Utilisation de l'effet overhauser,des mesures de relaxation en RMN 1H et des deplacements chimiques en RMN 13C dans l'etude de la conformation des amides—II: Application a l'etude conformationnelle d'amides alicycliques: acyl-prolines

¹H and ¹³C chemical shifts in the two decoalesced rotamers of the deuteromethyl ester of N-acetyl-l-proline are established by use of NOE and relaxation measurements. This requires the previous determination of the rotational barrier ΔG_c^≠ and the measurement of the relaxation times T₁ of the α-proton in the two conformers. The observed results reinforce the previous structural conclusions inferred from ¹³C studies on several acyl l-prolines and hydroxy l-prolines. The introduction of an acyl group in the prolines does not affect the different carbon atoms of the cycle in the two rotamers in the same way. These results can be interpreted in terms of electric field effects by the examination of X-Pro dipeptides with the N-terminal amino group shifted from α (COCH₂NH₂) to δ position (CO(CH₂)₄NH₂).     

Design, synthesis, and evaluation of a novel class of enantioselective electrophilic fluorinating agents: N-fluoro ammonium salts of cinchona alkaloids (F-CA-BF(4))

The first enantiopure N-fluoro quaternary ammonium salts of cinchona alkaloids as enantioselective fluorinating agents are reported. A one-step transfer-fluorination on the naturally occurring cinchona alkaloids gave the fluorinating agents F-CA-BF(4). This new generation of fluorinating agents exhibited asymmetric induction up to 61% on fluorination of enolates and silyl enol ethers of 2-methyl-1-tetralone.     

Bismuth(III) bis(trifluoromethanesulfonyl)amide

Bismuth(III) bis(trifluoromethanesulfonyl)amide (Bi(NTf₂)₃, 3) has been prepared from the reaction of protiodemetallation of tri-p-tolylbismuth by a stoichiometric amount of bis(trifluoromethanesulfonyl)amine (1). The intermediates BiPh_3−n(NTf₂)_n (n=2 (4), 1 (5)) resulting from the reaction of 1 with triphenylbismuth have also been isolated. The amide 3 was able to catalyze the benzoylation and the benzenesulfonylation of toluene.     

Theoretical investigation of the uranyl ion sorption on the rutile TiO2(110) face

Canister integrity and radionuclide retention is of first importance for assessing the long-term safety of nuclear waste stored in engineered geologic depositories. Uranyl ion sorption on the TiO(2) rutile (110) face is investigated using periodic density functional theory (DFT) calculations. From experimental observations, only two uranyl surface complexes are observed and characterized. When the pH increases (from 1.5 to 4.5), the relative ratios of these two surface complexes are modified. From a crystallographic point of view, three sorption sites can be considered and have been studied with different protonation states of the surface to account for very acidic and low acidic conditions. The two surface complexes experimentally observed were calculated as the most stable ones, while the evolution of their sorption energies agrees with experimental data.     

Comparative behavior of britholites and monazite/brabantite solid solutions during leaching tests: a combined experimental and DFT approach

In the field of the specific immobilization of actinides, several phosphate-based ceramics have already been proposed as suitable candidates. Among them, britholite and monazite/brabantite (now called monazite/cheralite) solid solutions have been considered as serious candidates on the basis of several properties of interest. Although both matrices appear almost similar from a chemical point of view, their chemical behavior during leaching tests appear to be strongly different with normalized dissolution rates of typically (2.1 +/- 0.2) g.m(-2).day(-1) for Th-britholites (10(-1)M HNO(3), theta = 25 degrees C, dynamic conditions) and (2.2 +/- 0.2) 10(-5) g.m(-2).day(-1) for Th-brabantites (10(-1)M HNO(3), theta = 90 degrees C, dynamic conditions). To understand such difference from a crystallographic point of view, comparative leaching tests have been performed using either high or low renewal of the leachate. The results obtained clearly revealed a lower chemical durability of An-britholites compared to that of (Ln, Ca, An)-monazite/brabantite solid solutions. As a confirmation of this point, density functional theory calculations clearly showed some great differences in the cohesive energy of calcium in both crystal structures, which can explain this strong difference in the chemical durability of both materials.     

Modulation of the Sorption Characteristics for an H-bonded porous Architecture by Varying the Chemical Functionalization of the Channel Walls

Five isostructural microporous supramolecular architectures prepared by H-bonded assembly between the hexa-anionic complex [Zr₂(Ox)₇]⁶⁻ (Ox=oxalate, (C₂O₄)²⁻) and tripodal cations (H₃-TripCH₂-R)³⁺ with R=H, CH₃, OH and OBn (Bn=CH₂Ph) are reported. The possibility to obtain the same structure using a mixture of tripodal cations with different R group (R=OH and R=CH₃) has also been successfully explored, providing a unique example of three-component H-bonded porous framework. The resulting SPA-1(R) materials feature 1D pores decorated by R groups, with apparent pore diameters ranging from 3.0 to 8.5 Å. Influence of R groups on the sorption properties of these materials is evidenced through CO₂ and H₂O vapor sorption/desorption experiments, as well as with I₂ capture/release experiments in liquid media. This study is one of the first to demonstrate the possibility of tuning the porosity and exerting precise control over the chemical functionalization of the pores in a given H-bonded structure, without modifying the topology of the reference structure, and thus finely adjusting the sorption characteristics of the material.     

Controlled Growth of Ag Nanocrystals in a H-Bonded Open Framework

A procedure that enabled rational access to the first example of hybrid material made of NPs grown within a H-bonded framework is reported. To avoid competitive reactions with the framework units, the metal precursor was chemically trapped in the porous structure and subsequently photo-reduced to afford the hybrid material Ag@SPA-2 , which consists of Ag NPs of nanometric sizes (<15 nm) homogeneously distributed in the crystals of the host material. In a subsequent step, taking advantage of the porous matrix the silver NPs have been transformed in situ to Ag₂S NP by simple infiltration of H₂S. The supramolecular network is shown to play an important role in stabilizing the inorganic nanomaterials and thus in controlling their growth.     

Joint quantum chemical and polarizable molecular mechanics investigation of formate complexes with penta- and hexahydrated Zn2+: Comparison between energetics of model bidentate,monodentate, and through-water Zn2+ binding modes and evaluation of nonadditivity effects

In order to gain an understanding of the energetics of polycoordinated Zn²⁺ binding to the formate anion (the end side chain of the Asp and Glu residues of proteins), we compare three competing binding modes in the presence of five and six water molecules: a, bidentate binding of Zn²⁺ to both formate oxygens; b, monodentate binding of Zn²⁺ to one formate oxygen; and c, through-water binding of Zn²⁺ to formate, in which the cation remains bound to its first-hydration shell waters and interacts with both formate oxygens through three water molecules. We also investigate a complex d, which is similar to c, in which formate is protonated into formic acid and one water molecule is deprotonated. The computations are carried out using the ab initio self-consistent field/MP2 with three basis sets of increasing size density functional theory, semiempirical AM1 and PM3, and the sum of interactions between fragments ab initio computed (SIBFA) molecular mechanics procedures. The summed energies of the isolated molecules making up the complexes disfavor tautomer d compared to a–c. On the other hand, the ab initio computations give the ordering of intermolecular interaction energies as d formic acid tautomer >b monodentate >a bidentate >c through-water. Whereas the first-order energy E₁ favors both inner-shell Zn²⁺ complexes with formate over the outer-shell complex, the polarization and the charge-transfer components of the second-order energy E₂ both favor the outer-shell complex over the inner-shell one, despite the increased separation between the cation and the highly polarizable formate ion. Energy balances including continuum solvation enthalpies produce an equilibration of complexes a–d. The preference favoring the monodentate complex over the bidentate one is consistent with other ab initio results for formate binding by a fully coordinated Zn²⁺ cation and with structural results from X-ray crystallography. The SIBFA results are consistent with the ab initio results, and the computed interaction energy values match the ab initio ones to within 3%. The effects of nonadditivity are analyzed in the ab initio, SIBFA, and semiempirical computations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1379–1390, 1999