres. For larger PbS crystals, several lead oxysulphates including 4PbO·PbSO4 and 3PbO·PbSO4 were located in the vicinity of the illuminated spot by Raman mapping. Copyright

Structural study of quercetin by vibrational and electronic spectroscopies combined with semiempirical calculations

As a follow-up to structural studies of monohydroxylated flavones, the structural and spectroscopic properties of a tetrahydroxylated flavone, the quercetin molecule, have been investigated. The molecular conformation of quercetin has been obtained from semiempirical treatment with the AM1 Hamiltonian. Some structural modifications have been observed between the molecule in the solid state and in an isolated state, notably in the rotation of the phenyl ring with respect to chromone part of the compound. The theoretical model has been validated by both vibrational and electronic spectroscopies. The calculated vibrational and UV-vis spectra are in good accordance with the experiments. The Raman spectra have been assigned, and the main electronic transitions involved in the absorption spectrum have been characterized.

Structural and spectroscopic investigation of 5-hydroxyflavone and its complex with aluminium

Abstract This study constitutes the third step in a program aimed to investigate the chelation of hydroxyflavones. After the 3-hydroxyflavone and 3′,4′-dihydroxyflavone, we now study the complexation of 5-hydroxyflavone (5HF) with Al(III). A stoichiometry of 1:1 was obtained from spectrophotometric methods for the complex in methanol. Important structural modifications of 5HF have been observed by density functional theory (DFT) calculation between free and complexed molecules. Whereas, 5HF adopts a staggered conformation, the complexed molecule is rather flat, involving participation of pyronium and cinnamoyl forms. Both γ-pyrone and B-rings are affected by complexation, and changes in the inter-ring linkage are observed. UV–vis and Raman spectroscopies have permitted to confirm the structural models of free and complexed 5HF as well as the modifications produced by complexation. The bathochromic shift of band I observed in the electronic spectrum of complex relative to the 5HF one, has been attributed to pronounced changes in the pattern of frontier orbitals involved in this transition.

Comparison of the chelating power of hydroxyflavones

A series of hydroxyflavones, widely occurring in nature, have been used as model compounds for the study of the behaviours of humic substances towards aluminium ion complexation. The chelating power of 3-hydroxy-4-keto, 5-hydroxy-4-keto and catechol functional groups have been compared in methanol solution, first when these binding sites are isolated, and then when they are in competition on the a same ligand. Electronic and vibrational spectroscopy investigations were performed to characterize the Al(III) complexes: stoichiometry, stability constants and involved groups in multi-site ligands. Quantum chemical calculations have been carried out in order to know the structural modifications generated to the ligand by complexation. The influence of the medium, acidic, neutral and alkaline, has been studied. The 3-hydroxy-4-keto group has the strongest chelating power in acidic and neutral media whereas catechol presents the greatest ability to chelate Al(III) in alkaline solution. This investigation has shown that capacity of different studied functions to chelate Al(III) is the same when they are in competition and when they are individually taken.

Structural and vibrational study of 3-hydroxyflavone and 3-methoxyflavone

Abstract Most stable molecular structures of 3-hydroxyflavone and 3-methoxyflavone have been investigated by semiempirical calculations (AMI method) at the state of isolated molecule. For 3OHF, the comparison between X-ray diffraction data and calculated structural parameters shows a major modification in the inter-ring linkage with a rotation of the phenyl ring with regard to the rest of the molecule. This rotation of the B-ring is not observed in the 3OMeF but the orientation of the methoxy group changes with the physical state. The predictions about the conformational modifications with the physical state of these molecules have been confirmed by vibrational spectroscopy. For the 3OHF molecule, the comparison between the solid and solution Raman spectra shows that the 1200–1400 cm−1 spectral range is particularly affected, seeing that the inter-ring stretching vibration contributes to a great number of normal modes. 3OMeF Raman spectra are less perturbed by the change of physical state, only two small frequencies shifts are observed. In order to assign the most important perturbation in the spectra of the two molecules, the vibrational frequencies have been computed to determine the atoms and groups involved in each normal mode.

Resonance raman spectroscopic studies of anthocyanins and anthocyanidins in aqueous solutions

Abstract The resonance Raman spectra of the six common anthocyanidins and some of their glycosylated derivatives have been recorded from acidic aqueous solutions. A molecular mechanics study of the three basic anthocyanidin types was undertaken in order to obtain a set of molecular parameters to search for correlation between molecular structure and measured resonance Raman data. The close electronical and geometrical configurations of benzopyrylium moieties, which appear to be the chromophore of the investigated pigments, explain the great similarity in the spectra of the six anthocyanidins. The main modifications, observed for the inter-ring stretching mode and the in-plane deformations of the carbon skeleton, are produced by a complex mechanism which involves a change in the delocalization pattern, a significant flexibility of the molecule around the inter-ring bond, as well as vibrational coupling between ring modes. However the substitution pattern can be easily recognized and the effect of glycosylation is clearly visible on the spectra; the substitution on position 5 seems to produce a more important perturbation in the vibrational properties than that on position 3.

Semi-empirical and vibrational studies of flavone and some deuterated analogues

Abstract The infrared solid state, Raman solid state and tetrachloride solution spectra of flavone have been obtained. Assignments of most of the vibrational data have been performed by comparison between the spectra of flavone and three isotopic species, deuterated on the A, B and C rings, respectively. The vibrational frequencies for all the investigated compounds have been calculated from the conformational analysis of flavone using the semi-empirical AM1 method and compared with experimental values. The correlation is more or less satisfactory; however, for some vibrational modes, the calculated isotopic shifts agree better with experiment than do the frequencies themselves. Specific vibrational modes which retain a benzene ring mono-substituted and ortho-distributed character have been recognized in the spectra, according to literature data, isotopic frequency shifts and graphic representation of the atomic displacements.

Conformational analysis of flavone: vibrational and quantum mechanical studies

Abstract Combined molecular mechanics (MMX), semi-empirical (AM1) and ab initio calculations were applied to an investigation of the conformational and electronic properties of the isolated flavone molecule in the ground state. The calculated molecular structure shows that the angle between the benzoypyrone moiety and the phenyl ring is about 30°. The barrier to rotation in this molecule is about 1.8 kcal mol −1 . The frequency shifts observed in the vibrational spectra of flavone in solution and in crystalline form are tentatively connected with a conformational change of the molecule with the physical state. The energy gap between a planar and a twisted configuration is very low and the intermolecular interactions in the crystal, which are expected to be larger than the modest intramolecular potential, must strongly reduce the dihedral angle. As a flat molecule should give more favorable packing, it is reasonable that flavone should exist in this form in the crystal. Electronic energy levels, charge distributions and bond orders were calculated using semi-empirical methods. Electronic interactions of the rings are discussed.

Theoretical investigation of the molecular structure of the isoquercitrin molecule

Abstract Isoquercitrin is a glycosilated flavonoid that has received a great deal of attention because of its numerous biological effects. We present a theoretical study on isoquercitrin using both empirical (Molecular Mechanics (MM), with MMX force field) and quantum chemical (AM1 semiempirical method) techniques. The most stable structures of the molecule obtained by MM calculations have been used as input data for the semiempirical treatment. The position and orientation of the glucose moiety with regard to the remainder of the molecule have been investigated. The flexibility of isoquercitrin principally lies in rotations around the inter-ring bond and the sugar link. In order to know the structural modifications generated by the substitution by a sugar, geometrical parameters of quercetin (aglycon) and isoquercitrin have been compared. The good accordance between theoretical and experimental electronic spectra permits to confirm the reliability of the structural model.