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Esboui M.,Laboratoire Of Spectroscopie Atomique | Jouvet C.,University Paris - Sud | Dedonder C.,University Paris - Sud | Ebata T.,Hiroshima University
Journal of Physical Chemistry A | Year: 2010

The excited-state lifetimes of different vibrational levels of the 2-hydroxypyridine-ammonia complex have been recorded with the picosecond pump probe technique. These lifetimes decrease with increasing energy. The decrease of the lifetime is discussed based on the possible enol keto isomerization computed using RICC2 methods. © 2010 American Chemical Society.


Ahmed J.B.,Laboratoire Of Spectroscopie Atomique | Fouad F.,University Mohammed Premier
IEEE Transactions on Plasma Science | Year: 2014

In this paper, we propose and use an experimental method permitting the measurement of optical depth of a self-absorbed spectral line emitted from laser-induced plasma on the surface of an aqueous solution. Using well-known equations describing the radiatif transfer and the evolution with the optical depth of the relative uncertainty on the measurement of line intensity and on the half width at half maximum, we have also deduced the uncertainty on the measurement of electron temperature and the electron density. An application to the diagnostics of this plasma has been realized, and we show the reliability of the spectroscopic measurements. 0018-9456 © 2014 IEEE.


Ruiz-Lopez M.F.,University of Lorraine | Fredj A.B.,University of Lorraine | Fredj A.B.,Laboratoire Of Spectroscopie Atomique
Journal of Physical Chemistry B | Year: 2010

theoretical analysis of chlorophyll a (Chla) hydration processes in aqueous organic solvents has been carried out by means of quantum chemistry calculations. A detailed knowledge of the thermodynamics of these processes is fundamental in order to better understand the organization of chlorophyll molecules in vivo, specifically the structure of chlorophyll pairs in photosystems I and II. In the present work, we assumed a Chla model in which the phytyl chain is replaced by a methyl group. Calculations were performed at the B3LYP/6-31G(d) level corrected for basis set superposition errors and dispersion interaction energy. This computational scheme was previously shown to provide data close to MP2/6-311++(2d,2p) results. Solvents effects were taken into account using either continuum (for nonpolar solvents) or discrete-continuum (for polar coordinating solvents) methods. In the latter case, we first examined the structure of Chla in rigorously dry solutions. Two types of solvents were characterized according to Mg-atom coordination: In type I solvents (acetone, acetonitrile, DMSO), Mg exhibits five-coordination, whereas in type II solvents (THF, pyridine), Mg exhibits six-coordination. Hydration processes are quite dependent on solvent nature. In nonpolar or low-polarity solvents such as cyclohexane or chloroform, hydration is always exothermic and exergonic, despite a large entropy term that strongly opposes hydration. In polar solvents of type II, hydration is quite unfavorable, and essentially no hydrates are expected in these media, except perhaps at very large water concentrations (although, in such a case, the medium cannot be simply described as an organic solvent). In polar solvents of type I, the situation is intermediate, and dihydration is favorable in some cases (acetone, acetonitrile) and unfavorable in others (DMSO). It is interesting to note that first hydration processes in coordinating solvents (of either type I or type II), where a water molecule must displace a solvent molecule coordinated to Mg, exhibit values of δH > 0 and AS > 0, in sharp contrast to first hydration processes in nonpolar media. The present results represent the first theoretical attempt to rationalize the large amount of experimental data on hydration and aggregation of Chla in aqueous organic media that have been accumulated over the past four decades. The data stress, in particular, the key role of Chla dihydrates, a point that has been the object of intense debate in the literature. Clearly, dihydrates are found to be more stable than monohydrates owing to a particular structure in which cooperative interactions occur between the water molecules and Chla. The calculations also explain the irregular behavior observed for Chla in aqueous THF or pyridine: In these media, Chla remains basically unhydrated because the Chla-solvent adducts are stabilized by strong dispersion interactions. © 2010 American Chemical Society.


Ahmed J.B.,Laboratoire Of Spectroscopie Atomique | Cowpe J.,University of Salford
Applied Optics | Year: 2010

Using well-known expressions describing radiative transfer, we have established an expression predicting the spectral profile of a self-absorbed Ca++ 393:4nm emission line as emitted by a transient laser-induced plasma. In this approach, the plasma was approximated as comprising five distinct layers, each of thickness 0:5mm, and each characterized by a unique uniform electron density, electron temperature, and optical depth. The validity of the theoretical model was confirmed by successful comparison with experimental data. Inhomogeneous laser-induced plasmas were produced on the surface of an aqueous CaCl2 (0:01mol=l) solution using a frequency-doubled Nd:YAG laser. Optical emission spectra were collected in such a way as to allow for temporal and spatial diagnostics of the plasma plumes. © 2010 Optical Society of America.


Esboui M.,Laboratoire Of Spectroscopie Atomique | Esboui M.,P.A. College
Journal of Chemical Physics | Year: 2015

The stepwise and concerted excited state intermolecular proton transfer (PT) and hydrogen transfer (HT) reactions in 2-hydroxypyridine-(NH3)2 complex in the gas phase under Cs symmetry constraint and without any symmetry constraints were performed using quantum chemical calculations. It shows that upon excitation, the hydrogen bonded in 2HP-(NH3)2 cluster facilitates the releasing of both hydrogen and proton transfer reactions along ammonia wire leading to the formation of the 2-pyridone tautomer. For the stepwise mechanism, it has been found that the proton and the hydrogen may transfer consecutively. These processes are distinguished from each other through charge translocation analysis and the coupling between the motion of the proton and the electron density distribution along ammonia wire. For the complex under Cs symmetry, the excited state HT occurs on the A″(1πσ∗) and A′(1nσ∗) states over two accessible energy barriers along reaction coordinates, and excited state PT proceeds mainly through the A′(1ππ∗) and A″(1nπ∗) potential energy surfaces. For the unconstrained complex, potential energy profiles show two 1ππ∗-1πσ∗ conical intersections along enol → keto reaction path indicating that proton and H atom are localized, respectively, on the first and second ammonia of the wire. Moreover, the concerted excited state PT is competitive to take place with the stepwise process, because it proceeds over low barriers of 0.14 eV and 0.11 eV with respect to the Franck-Condon excitation of enol tautomer, respectively, under Cs symmetry and without any symmetry constraints. These barriers can be probably overcome through tunneling effect. © 2015 AIP Publishing LLC.


Ben Fredj A.,Laboratoire Of Spectroscopie Atomique
Helvetica Chimica Acta | Year: 2016

Dimeric structures chlorophyll (a) (Chla) and their mono- and dihydrated have been suggested to play an important role in the mechanism of photoreaction center chlorophyll special pairs PSI and PSII. Despite their functional importance, the molecular basis structures for interacting two Chla molecules and the structural stabilization role of H2O in the formation of hydrated Chla dimer complexes is poorly understood. In this article, the different coordination modes between two interacting Chla molecules and the configurational (orientation and distance) features between the dimer and bound H2O molecules are characterized by means of super molecule approach the density functional theory DFT. An estimation of the thermodynamic quantities is made for Chla dimerization and hydration processes. The results indicate that structure including ester linkages via H2O hydrogen bonding is the most favorable conformation for the dihydrated chlorophyll (a) dimer at B3LYP/6-31G∗-DCP level of calculation. The dispersion interaction is shown to be of great significance for the Chla dimer stabilization. In aqueous nonpolar solvent, the thermodynamics show that Chla has a slightly stronger driving force for full hydration than for dimerization and that hydration of the dimers is rather weakly exergonic. The tetrahydrated dimers having a similar arrangement to that in crystals of ethyl chlorophyllide (a) dihydrate are found to be more stable than the Chla dihydrated dimer. The data underscore the key role of H-bonding in the stability of Chla-H2O adducts and, in particular, the great importance of the Chla monomeric dihydrated species in the hydration and dimerization of Chla in aqueous media. Clearly, the Chla dihydrates (Chla-2 H2O) are found more stable than the monohydrates (Chla-H2O) and the Chla dimers (Chla2), owing to a particular structure in which cooperative interactions occur between the H2O molecules and Chla. Calculations also indicate that the most thermodynamically preferred pathway for the formation of Chla dimer hydrates can be represented by two steps: the first corresponds to the formation of Chla monomeric dihydrates and the second is the dimerization of the dihydrates on to tetrahydrated Chla dimers. These results allow to obtain a new possible pathway for Chla dimer formation processes and could provide new insights to the aggregation of chlorophyll (a) in solution. © 2016 Verlag Helvetica Chimica Acta AG, Zürich.


PubMed | Laboratoire Of Spectroscopie Atomique
Type: Journal Article | Journal: The journal of physical chemistry. A | Year: 2010

The excited-state lifetimes of different vibrational levels of the 2-hydroxypyridine-ammonia complex have been recorded with the picosecond pump probe technique. These lifetimes decrease with increasing energy. The decrease of the lifetime is discussed based on the possible enol keto isomerization computed using RI-CC2 methods.


Esboui M.,Laboratoire Of Spectroscopie Atomique | Esboui M.,P.A. College | Jaidane N.,Laboratoire Of Spectroscopie Atomique
Photochemical and Photobiological Sciences | Year: 2015

Minimum energy structures of the ground and lowest excited states of the phenol (PhOH)-pyridine (Py) hydrogen-bonded complex in the gas phase were determined by ab initio calculations. Photophysical and photochemical features of the complex under Cs symmetry (planar (Pl) and perpendicular (Pe) conformers) and without any symmetry constraints (unconstrained (Un) conformer) were studied with respect to nonradiative decay processes to the ground state. The mechanism involves internal conversion (IC) and intersystem crossing (ISC) along the O-H bond elongation coordinate, where a coupled electron/proton-transfer reaction plays a decisive role in the photophysics of this complex. For the Pl conformer, nonradiative decay proceeds from a locally excited 1ππ∗(LE) minimum over a conical intersection barrier (0.12 eV) to a charge-transfer (CT) minimum, which corresponds to a hydrogen-bonded PhO⋯HPy biradical. Near this second minimum, a barrierless conical intersection 1A′(ππ∗(CT))-S0 funnels the electron population from the CT to the ground S0 state, completing the nonradiative deactivation. Calculations performed for the Pe and Un conformers confirmed that the same radiationless mechanism proceeds with no 1ππ∗(LE)/1ππ∗(CT) conical intersection near the Franck-Condon region. Furthermore, the population of the lowest triplet states via ISC and their contribution to the photophysics of PhOH-Py complex have been discussed. These findings appear to suggest that there is no single dominant path, but rather many distinct paths involving different quenching mechanisms. © The Royal Society of Chemistry and Owner Societies 2015.


PubMed | Laboratoire Of Spectroscopie Atomique
Type: Journal Article | Journal: Photochemical & photobiological sciences : Official journal of the European Photochemistry Association and the European Society for Photobiology | Year: 2015

Minimum energy structures of the ground and lowest excited states of the phenol (PhOH)-pyridine (Py) hydrogen-bonded complex in the gas phase were determined by ab initio calculations. Photophysical and photochemical features of the complex under Cs symmetry (planar (Pl) and perpendicular (Pe) conformers) and without any symmetry constraints (unconstrained (Un) conformer) were studied with respect to nonradiative decay processes to the ground state. The mechanism involves internal conversion (IC) and intersystem crossing (ISC) along the O-H bond elongation coordinate, where a coupled electron/proton-transfer reaction plays a decisive role in the photophysics of this complex. For the Pl conformer, nonradiative decay proceeds from a locally excited (1)*(LE) minimum over a conical intersection barrier (0.12 eV) to a charge-transfer (CT) minimum, which corresponds to a hydrogen-bonded PhOHPy biradical. Near this second minimum, a barrierless conical intersection (1)A(*(CT))-S0 funnels the electron population from the CT to the ground S0 state, completing the nonradiative deactivation. Calculations performed for the Pe and Un conformers confirmed that the same radiationless mechanism proceeds with no (1)*(LE)/(1)*(CT) conical intersection near the Franck-Condon region. Furthermore, the population of the lowest triplet states via ISC and their contribution to the photophysics of PhOH-Py complex have been discussed. These findings appear to suggest that there is no single dominant path, but rather many distinct paths involving different quenching mechanisms.

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