Claude Bremard

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

Vibrational spectra of cation-exchanged zeolite A. Experimental and molecular dynamics study

Abstract A simple potential representing the aluminosilicate framework with a simplified general valence force field and the cation-framework and cation-cation interactions with the help of changes, polarizabilities, and diamagnetic susceptibilities is used in a molecular dynamics study of the infrared and Raman spectra of cation-exchanged zeolite A with Si/Al ratio of 1. The results are compared to experimental Raman spectra of a series of dehydrated samples. The influence of the cations is evident up to 500 cm −1 . Couplings of cation and framework motions are observed, as well as couplings of cations in different sites.

Sorption of anthracene, phenanthrene and 9,10-dimethylanthracene on activated acid HZSM-5 zeolite. Effect of sorbate size on spontaneous ionization yield

In situ diffuse reflectance UV visible and Raman scattering spectroscopies were used to monitor the spontaneous ionization of anthracene, phenanthrene and 9,10-dimethylanthracene by direct exposure to thermally activated acid HZSM-5 zeolite. Calcination of the zeolite under oxygen is a prerequisite for the spontaneous ionization through the formation of Lewis acid sites. The framework structure of ZSM-5 zeolites contains two types of intersecting channels sufficiently wide to allow rod-shaped molecules like anthracene to pass through and to diffuse into the void space. As expected, molecular modeling calculations suggest that anthracene is able to penetrate into the void space of zeolite and that phenanthrene and dimethylanthracene cannot pass through due to steric constraints. The anthracene radical cation is found to be generated in high yield and to be stable over several months. The tight fit between the shape of anthracene and the pore size of the straight channels of the zeolite is considered to be the main factor responsible for the stabilization of the anthracene radical ion. Dimethylanthracene is found to generate dimethylanthracene radical cation in very low yield. Dimethylanthracene radical cation is probably located at the pore openings and is not stabilized in the void space. Ionization of phenanthrene occurs in low yield. However the resulting stable radical cation is identified as anthracene radical cation. The assumption is made that phenanthrene isomerizes readily at the openings of pores via its radical cation.

Electrochemical oxidation of nitrogen dioxide in aprotic media: Kinetic and thermodynamic constants relative to the equilibria N2O4 ⇄ 2 NO2 and N2O4 ⇄ NO2+ + NO2−

The equilibrium constant value of the homolytic dissociation: N2O4 ⇄ 2 NO2 has been determined in some aprotic media using NMR and ESR techniques. From these results, we have undertaken the electrochemical study of the N2O4 oxidation in these solvents. N2O4 undergoes an electron transfer preceded by a monomerization reaction as follows: Mathematical models applied to this process, have allowed us to obtain the rate constants k1, k−1, and the standard potential of the NO2+/NO2 and NO2+/12 N2O4 systems. Hence, it has been possible to derive the value of the equilibrium constant for the ionic dissociation: N2O4 ⇄ NO2+ + NO2−.

Dicopper(II) complexes of novel polyfunctional pyridazines: crystal structure and magnetic properties of bis[µ-pyridazine-3,6-dicarbaldehyde dioximato(1–)-κN1,N′:N2,N″]-bis[aqua(perchlorato-κO)copper(II)]

Two novel pyridazine ligands with 3,6-CRNOH (R = H, H2L1; Ph, H2L2) oxime side chains were synthesised. The related copper(II) dinuclear complexes [Cu2(HL1)2(ClO4)2(H2O)2]1 and [Cu2(HL2)2(ClO4)2(MeOH)n]2 were obtained subsequently. The crystal structure of 1 was determined: space group P21/n, a= 12.031(6), b= 9.517(4), c= 9.973(5)A, β= 100.16(4)° and Z= 2. The copper(II) ions of the binuclear unit are bridged by the two diazine fragments of the two essentially planar tetradentate ligands with the oxime nitrogen atoms completing the equatorial co-ordination. Two intra-complex hydrogen bridges link the terminal oximato moieties to give dinucleating macrocyclic complexes. In complexes 1 and 2 the copper(II) is in a classical 4 + 2 environment. The magnetic properties of those compounds revealed a spin-singlet ground state in each case. The singlet-triplet energy gaps were found to be –536(2) for 1 and –545(4) cm–1 for 2. The low-lying states of the complexes are discussed in relation to the nearly planar structure of the macrocyclic complexes.

Sorption of biphenyl in non-acidic MFI-type zeolites: spectroscopic and modeling studies

Abstract The diffuse reflectance UV–VIS and IR absorption, Raman scattering spectrometries as well as 13 C-MAS-NMR experiments provide evidence for the sorption, structure and dynamics of biphenyl (BP, C 12 H 10 ) as intact molecule in non-acidic MFI zeolites, silicalite-1 and Al-ZSM-5. The effect of the aluminum content n , the nature of the charge-balancing cation M n + and the BP loading in M m / n (AlO 2 ) m (SiO 2 ) 96− m ( M m / n ZSM-5; m =0, 3, 6; n =1, 2; M=Li + , Na + , K + , Rb + , Cs + , Tl + , Zn 2+ , Cd 2+ ) was examined. X-ray powder diffraction, 29 Si, 27 Al, MAS-NMR as well as Raman scattering experiments provide evidence for structural changes of the host upon BP sorption even at low loading. From Monte Carlo simulations, the preferential sites of BP were determined in the straight channels of purely siliceous silicalite-1 with orthorhombic crystal symmetry, whereas well defined sites are energetically favored in the vicinity of extra-framework cations of aluminated ZSM-5. Molecular mechanics calculations indicate that BP lies in the straight channels in a twisted conformation with a phenyl group facially coordinated to the extra-framework cations. From the molecular dynamics calculations, the BP mean square displacements (MSD) indicate that on the simulation time (1 ns) BP is occluded in its sorption site. The calculated frequencies of vibrational densities of states of selected atoms of BP were found to be in agreement with the frequencies of IR and Raman bands assigned to corresponding vibrational modes.

Spectroscopic Investigations of Malaria Pigment

Malaria pigment is generated during the growth of the intraerythrocytic parasite plasmodium. This compound is postulated as a product of haemoglobin degradation and consists of an unknown iron porphyrin compound and an apoprotein. We report here a new spectroscopic investigation on the porphyrin component of intact malaria pigment obtained from in vitro cultures of Plasmodium falciparum without any previous chemical or biochemical treatment of the sample. We demonstrate the use of resonance Raman microspectrometry, electron paramagnetic resonance spectrometry, and magnetic susceptibility measurements that show the iron porphyrin moiety of the malaria pigment to be a high-spin monomeric iron(III) protoporphyrin hydroxide.

Effects of spatial constraints and Brønsted acid site locations on para-terphenyl ionization and charge transfer in zeolites.

The locations of Brønsted acid sites (BAS) in the channels of medium-pore zeolites have a significant effect on the spontaneous ionization of para-terphenyl (PP(3)) insofar as spatial constraints determine the stability of transition states and charge-transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H(+) polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP(3) incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H-FER, H-MFI, H-MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP(3)(.+) decay is hole transfer to form AlO(4)H(.+) ⋅⋅⋅PP(3) charge-transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron-transfer theory, the small reorganization energy in constrained channels explains the slow hole-transfer rate.

ORGANOMETALLIC CHEMISTRY OF GROUP VI METALS IN THE VOID SPACE OF ZEOLITES

Abstract Many salient features about the structures, dynamics and transport of organometallic complexes of group VI metals (M(CO) 6 , M=Cr, Mo, W) and ligands (C 6 H 6 , C 5 H 5 N, P(CH 3 ) 3 ) occluded within the void space of faujasitic zeolites have been deduced from X-ray and neutron diffraction, EXAFS, inelastic neutron scattering, RMN, IR absorption, Raman scattering investigations as well as from theoretical methods. The faujasitic zeolites are porous crystalline aluminosilicates with M ′ n [(SiO 2 ) 192− n (AlO 2 ) n ] (M ′ n FAU) formulae per unit cell, the M(CO) 6 and ligand molecules can gain free access to the inner void space (supercages) through the smallest section called the windows. The silicon–aluminum ratios (Si/Al) of the framework are found to be in the 1–100 range whereas M′ + cations compensate the AlO − 2 charges of the framework (M + ′=H + , Li + , Na + , K + , Rb + , Cs + ). From the in situ spectroscopic investigations and molecular modeling a coherent picture of the reactions between M(CO) 6 and ligands under thermal activation has been drawn as a function of the aluminum content of the zeolite. Among the available in situ spectroscopic investigations (far-IR, Raman, UV–visible, NMR) diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in the mid-IR region provides an efficient tool for dehydration, sorption and cosorption as well as subsequent reactions under thermal activation even at low coverage. In siliceous faujasite (Si/Al=100) the M(CO) 6 and ligand molecules are randomly distributed within the void space and the molecular motions approach the rapid isotropic limits of liquids. The chemical behavior upon thermal activation is found to be analogous to that observed in solution. In Na 56 FAU (Si/Al=2.5) the reagents are trapped in well-defined sorption sites in close proximity. Upon gentle thermal activation a fast reaction occurs to form M(CO) 3 (η 6 -C 6 H 6 ) or Mo(CO) 3 (C 5 H 5 N) 3 , or Mo(CO) 4 (P(CH 3 ) 3 ) 2 as major species inside the supercage through a concerted mechanism including the electrostatic field and the basicity of the framework oxygens. In Na 85 FAU (Si/Al=1.25) the M(CO) 6 and ligand molecules are not encapsulated in close proximity. M(CO) 6 reacts thermally in the void space in the absence of ligand to lose sequentially three CO ligands and form predominantly anchored M(CO) 3 (O z ) 3 species in which the three vacant coordination sites are occupied by three O z framework oxygens. The substitution kinetics have been interpreted on the basis of structure and dynamics of reactant and product guests in the zeolites. The kinetics data reveal that the available porous volume of zeolite can provide a precisely defined activating environment to obtain faster reactions than in homogeneous media. The dynamics of the supramolecular assembly of M(CO) 6 , ligands and extraframework cations M′ + in the vicinity of the oxygen atoms of the inner surface can undergo rapid and specific organometallic reactions under geometry, size, shape and electrostatic constraints.

Electrocatalysis of the nitration of naphthalene by N2O4 in aprotic media

Abstract In order to examine the possibility of effective nitration of aromatics with electrochemically generated catalysts, we studied the nitration of naphthalene by N 2 O 4 . Solutions of small amounts of nitronium ion NO 2 + in dry sulfolane pepared by partially controlled potential electrolysis of dinitrogen tetroxide, nitrate naphthalene efficiently through a catalytic process involving the well-known species NO + . Likewise, exhaustive electrolysis of N 2 O 4 solutions (with or without naphthalene) containing water at trace levels at potentials insufficient to oxidise nitrogen dioxide (and naphthalene), also generates the catalytic species NO + . Using Raman spectroscopy, voltammetric techniques and gas chromatography, the mechanistic aspects of the catalytic reactions are discussed, taking into account the thermodynamic, kinetic and electrochemical properties of oxygenated nitrogen compounds in aprotic media.