The properties of halides from the lightest, fluoride (F(-)), to the heaviest, astatide (At(-)), have been studied in water using a polarizable force-field approach based on molecular dynamics (MD) simulations at the 10 ns scale. The selected force-field explicitly treats the cooperativity within the halide-water hydrogen bond networks. The force-field parameters have been adjusted to ab initio data on anion/water clusters computed at the relativistic Möller-Plesset second-order perturbation theory level of theory. The anion static polarizabilities of the two heaviest halides, I(-) and At(-), were computed in the gas phase using large and diffuse atomic basis sets, and taking into account both electron correlation and spin-orbit coupling within a four-component framework. Our MD simulation results show the solvation properties of I(-) and At(-) in aqueous phase to be very close. For instance, their first hydration shells are structured and encompass 9.2 and 9.1 water molecules at about 3.70 ± 0.05 Å, respectively. These values have to be compared to the F(-), Cl(-), and Br(-) ones, i.e., 6.3, 8.4, and 9.0 water molecules at 2.74, 3.38, and 3.55 Å, respectively. Moreover our computations predict the solvation free energy of At(-) in liquid water at ambient conditions to be 68 kcal mol(-1), a value also close the I(-) one, about 70 kcal mol(-1). In all, our simulation results for I(-) are in excellent agreement with the latest neutron- and X-ray diffraction studies. Those for the At(-) ion are predictive, as no theoretical or experimental data are available to date.

The application of (211)At to targeted cancer therapy is currently hindered by the rapid deastatination that occurs in vivo. As the deastatination mechanism is unknown, we tackled this issue from the viewpoint of the intrinsic properties of At-involving chemical bonds. An apparent correlation has been evidenced between in vivo stability of (211)At-labeled compounds and the At-R (R = C, B) bond enthalpies obtained from relativistic quantum mechanical calculations. Furthermore, we highlight important differences in the nature of the At-C and At-B bonds of interest, e.g. the opposite signs of the effective astatine charges, which implies different stabilities with respect to the biological medium. Beyond their practical use for rationalizing the labeling protocols used for (211)At, the proposed computational approach can readily be used to investigate bioactive molecules labeled with other heavy radionuclides.

This work aims to resolve some controversies about astatine(III) hydroxide species present in oxidant aqueous solution. AtO(+) is the dominant species existing under oxidizing and acidic pH conditions. This is consistent with high-performance ion-exchange chromatography data showing the existence of one species holding one positive charge. A change in speciation occurs as the pH changes from 1 to 4, while remaining under oxidizing conditions. Dynamic experiments with ion-exchange resins evidence the existence of a neutral species witnessed by its elution in the void volume. Batch-experiments using a competition method show the exchange of one proton indicating the formation of the AtO(OH) species. The hydrolysis thermodynamic constant, extrapolated to zero ionic strength, was determined to be 10(-1.9). This value is supported by two-component relativistic quantum calculations and therefore allows disclosing unambiguously the structure of the formed species.

The AtO(+) molecular ion, a potential precursor for the synthesis of radiotherapeutic agents in nuclear medicine, readily reacts in aqueous solution with organic and inorganic compounds, but at first glance, these reactions must be hindered by spin restriction quantum rules. Using relativistic quantum calculations, coupled to implicit solvation models, on the most stable AtO(+)(H2O)6 clusters, we demonstrate that specific interactions with water molecules of the first solvation shell induce a spin change for the AtO(+) ground state, from a spin state of triplet character in the gas phase to a Kramers-restricted closed-shell configuration in solution. This peculiarity allows rationalization of the AtO(+) reactivity with closed-shell species in aqueous solution and may explain the differences in astatine reactivity observed in (211)At production protocols based on "wet" and "dry" processes.

(211)At radionuclide is of considerable interest as a radiotherapeutic agent for targeted alpha therapy in nuclear medicine, but major obstacles remain because the basic chemistry of astatine (At) is not well understood. The AtO(+) cationic form might be currently used for (211)At-labeling protocols in aqueous solution and has proved to readily react with inorganic/organic ligands. But AtO(+) reactivity must be hindered at first glance by spin restriction quantum rules: the ground state of the free cation has a dominant triplet character. Investigating AtO(+) clustered with an increasing number of water molecules and using various flavors of relativistic quantum methods, we found that AtO(+) adopts in solution a Kramers restricted closed-shell configuration resembling a scalar-relativistic singlet. The ground-state change was traced back to strong interactions, namely, attractive electrostatic interactions and charge transfer, with water molecules of the first solvation shell that lift up the degeneracy of the frontier π* molecular orbitals (MOs). This peculiarity brings an alternative explanation to the highly variable reproducibility reported for some astatine reactions: depending on the production protocols (with distillation in gas-phase or "wet chemistry" extraction), (211)At may or may not readily react.

Valence electronic excitation spectra are calculated for the H(2)O···Br(2) complex using highly correlated ab initio potentials for both the ground and the valence electronic excited states and a 2-D approximation for vibrational motion. Due to the strong interaction between the O-Br and the Br-Br stretching motions, inclusion of these vibrations is the minimum necessary for the spectrum calculation. A basis set calculation is performed to determine the vibrational wave functions for the ground electronic state and a wave packet simulation is conducted for the nuclear dynamics on the excited state surfaces. The effects of both the spin-orbit interaction and temperature on the spectra are explored. The interaction of Br(2) with a single water molecule induces nearly as large a shift in the spectrum as is observed for an aqueous solution. In contrast, complex formation has a remarkably small effect on the T = 0 K width of the valence bands due to the fast dissociation of the dihalogen bond upon excitation. We therefore conclude that the widths of the spectra in aqueous solution are mostly due to inhomogeneous broadening.

N,N′-Bis(4-aminophenyl)1,4-quinonenediimine doped with silicotungsten polyacid were investigated by Raman, IR and UV–Vis absorption methods and the results analyzed based on numerical approaches. In this aim, density functional theory (DFT) calculations were performed to optimize the geometry and to clarify the electronic and optical features of the investigated molecules. Three main systems were considered and consist in a conjugated organic group N,N′-bis(4-aminophenyl)1,4-quinonenediimine, a silicotungsten polyacid group and an association of these molecules taken in their ground state. Theoretical models with functionals based on judicious choice of potentials and exchange interactions were implemented in the calculation routines in order to achieve consistent analysis of the experimental results. As a consequence, Hartree–Fock (HF) exchange term in the hybrid functional leads to different organic group conformations while the DFT/B3LYP potential reproduces the main features of the experimental IR spectra. The time dependant TD-DFT was used in order to calculate the characteristic electronic transitions of the considered molecules and the results compared with the experimental values.

Valence electronic excitation spectra are calculated for the H(2)O...Cl(2) dimer using state-of-the art ab initio potentials for both the ground and the valence excited states, a basis set calculation of the ground state nuclear wave function, and a wave packet analysis to simulate the dynamics on the excited state surface. The peak of the H(2)O...Cl(2) dimer spectrum is blue-shifted by 1250 cm(-1) from that of the free Cl(2) molecule. This is less than the value previously estimated from vertical excitation energies but still significantly more than the blue shift in aqueous solution and clathrate-hydrate solid. Seventy percent of the blue shift is attributed to ground state stabilization, the rest to excited state repulsion. Spin-orbit effects are found to be small for this dimer. Homogeneous broadening is found to be slightly smaller for the dimer than for the free Cl(2). The reflection principle and spectator model approximations were tested and found to be quite satisfactory. This is promising for an eventual simulation of the condensed phase spectra.

The novel disilazane compound 2-pyridinetetramethyldisilazane (1) has been synthesized. The competition between N-pyridine coordination and Si-H bond activation was studied through its reactivity with two ruthenium complexes. The reaction between 1 and RuH(2)(H(2))(2)(PCy(3))(2) led to the isolation of the new complex RuH(2){(eta(2)-HSiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(PCy(3))(2) (2) resulting from the loss of two dihydrogen ligands and coordination of 1 to the ruthenium center via a kappa(2)N,(eta(2)-Si-H) mode. Complex 2 has been characterized by multinuclear NMR experiments ((1)H, (31)P, (13)C, (29)Si), X-ray diffraction and DFT studies. In particular, the HMBC (29)Si-(1)H spectrum supports the presence of two different silicon environments: one Si-H bond is dangling, whereas the other one is eta(2)-coordinated to the ruthenium with a J(SiH) value of 50 Hz. DFT calculations (B3PW91) were also carried out to evaluate the stability of the agostic species versus a formulation corresponding to a bis(sigma-Si-H) isomer and confirmed that N-coordination overcomes any stabilization that could be gained by the establishment of SISHA interactions. There is no exchange between the two Si-H bonds present in 2, as demonstrated by deuterium-labeling experiments. Heating 2 at 70 degrees C under vacuum for 24 h, leads to the formal loss of one equivalent of H(2) from 2 and formation of the 16-electron complex RuH{(SiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(PCy(3))(2) (3) formulated as a hydrido(silyl) species on the basis of multinuclear NMR experiments. The dehydrogenation reaction is fully reversible under dihydrogen atmosphere. Reaction of Ru(COD)(COT) with 3 equiv of 1 under a H(2) pressure led to the isolation of the new complex RuH{(SiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(3) (4) characterized as a hydridotrisilyl complex by multinuclear NMR techniques, X-ray and neutron diffractions, as well as DFT calculations. The (29)Si HMBC experiments confirm the presence of two different silicon atoms in 4, with a signal at -14.64 ppm for three dangling Si-Me(2)H fragments and a signal at 64.94 ppm (correlating with the hydride signal) assigned to three Si-Me(2)N groups bound to Ru. Comparison of DFT and neutron parameters involving the hydride clearly indicates an excellent correlation. The Si-H distance of approximately 2.15 A is much shorter than the sum of the van der Waals radii and typically in the range of a significant interaction between a silicon and a hydrogen atom (SISHA interactions). In 4, three dangling Si-H groups remain accessible for further functionalization.

The thermolysis of N-acyl-2,2-dimethyl aziridines 1 yielded N-methallyl amides 2 and/or oxazolines 3 depending on their substitution. These results suggest the intermediate formation of a zwitterion 1' by heterolytic cleavage of the CMe2-N bond of aziridines 1, which could evolve either through hydrogen abstraction from one of the methyl groups or cyclization yielding, respectively, N-methallyl amides 2 or oxazolines 3. Theoretical calculations by the DFT/B3LYP method confirmed the proposed mechanism for the determination of transition states. The kinetic/thermodynamic competition which governs the formation of the products shows that the oxazoline 3 is the kinetic product and the N-methallyl amide 2 e is the thermodynamic one. To cite this article: N. Besbes et al., C. R. Chimie 11 (2008). (C) 2008 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

An ab initio study of the interactions between H2O and Cl2+ and H2O and Br2+ has been performed. We present calculations using both the UMP2 level and the UCCSD(T) level of correlation with the aug-cc-pVTZ basis. The aug-cc-pVQZ basis was tested for selected geometries and was found to yield results similar to the smaller basis. For the H2O-Cl2+ cation, a C2v structure has been identified as the minimum, with De = 6500 cm-1 (78 kJ/mol). A low-lying excited state has De = 6000 cm-1 (72 kJ/mol). The adiabatic and vertical ionization energies of the complex are 10.7 and 11.0 eV, compared to the experimental adiabatic value, 11.5 eV, for free chlorine. For the H2O-Br2+ cation, the calculations are more subtle due to second-order Jahn-Teller effects and result in a Cs structure at the minimum, with De = 6300 cm-1 (75 kJ/mol), yielding an adiabatic ionization energy of 9.9 eV compared to the corresponding experimental value, 10.5 eV, for free bromine. The relatively large binding energies give rise to strong normal mode couplings such that the halogen stretching mode becomes mixed with the water bending and other intermolecular modes, resulting in very large frequency shifts. Vertical ionization energies and ion vibrational frequencies also are reported and used to discuss possible experiments to obtain more precise data for each of the complexes.

The new pyridine-borane compound (2-picolyl)BCy2, readily prepared from 2-picolyllithium and CIBCy2, adopts a head-to-tail dimeric structure in the solid state as indicated by X-ray diffraction analysis and according to NMR and DFT studies, the dimeric form equilibrates in solution with a strained monomeric, structure; the ambiphilic behavior of the new compound is illustrated by its bridging coordination to the (p-cymene)RuCl2 unit.

Reaction of a phosphinobenzylsilane compound with ruthenium complexes leads to C-H and/or Si-H activation. The new complex Ru{eta(2)-H-SiMe2CH(o-C(6)H(4))PPh2}2 (5) was isolated and X-ray, NMR and DFT studies reveal that 5 displays two agostic Si-H interactions and two carbon-metallated bonds.

Correlated ab initio calculations have been performed on three dipalladium(I) complexes. These compounds differ both by the metal–metal interaction and by the metal–ligand interaction. The [Pd 2 Cl 2 (μ −H 2 PCH 2 PH 2 ) 2 ] complex exhibits a σ overlap between the two binding metallic orbitals and has no bridging ligand. In [Pd 2 Cl 4 (μ −CO) 2 ] 2− , the leading interaction between the two palladium involves a π overlap between the metallic orbitals and goes through the two bridging CO ligands. In [Pd 2 Cl 2 (μ −CO)(μ −H 2 PCH 2 PH 2 ) 2 ], a single CO ligand bridges the two palladium atoms which interact through a hybrid σ–δ overlap. The three compounds also differ by the metal–metal distances. Surprisingly enough, while the palladium atoms are formally d 9 in all these complexes, none of them is paramagnetic. We propose here a detailed analysis of the electronic structures of these compounds and rationalize their chemical structures as well as the role of back-donation in the CO bridged compounds. Finally, since highly correlated treatments are used to describe these complexes, a detailed study of the role of both non-dynamical and dynamical correlations is performed. Concerning the [Pd 2 Cl 4 (μ −CO) 2 ] 2− complex, this analysis has revealed that the complex is not bound at the lowest correlated levels of calculation and therefore dynamical correlation is alone responsible for its binding energy.