Isomer-Dependent Escape Rate of Xenon from a Water-Soluble Cryptophane Cage Studied by Ab Initio Molecular Dynamics.

The escape of xenon from the anti and syn diastereomers of hexacarboxylic-cryptophane-222 in water has been studied by ab initio molecular dynamics simulations. The structures of both complexes, when the xenon atom is trapped inside their cages, have been compared and show no major differences. The free-energy profiles corresponding to the escape reaction have been calculated with the Blue Moon ensemble method using the distance between Xe and the center of mass of the cage as the reaction coordinate. The resulting free-energy barriers are very different; the escape rate is much faster in the case of the syn diastereomer, in agreement with experimental data obtained in hyperpolarized 129 Xe NMR. Our simulations reveal the mechanistic details for each diastereomer and provide an explanation for the different in-out xenon rates based on the solvation structure around the cages.

Simultaneously measuring the dichroism and the dielectric response of an azobenzene-doped organic glass former.

We have designed an experimental setup allowing to simultaneously measure both the dielectric response of a supercooled liquid and the dynamics of azobenzene chromophores dispersed in it. Both the azobenzene chromophores and the organic glass former have been synthesized with similar reaction paths: they are chemically similar, apart from the azobenzene group responsible for the strong optical absorption in the [350; 450 nm] range for the chromophores, while the embedding supercooled liquid is optically transparent. This material is deposited on transparent electrodes with an inter-electrode gap as small as 4 µm-obtained thanks to optical lithographic techniques. We show that our setup is sensitive enough to measure the coupling between the dielectric macroscopic response and the isomerization dynamics of 1% of chromophores excited by a 0.5-5 mW/cm2 light beam. We demonstrate that this coupling neither comes from the heating of the sample due to the light absorption nor from changes of the sample shape with light. Finally, we discuss the few physical effects, which may give rise to this coupling, and show that our experiment could test some recent predictions done in the framework of random first order transition theory of the glassy state.

Large Shape Staggering in Neutron-Deficient Bi Isotopes.

The changes in the mean-square charge radius (relative to ^{209}Bi), magnetic dipole, and electric quadrupole moments of ^{187,188,189,191}Bi were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in ^{187,188,189}Bi^{g}, manifested by a sharp radius increase for the ground state of ^{188}Bi relative to the neighboring ^{187,189}Bi^{g}. A large isomer shift was also observed for ^{188}Bi^{m}. Both effects happen at the same neutron number, N=105, where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.

Quantum chemical study of the photocoloration reaction in the napthoxazine series.

Ab initio and semiempirical quantum mechanical calculations were performed to study the electronic spectra of spiroxazine photochromic compounds as well as the corresponding photoisomers. Ground-state geometries were optimized based on density functional theory (DFT). Excitation energies of the different forms were calculated using the time-dependent density functional theory (TD-DFT) method. Semiempirical calculations including configuration interactions were performed to detail the mechanism of ring opening in excited states. On the basis of the obtained potential energy profile, a complete mechanism of photocoloration able to clarify some experimental findings is provided. A correlation of the experimental quantum yield of photocoloration with the calculated properties as a function of substituent effects is proposed.

Molecular dynamics study of the hydration of lanthanum(III) and europium(III) including many-body effects.

Lanthanides complexes are widely used as contrast agents in magnetic resonance imaging (MRI) and are involved in many fields such as organic synthesis, catalysis, and nuclear waste management. The complexation of the ion by the solvent or an organic ligand and the resulting properties (for example the relaxivity in MRI) are mainly governed by the structure and dynamics of the coordination shells. All of the MD approaches already carried out for the lanthanide(III) hydration failed due to the lack of accurate representation of many-body effects. We present the first molecular dynamics simulation including these effects that accounts for the experimental results from a structural and dynamic (water exchange rate) point of view.

Can we understand the different coordinations and structures of closed-shell metal cation-water clusters?

We present a model potential for studying M(q+)(H(2)O)(n=1,9) clusters where M stands for either Na(+), Cs(+), Ca(2+), Ba(2+), or La(3+). The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.