Joëlle Mascetti

Concentration quenching of the Nd3+ emission in alkali rare earth borates

Four new ternary compounds with compositions Li6Nd(BO3)3, Na3Nd(BO3)2, and Na18Nd(BO3)7 have been found in the M2ONd2O3B2O3 systems, where M is either Li or Na. Concentration quenching of the neodymium emission in homologous lanthanum or gadolinium borates has been investigated. While in the Li6Gd(BO3)3, Na3La2(BO3)3, and Na3La(BO3)2 host lattices the quenching rate shows a quadratic dependence on Nd3+ concentration, as expected since the coordination polyhedra are connected by common faces or edges, in Na18La(BO3)7 the luminescent lifetime is not influenced by neodymium concentration. Lifetimes and crystal field splitting of the J levels are compared to those of other oxide host lattices.

The photodimerisation of trans-cinnamic acid and its derivatives: a study by vibrational microspectroscopy.

Infrared and Raman spectroscopies have been used to monitor the [2 + 2] photodimerisation reactions of alpha-trans-cinnamic acid and of a number of its derivatives. The principal changes observed in the spectra upon dimerisation are decay of a band around 1637 cm(-1), which is assigned to v(C=C) of the ethene bond of the monomer, and growth of bands just above 3000 cm(-1), which result from v(C-H) of saturated carbon atoms of the dimer. The use of microscope attachments has allowed us to follow the reactions of single crystals and we conclude that the reactions are topotactic in nature. We have carried out preliminary kinetic experiments in which spectra of one single crystal are recorded after sequential periods of photolysis. We find that the rates of dimerisation of differently substituted cinnamic acids are similar, with physical effects, such as the thickness of an individual crystal, outweighing any observed electronic effects (inductive or mesomeric).

Carbon dioxide interaction with metal atoms: Matrix isolation spectroscopic study and DFT calculations

Abstract This paper collects the results obtained in different studies on the interaction of the CO 2 molecule with transition metal atoms, using matrix isolation FTIR spectroscopy and density functional theory (DFT). Late-transition metal atoms (Fe, Co, Ni and Cu) form one-to-one M(CO 2 ) complexes, while those from the left-hand side in the periodic table (Ti, V, and Cr) insert spontaneously into a CO bond yielding oxocarbonyl species. Owing to isotopic experiments with 13 CO 2 and C 18 O 2 , these results allow spectroscopic identification of carbon dioxide bonding modes in organometallic species containing CO 2 moiety. Special attention is paid to the interaction of CO 2 molecule with Ni and Ti atoms. In neat CO 2 matrices, it is shown that CO 2 is side-on coordinated to nickel in a 1:1 complex. The binding energy is weak (18 kcal mol −1 ). In argon diluted matrices, no reaction occurs, even after annealing. Interestingly, the coordination of CO 2 is promoted by adding N 2 in the rare gas matrix. This is rationalized by comparing the potential energy curves corresponding to the interaction of the Ni atom or the NiN 2 moiety with CO 2 . The binding energy is then 32 kcal mol −1 . DFT calculations show that Ti inserts with no energy barrier into a CO bond, resulting in an OTiCO insertion product, which is far more stable than any of the possible Ti(CO 2 ) complexes and reactive towards CO 2 . An intrinsic reaction path for the insertion process is investigated.

The crystal chemistry of the new rare-earth sodium borates Na3Ln(BO3)2(Ln = La, Nd)

Abstract The ternary borate systems Na2OLn2O3B2O3 (Ln = La, Nd) have been investigated in view of obtaining high-neodymium-concentration materials with weak concentration quenching. A ternary phase of composition Na3Ln(BO3)2 (Ln = La, Nd) has been found. It crystallizes in the monoclinic space group P2 1 c . The structure has been determined for Na3Nd(BO3)2. A full-matrix least-squares refinement led to R = 0.040. The structure is formed by isolated Bo3 triangles held together by the neodymium and sodium ions. The rare-earth atoms have a complex eightfold coordination in a covalent BO3 matrix.

IR evidence for the formation of CO2 transition-metal atom complexes in low-temperature matrices

The interactions of CO2 with several first-row transition-metal atoms in low-temperature matrices (15 K) have been investigated by infrared spectroscopy, In pure CO2 matrices, Ti, V, Cr, Fe, Co, Ni and Cu form complexes with CO2 in a bent arrangement, as shown by the new absorptions arising after deposition and isotopic substitutions with 13CO2 and C18O2. These complexes are stable only at very low temperatures. In the case of Ni, annealing above 80 K leads to the reduction of carbon dioxide into carbon monoxide with simultaneous formation of nickel carbonyl complexes.

Efficient photochemistry of coronene:water complexes

The photochemistry of ices with polyaromatic hydrocarbons (PAHs) has been extensively studied, but to date no investigation has been made of PAHs in interaction with few (n < 4) molecules of water. We have performed photochemical matrix isolation studies of coronene:water complexes, probing the argon matrix with FTIR spectroscopy. We find that coronene readily reacts with water upon irradiation with a mercury vapour lamp to produce oxygenated PAH photoproducts, and we postulate a reaction mechanism via a charge transfer Rydberg state. This result suggests that oxygenated PAHs should be widely observed in regions of the ISM with sufficiently high water abundances. In order to explain the low derived observational abundances of oxygenated PAHs, additional destruction routes must be invoked.

Water clusters in an argon matrix: infrared spectra from molecular dynamics simulations with a self-consistent charge density functional-based tight binding/force-field potential.

The present theoretical study aims at investigating the effects of an argon matrix on the structures, energetics, dynamics, and infrared (IR) spectra of small water clusters (H2O)n (n = 1-6). The potential energy surface is obtained from a hybrid self-consistent charge density functional-based tight binding/force-field approach (SCC-DFTB/FF) in which the water clusters are treated at the SCC-DFTB level and the matrix is modeled at the FF level by a cluster consisting of ∼340 Ar atoms with a face centered cubic (fcc) structure, namely (H2O)n/Ar. With respect to a pure FF scheme, this allows a quantum description of the molecular system embedded in the matrix, along with all-atom geometry optimization and molecular dynamics (MD) simulations of the (H2O)n/Ar system. Finite-temperature IR spectra are derived from the MD simulations. The SCC-DFTB/FF scheme is first benchmarked on (H2O)Arn clusters against correlated wave function results and DFT calculations performed in the present work, and against FF data available in the literature. Regarding (H2O)n/Ar systems, the geometries of the water clusters are found to adapt to the fcc environment, possibly leading to intermolecular distortion and matrix perturbation. Several energetical quantities are estimated to characterize the water clusters in the matrix. In the particular case of the water hexamer, substitution and insertion energies for the prism, bag, and cage are found to be lower than that for the 6-member ring isomer. Finite-temperature MD simulations show that the water monomer has a quasifree rotation motion at 13 K, in agreement with experimental data. In the case of the water dimer, the only large-amplitude motion is a distortion-rotation intermolecular motion, whereas only vibration motions around the nuclei equilibrium positions are observed for clusters with larger sizes. Regarding the IR spectra, we find that the matrix environment leads to redshifts of the stretching modes and almost no shift of the bending modes. This is in agreement with experimental data. Furthermore, in the case of the water monomer and dimer, the magnitudes of the computed shifts are in fair agreement with the experimental values. The complex case of the water hexamer, which presents several low-energy isomers, is discussed.

Formation of peroxocarbonates from L3Rh(O2)Cl and L2Ni(CO2) : a unique reaction mechanism with carbon dioxide insertion into the O-O bond

Publisher Summary Transition-metal peroxocarbonates of formula L n M(CO 4 )X m are known since a long time. If the peroxocarbonate complexes are prepared by reaction of dioxygen complexes of transition metals L n M(O 2 )X m (M =Pd, Pt, Rh, Ir) with carbon dioxide, [1a] in principle, two ways are possible, that imply the formal insertion of carbon dioxide into the O-O or M-O bond.

Electronic spectroscopy of protonated 1-aminopyrene in a cold ion trap

In aromatic systems that contain an amino group, there is competition between protonation on a carbon atom of the skeleton and protonation on the amino group. Herein, we studied the photofragmentation of protonated 1-aminopyrene in a cold ion trap and mainly observed the protonated amino tautomer, which led to fragmentation pathways through the loss of H or NH3 groups. Several excited states were assigned, among which the fourth excited state showed broadened bands, thus indicating a fast decay that was attributed to the presence of a πσ* charge-transfer state by comparison of the experimental results with ab initio calculations. We deduced the πσ* transition energies in protonated aromatic amino compounds of increasing size directly from the ionization potentials of the neutral aromatic unsubstituted molecules. Tautomers that were protonated on a carbon atom of the pyrene skeleton were also weakly observed, and we showed that two tautomers that were protonated on a carbon atom of the aromatic ring could be distinguished by using electronic spectroscopy.

Photochemistry of Fe:H2O Adducts in Argon Matrixes: A Combined Experimental and Theoretical Study in the Mid-IR and UV–Visible Regions

The photochemistry of Fe:H2O adducts is of interest in fields as diverse as catalysis and astrochemistry. Industrially, iron can be used as a catalyst to convert H2O to H2, whereas in the interstellar medium it may be an important component of dust grains, influencing the chemistry on their icy surfaces. This study consisted of the deposition and spectral characterization of binary systems of atomic iron with H2O in cryogenic argon matrixes. In this way, we were able to obtain information about the interaction of the two species; we observed the formation of adducts of iron monomers and dimers with water molecules in the mid-IR and UV-visible spectral domains. Upon irradiation with a UV radiation source, the iron species were inserted into the water molecules to form HFeOH and HFe2OH, leading in some cases to the formation of FeO possibly accompanied by the production of H2. DFT and correlated multireference wave function calculations confirmed our attributions. This combination of IR and UV-visible spectroscopy with theoretical calculations allowed us to determine, for the first time, the spectral characteristics of iron adducts and their photoproducts in the UV-visible and in the OH stretching region of the mid-IR domain.