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 129Xe 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.
Comparative study of the complexes of two cryptophane diastereomers with xenon via accelerated methods of ab‐initio molecular dynamics reveals totally distinctive behavior for the escape of xenon from the cages, thereby explaining their difference in detectability through hyperpolarized 129Xe NMR.
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The CaLIPSO project is an innovative high-energy photon detector concept using trimethylbismuth as sensitive medium in a liquid ionization chamber. The detector, designed for high precision brain PET ...imaging, works as a time-projection chamber and detects Cherenkov light and charge signal. We measured the free ion yield of trimethylbismuth, which represents the number of electron-ion pairs released by the incident photon. To do so, we developed a low-noise measuring system to determine the current induced by a \Co source in the liquid with an accuracy better than 5 fA for an electric field up to 7 kV/cm. We used tetramethylsilane as benchmark liquid to validate the apparatus and we measured a zero-field free ion yield of 0.53±0.03 in agreement with measurements in literature. However, we found a zero-field free ion yield of 0.083±0.003 for trimethylbismuth, which is a factor 7 lower than the typical values for similar dielectric liquids. Quantum chemistry computations on heavy atoms tend to demonstrate a high capacity of trimethylbismuth to capture electrons which could explain the weak value. The consequences of a low free ion yield in terms of high-energy photon detection and brain PET imaging are finally discussed.
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 approachs 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.
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Polarization energy of the Lr–OH23+ dimer systems as calculated with both a small and large core pseudopotential as a function of the Lr–OH2 distance. Display omitted
► Energy decomposition analysis ...of lanthanide and actinide monoaqua systems. ► Open-shell and closed-shell complexes. ► Comparison of large f-in-core or small core quasi relativistic pseudopotentials. ► Computation of the unpaired electrons contribution to the polarization energy component.
We propose an energy decomposition analysis of mono aqua systems of both open and closed shell lanthanide and actinide cations using the CSOV scheme. We compared the values obtained with either large f-in-core or small core quasi relativistic pseudopotentials and computed the unpaired electrons contribution to the polarization energy component. Through a quasi-systematic approach on a number of chosen f-element cations, we quantified the different trends across both series for each contribution. This work is an important preliminary step for the acquisition of reference ab initio data for further parameterization of polarizable force fields for lanthanides and actinides.
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CaLIPSO is an innovative photon detector concept designed for high precision brain PET imaging. For the first time, liquid trimethylbismuth is used as sensitive medium. The detector operates as a ...time-projection chamber and detects both Cherenkov light and charge signal. Indeed, each 511-keV photon releases a single primary electron that triggers a Cherenkov radiation and ionizes the medium. As trimethylbismuth has never been studied before, we measured its free ion yield defined as the number of electron–ion pairs released by the primary electron. To this end, we developed a low-noise measuring system to determine the weak current induced by a 60Co source in the liquid with an accuracy better than 5 fA for an electric field up to 7 kV/cm. We used tetramethylsilane as benchmark liquid to validate the apparatus and we measured a zero-field free ion yield of 0.53 ＋/−0.03 in agreement with literature. However, we found a zero-field free ion yield of 0.083 ＋/−0.003 for trimethylbismuth, which is a factor 7 lower than the typical values for similar dielectric liquids. Quantum chemistry computations on heavy atoms tend to demonstrate the high ability of trimethylbismuth to capture electrons, which could explain this weak value. This recombination mechanism marks a new step in understanding charge transport in liquid detectors. Finally, to verify the detectability of individual charge pulses, we developed a charge pulse measurement system which has been successfully validated with TMSi. Measurements with TMBi are ongoing.
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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.
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The spectroscopic and thermodynamic properties of the molecules M@C28 (M = Ce, Th, Pa+, U2+, Pu4+) are calculated using density functional theory. The systems have considerable energetic stability. ...It is shown that the actinide cases can be classified as “32-electron” systems, using the bonding s-, p-, d-, and f-type orbitals of the central metal. The rest of the valence molecular orbitals have purely carbon character.
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9.
Modeling of Uranyl Cation−Water Clusters Clavaguéra-Sarrio, C; Brenner, V; Hoyau, S ...
The journal of physical chemistry. B,
04/2003, Volume:
107, Issue:
13
Journal Article
Peer reviewed
Polarization and charge-transfer contributions have been shown to be nonnegligible in the binding energy of UO2(H2O)2+ and have been quantified by Coordination Energy Partitioning. In this context, ...two types of model potential for uranyl cation−water clusters, i.e., one including polarization effects explicitly and charge-transfer effects implicitly, and the other including these two effects explicitly, have been derived from ab initio calculations on UO2(H2O)2+. Only the model with the explicit charge-transfer term reproduces accurately the ab initio geometries and energies of small clusters containing up to five water molecules. An exploration of the potential energy surface of clusters with up to eight water molecules has been performed with the Monte Carlo growth method. The first coordination sphere in clusters contains five water molecules, as in experimental liquid and solid phases. The presence of further water molecules in the second and third coordination spheres reinforces the preference for five in the first shell. The coordination number five results from a subtle competition between polarization and repulsion contributions on one hand, which favor molecules in the second coordination sphere, and electrostatic and charge-transfer contributions on the other hand, which obviously favor molecules in the first coordination sphere. In addition, the water molecules in the second coordination sphere are principally oriented by a strong electrostatic interaction with the uranyl cation, but also interact weakly with two water molecules of the first coordination sphere.
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The changes in the mean-square charge radius (relative to 209Bi), magnetic dipole, and electric quadrupole moments of 187, 188, 189, 191Bi were measured using the in-source resonance-ionization ...spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in 187, 188, 189Big, manifested by a sharp radius increase for the ground state of 188Bi relative to the neighboring 187, 189Big. A large isomer shift was also observed for 188Bim. 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.
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