Influence of steric hindrance on the molecular packing and the anchoring of quinonoid zwitterions on gold surfaces

Driven by the huge potential of engineering the molecular band offset with highly dipolar molecules for improving charge injection into organic electrics, the anchoring of various N-alkyl substituted quinonoid zwitterions of formula C6H2 (···NHR)2 (···O)2 (R = iPr, Cy, CH2CH(Et)CH2CH2CH2CH3,. . .) on gold surfaces is studied. The N–Au interactions result in an orthogonal arrangement of the zwitterions cores with respect to the surface, and stabilize adsorbed compact rows of molecules. IR spectroscopy is used as a straightforward diagnostic tool to validate the presence of ultra-thin molecular films. When combined with computational studies, IR measurements indicate that the presence of a CH2group in α position to the nitrogen atom is important for a successful anchoring through N–Au interactions. The presence of such a flexible CH2 spacer, or of aryl groups, enables π-interactions with the surface, making possible the anchoring of enantiopure or sterically-hindered zwitterions. X-ray diffraction analyses indicate that the intermolecular spacing within a row of molecules can be modulated by the nature of the alkyl substituent R. This modulation is directly relevant to the electronic properties of the corresponding molecular films since these zwitterions are expected to form rows on gold surfaces similar to those observed in the bulk crystalline state

Simulations par dynamique moléculaire de la solvatation et du comportement interfacial d'espèces hydrophobes.<br />Application à l'hypothèse TATB et à l'extraction liquide/liquide de cations par le CO2 supercritique.

We report molecular dynamics studies on the solvation of charged hydrophobic molecules in pure liquids and at liquid / liquid interfaces. The first part of the thesis deals with the TATB hypothesis according to which the AsΦ4+ (TA+) and BΦ4- (TB-) ions have the same free energy of solvation in any solvent. The two ions are found to be solvated differently in pure liquids (water, chloroform, acetonitrile) as at a chloroform / water interface. These results are confirmed by free energy calculations and by simulations on isovolumic spherical S+ and S- ions, which perfectly meet the TATB criteria. The many methodological tests performed show the importance of (i) the corrected treatment of "long range interactions" (ii) the precise repartition of atomic charges (iii) the solvent models, especially for water, on the + / - charge discrimination by solvent. In the second part, in relation to the liquid / liquid extraction of cations from water to supercritical CO2, we report the behaviour of ions (Cs+, UO22+, Eu3+), of uncomplexed extractants molecules (tri-n-butylphosphate, calixarene), of their complexes with the cations and nitric acid at a preformed chloroform / water interface and during demixing simulations which started from a perfectly mixed CO2 / water solutions. These studies demonstrate the importance of interfacial phenomena, of simulation conditions and acid and extractant concentrations, in assisted ion extraction to supercritical CO2.Nous avons étudié par simulations de dynamique moléculaire la solvatation de molécules hydrophobes chargées dans des liquides purs et à des interfaces liquide / liquide. La première partie concerne l'hypothèse TATB qui suppose que les deux ions AsΦ4+ (TA+) et BΦ4- (TB-) ont la même énergie de solvatation dans tout solvant. Nous avons montré que les deux ions étaient solvatés différemment dans des liquides purs (eau, chloroforme, acétonitrile) ainsi qu'à une interface chloroforme / eau. Des calculs de différences d'énergie libre de transfert ont confirmé cette tendance, de même que des simulations sur des ions "hypothétiques" S+ et S-, analogues sphériques de AsΦ4+ et BΦ4- qui répondent exactement aux critères de l'hypothèse. De nombreux tests méthodologiques ont été effectués et ont permis de montrer l'importance (i) d'une description correcte des interactions à "longue distance", (ii) de la répartition précise des charges atomiques et (iii) du modèle de solvant utilisé notamment pour l'eau, sur la différence de solvatation de "gros" ions hydrophobes selon leur charge. La seconde partie décrit les premières simulations avec le CO2 supercritique dans le cadre de l'extraction liquide / liquide de cations métalliques. Nous avons étudié le comportement d'ions (Cs+, UO22+, Eu3+), de molécules extractantes (tri-n-butylphosphate, calixarène), de complexes de ces cations avec ces molécules extractantes et d'acide nitrique à une interface préformée CO2 / eau et lors de simulations de séparation de phase, en partant de solutions binaires homogènes CO2 / eau. Ces études démontrent l'importance des phénomènes interfaciaux, des conditions de simulations, ainsi que de la concentration en acide et en extractant, dans les processus d'extraction vers le CO2 supercritique

Actual Chimique Actual Chimique

Ion extraction to supercritical CO2. Contribution from the molecular dynamics simulations Supercritical fluid carbon dioxide (SC-CO2) With its moderate critical constants, nonflammable nature and low cost provides an attractive alternative for replacing organic solvents traditionally used in liquid/liquid processes. A recently developed technique uses SC-CO2 as organic phase with suitable complexant molecules to extract metallic cations from water. We illustrate the contribution from molecular dynamics simulations to describe, at the molecular level, the solvation, complexation and migration of various species involved in the supercritical fluid extraction of cesium by calixarene, and of uranyl nitrate by TBP. Both systems are important in the context of nuclear waste partitioning with > solvents. The simulations demonstrate the importance of the interfacial phenomena in ion extraction to CO2. Water and CO2 do not mix and display an interface where the ligands adsorb and the complexation takes place. In the case of the uranyl extraction, spontaneous complexation and extraction has been observed in the simulations, which show the importance of the concentration in extractant and nitric acid. At high TBP and acid concentrations, the interface evolves from a well-defined border to a > where CO2, TBP, acid and complexes mix, allowing the final extraction of uranyl

Liquid−Liquid Extraction of Pertechnetic Acid (Tc VII ) by Tri- n -butyl Phosphate: Where Is the Proton? A Molecular Dynamics Investigation

International audienc

Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

International audienc

Simulations par dynamique moléculaire de la solvatation et du comportement interfacial d'espèces hydrophobes (application à l'hypothèse TATB et à l'extraction liquide/liquide de cations par le CO2 supercritique)

Nous avons étudié par simulations de dynamique moléculaire la solvatation de molécules hydrophobes chargées dans des liquides purs et à des interfaces liquide / liquide. La première partie concerne l'hypothèse TATB qui suppose que les deux ions Asf4+ (TA+) et Bf4- (TB-) ont la même énergie de solvatation dans tout solvant. Nous avons montré que les deux ions étaient solvatés différemment dans des liquides purs (eau, chloroforme, acétonitrile) ainsi qu'à une interface chloroforme / eau. Des calculs de différences d'énergie libre de transfert ont confirmé cette tendance, de même que des simulations sur des ions "hypothétiques" S+ et S-, analogues sphériques de Asf4+ et Bf4- qui répondent exactement aux critères de l'hypothèse. De nombreux tests méthodologiques ont été effectués et ont permis de montrer l'importance (i) d'une description correcte des interactions à "longue distance", (ii) de la répartition précise des charges atomiques et (iii) du modèle de solvant utilisé notamment pour l'eau, sur la différence de solvatation de "gros" ions hydrophobes selon leur charge. La seconde partie décrit les premières simulations avec le CO2 supercritique dans le cadre de l'extraction liquide / liquide de cations métalliques. Nous avons étudié le comportement d'ions (Cs+, UO22+, Eu3+), de molécules extractantes (tri-n-butylphosphate, calixarène), de complexes de ces cations avec ces molécules extractantes et d'acide nitrique à une interface préformée CO2 / eau et lors de simulations de séparation de phase, en partant de solutions binaires homogènes CO2 / eau. Ces études démontrent l'importance des phénomènes interfaciaux, des conditions de simulations, ainsi que de la concentration en acide et en extractant, dans les processus d'extraction vers le CO2 supercritiqueSTRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

J Phys Chem B J Phys Chem B

We report a molecular dynamics study of the solvation of UCl6-, UCl62-, and UCl63- complexes in the [BMI][Tf2N] and [MeBu3N][Tf2N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf2N-)) and the butyl-3-methyl-imidazolium(+) (BMI+) or methyl-tri-n-butyl-ammonium (MeBu3N+) cation, respectively. The comparison of two electrostatic models of the complexes (ionic model with -1 charged halides versus quantum mechanically derived charges) yields similar solvation features of a given solute. In the two liquids, the first solvation shell of the complexes is positively charged and evolves from purely cationic in the case of UCl63- to a mixture of cations and anions in the case of UCl6-. UCl63- is exclusively "coordinated" to BMI+ or MeBu3N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl6- complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu3N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl63- interacts more attractively with the [BMI][Tf2N] liquid than with [MeBu3N][Tf2N], while UCl6- does not show any preference, suggesting a significant solvation effect of the redox properties of uranium, also supported by free energy perturbation simulations. The effect of ionic liquid (IL) humidity is investigated by simulating the three complexes in 1:8 water/IL mixtures. In contrast to the case of "naked" ions (e.g., lanthanide(3+), UO22+, alkali, or halides), water has little influence on the solvation of the UCl6n- complexes in the two simulated ILs, as indicated by structural and energy analysis.163va Times Cited:30 Cited References Count:7

J Phys Chem A J Phys Chem A

We present a molecular dynamics study of the solvation properties of the tetrahedral AsPh4+ and BPh4- ions in water and chloroform solutions. According to the "extrathermodynamic" TATE (telraphenylarsonium tetraphenylborate) hypothesis, these nearly isosterical ions have identical free energies of solvation in any solvent, as the latter are generally assumed to display little dependence on the details of the charge repartition, provided that the total +/- charge is delocalized and that the ion's periphery is relatively inert. We compare eight different sets of charges obtained consistently for both ions and find that the anion is always better hydrated than the cation, as evidenced by ion-solvent interaction energies and changes in free energies of ion charging. This is explained by specific OH-Jr bridging interactions in the anion and the positive electrostatic potential at the center of the fictitious AsPh40 and BPh40 all-neutral species. With all models, the cation is also predicted to be more easily transferred from water to dry chloroform. The conclusions obtained with standard solvent models (TIP3P water and OPLS chloroform) are validated by tests with the polarizable Wallqvist and Berne water model and the Chang et al. chloroform model, and with computer simulations on a "wet chloroform" solution. The recently developed TIP5P water model yields, however, much closer hydration energies of AsPh4+ and BPh4-. The importance of "long-rangre" electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate the Coulombic interactions. These results are important in the context of the "TATB hypothesis" and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous liquid environments.378tn Times Cited:29 Cited References Count:6