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).     

FT-IR, FT-Raman spectra and quantum chemical calculations of 3,4-dimethoxyaniline

In this work, we will report a combined experimental and theoretical study on molecular and vibrational structure of 3,4-dimethoxyaniline (3,4-DMA). The Fourier transform infrared and Fourier transform Raman spectra of 3,4-DMA was recorded in the solid phase. The optimized geometry was calculated by HF and B3LYP methods using 6-31G(d,p) and 6-311++G(d,p) basis sets. The harmonic vibrational frequencies, infrared intensities, Raman scattering activities and the thermodynamic functions of the title compound were performed at and HF/B3LYP/6-311++G(d,p) level of theories. The scaled theoretical wavenumber showed very good agreement with the experimental values. A detailed interpretation of the infrared and Raman spectra of 3,4-DMA was reported. The theoretical spectrograms for IR and Raman spectra of the title molecule have been constructed.     

Vibrational spectra and quantum chemical calculations of 3,4-diaminobenzoic acid.

The Fourier Transform Raman and Fourier Transform infrared spectra of 3,4-diaminobenzoic acid (3,4-DABA) were recorded in the solid phase. Geometry optimizations were done without any constraint and harmonic-vibrational wave numbers and several thermodynamic parameters were calculated for the minimum energy conformer at ab initio and DFT levels invoking 6-311++G(d,p) basis set. The results were compared with the experimental values with the help of specific scaling procedures, the observed vibrational wavenumbers in FT-IR and FT-Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range, the error obtained was in general very low. The appropriate theoretical spectrograms for the IR and Raman spectra of the title molecule were also constructed.     

Complexation of lead(II) by chlorogenic acid: experimental and theoretical study

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations have been performed on chlorogenic acid (H(3)CGA), a polyphenolic compound, used as a model molecule of humic substances. The different deprotonated forms of H(3)CGA have also been investigated. H(3)CGA is a multisite ligand that presents several metal complexing sites in competition, notably the carboxylic and catechol moieties. In low acidic aqueous medium, the complexation of Pb(II) has been followed by electronic absorption spectrometry. The formation of two complexes of stoichiometry metal:ligand 1:1 (log beta(1:1) = 3.39) and 2:1 (log beta(2:1) = 7.12) has been highlighted with use of chemometric methods. The theoretical spectrum of the 1:1 complex obtained by TD-DFT methodology shows the formation of a chelate [Pb(H(2)CGA)(H(2)O)(3)](+) with the metal fixation at the level of the carboxylate function. The second complexing site, the catechol moiety, is rapidly involved in the formation of the 2:1 complex from molar ratios [metal]/[ligand] higher than 0.1. The electronic transitions calculated for both free ligand and complexes involved the same molecular orbitals, and no ligand-metal or metal-ligand charge transfer is observed.     

A DFT/TDDFT study of the structural and spectroscopic properties of Al(III) complexes with 4-nitrocatechol in acidic aqueous solution

The complexation of 4-nitrocatechol in aqueous solution at pH 5 has been studied by molecular spectroscopy combined with quantum chemical calculations. In these physico-chemical conditions, the formation of the two complexes [4ncatAl(H₂O)₄]⁺ and [(4ncat)₂Al(H₂O)₂]⁻ has been highlighted. The electronic absorption spectra of the 1:1 and 1:2 complexes of Al(III) with 4-nitrocatechol have been computed using the time-dependent density functional theory and the polarizable continuum model. It turns out that the 6-311+G(d,p) basis set provides a good agreement between experimental and theoretical absorption spectra. This good agreement has allowed the determination of the preferential conformation of the 1:2 complex in aqueous solution. A complete assignment of the UV–Vis absorption and Raman spectra of the complexes has been proposed.     

Time dependent density functional theory study of electronic absorption properties of lead(II) complexes with a series of hydroxyflavones

The structural changes occurring with the chelation of lead(II) to 3-hydroxyflavone, 5-hydroxyflavone, and 3',4'-dihydroxyflavone have been investigated by the density functional theory (DFT) method with the B3LYP functional and the 6-31G(d,p) basis set. The two effective core potentials Lanl2dz (Los Alamos) and MWB78 (Stuttgart/Dresden) were used for the Pb ion. Only the 3',4'-dihydroxyflavone ligand shows minor geometrical modifications upon chelation, whereas the two other ligands present important changes of their chromone moiety. The time dependent density functional theory (TD-DFT) has been employed to calculate the electronic absorption spectra of the 1:1 complexes of lead(II) with the three hydroxyflavones, as well in a vacuum as in methanol. The solvent effect is modeled using the self-consistent reaction field (SCRF) method with the polarized continuum model (PCM). Comparison with experimental data allows a precise assessment of the performances of the method, which appears competitive and suitable to reproduce the spectral measurements when the solvent effect is taken into account. These calculations and the molecular orbital analysis have allowed an explanation of the different behaviors of the three ligands toward Pb(II) and particularly the fact that no bathochromic shift is observed with the addition of lead(II) to a 5-hydroxyflavone solution. A complete assignment of the electronic absorption spectra of both free and complexed ligands has been carried out.     

Determination of the chelating site preferentially involved in the complex of lead(II) with caffeic acid: a spectroscopic and structural study

The complexation of lead(II) with mono-deprotonated caffeic acid in aqueous solution (pH = 6.50) has been investigated by UV-visible, fluorescence, and vibrational spectroscopies combined with quantum chemical calculations (DFT). The caffeate ion presents two chelating sites in competition: the carboxylate and the catechol functions. Electronic spectroscopies highlighted two different complexed forms with, respectively, 1:1 and 2:1 stoichiometry. The 1:1 complex predominates for low lead concentrations, even if the second complexed form appears before the first chelating site is fully occupied. Both spectroscopic data and calculations reveal that Pb(II) preferentially coordinates with the carboxylate function, in opposition with previous results found for the Al(III) complexation, where the catechol group presents the greater complexing power. The structural and vibrational modifications between the mono-deprotonated ligand and 1:1 complex engendered by the chelation are discussed. Water molecules have been added on the Pb ion to modify its coordination, and structures of Pb(H(2)CA)(H(2)O)(n)(+) with n = 0-4 were optimized. Calculations of theoretical frequencies have permitted us to propose a tentative assignment of infrared and Raman spectra of complexed species.