Reentrant Phase Coherence in Superconducting Nanowire Composites.

The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature T, magnetic field H, or current I. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na2-δMo6Se6 featuring Na vacancy disorder (δ ≈ 0.2) are shown to behave as percolative networks of superconducting nanowires. Long-range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero (T,H,I) limit. However, a region of reentrant phase coherence develops upon raising (T,H,I). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy EJ at nonzero (T,H,I), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na2-δMo6Se6 is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated (T,H,I).

Stereodynamics of nitrogen chiral centers in aza-ß3-cyclodipeptides.

The present work is devoted to the synthesis, conformational analysis, and stereodynamic study of aza-ß(3)-cyclodipeptides. This pseudopeptidic ring shows E/Z hydrazide bond isomerism, eight-membered ring conformation, and chirotopic nitrogen atoms, all of which are elements that are prone to modulate the ring shape. The (E,E) twist boat conformation observed in the solid state by X-ray diffraction is also the ground conformation in solution, and emerges as the lowest in energy when using quantum chemical calculations. The relative configuration associated with ring chirality and with the two nitrogen chiral centers is governed by steric crowding and adopts the (P)S(N) S(N)/(M)R(N)R(N) combination which projects side chains in equatorial position. The nitrogen pyramidal inversion (NPI) at the two chiral centers is correlated with the ring reversal. The process is significantly hindered as was shown by VT-NMR experiments run in C2D2Cl4, which did not make it possible to determine the barrier to inversion. Finally, these findings make it conceivable to resolve enantiomers of aza-ß(3)-cyclodipeptides by modulating the backbone decoration appropriately.

Sc(0.43(2))Rb2Mo15S19, a partially Sc-filled variant of Rb2Mo15S19.

The structure of scandium dirubidium pentadecamolybdenum nonadecasulfide, Sc(0.43(2))Rb(2)Mo(15)S(19), constitutes a partially Sc-filled variant of Rb(2)Mo(15)S(19) [Picard, Saillard, Gougeon, Noel & Potel (2000), J. Solid State Chem. 155, 417-426]. In the two compounds, which both crystallize in the R ̅3c space group, the structural motif is characterized by a mixture of Mo(6)S(i)(8)S(a)(6) and Mo(9)S(i)(11)S(a)(6) cluster units (`i' is inner and `a' is apical) in a 1:1 ratio. The two components are interconnected through interunit Mo-S bonds. The cluster units are centred at Wyckoff positions 6b and 6a (point-group symmetries ̅3. and 32, respectively). The Rb(+) cations occupy large voids between the different cluster units. The Rb and the two inner S atoms lie on sites with 3. symmetry (Wyckoff site 12c), and the Mo and S atoms of the median plane of the Mo(9)S(11)S(6) cluster unit lie on sites with .2 symmetry (Wyckoff site 18e). A unique feature of the structure is a partially filled octahedral Sc site with ̅1 symmetry. Extended Hückel tight-binding calculations provide an understanding of the variation in the Mo-Mo distances within the Mo clusters induced by the increase in the cationic charge transfer due to the insertion of Sc.

Z/E isomerism in Nα-Nα-disubstituted hydrazides and the amidoxy bond: application to the conformational analysis of pseudopeptides built of hydrazinoacids and α-aminoxyacids.

We have investigated the Z/E isomerism of the hydrazide link (CO-NH-N) and amidoxy link (CO-NH-O). The study was first focused on small molecular models using NMR and X-ray diffraction. It allowed determination of simple NMR criterions to differentiate easily the Z and E forms, which were then applied to investigate the behavior of these links inside the corresponding oligomers. Our results concerning the hydrazide link corroborate pioneering work that had been done in the 1970s except in the case were it is located inside aza-ß(3)-cyclopeptides, where the old empirical rules failed to predict the right geometry of the link. The geometrical preference of the amidoxy bond is also unambiguously established and differs clearly from recent theoretical calculations. Our findings help rationalize the close self-organization ability of aza-ß(3)-peptides and α-aminoxypeptides, two recently described foldamers.

V(1.42)in(1.83)mo(15)se(19).

The structure of the title compound, vanadium indium penta-deca-molybdenum nona-deca-selenide, V(1.42)In(1.83)Mo(15)Se(19), is isotypic with In(2.9)Mo(15)Se(19) [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285-292]. It is characterized by two cluster units Mo(6)Se(i) (8)Se(a) (6) and Mo(9)Se(i) (11)Se(a) (6) (where i represents inner and a apical atoms) that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry and , respectively. The clusters are inter-connected through additional Mo-Se bonds. In the title compound, the V(3+) cations replace the trivalent indium atoms present in In(2.9)Mo(15)Se(19), and a deficiency is observed on the monovalent indium site. One Mo, one Se and the V atom are situated on mirror planes, and two other Se atoms and the In atom are situated on threefold rotation axes.

Tl(2)Mo(9)Se(11).

The structure of Tl(2)Mo(9)Se(11), dithallium nona-molybdenum undeca-selenide, is isotypic with Tl(2)Mo(9)S(11) [Potel et al. (1980 ▶). Acta Cryst. B36, 1319-1322]. The structural set-up is characterized by a mixture of Mo(6)Se(i) (8)Se(a) (6) and Mo(12)Se(i) (14)Se(a) (6) cluster units in a 1:1 ratio. Both components are inter-connected through inter-unit Mo-Se bonds. The cluster units are centered at Wyckoff positions 3a and 3b (point-group symmetry .). The two Tl(I) atoms are situated in the voids of the three-dimensional arrangement. Two of the five independent Se atoms and the Tl atoms lie on sites with 3. symmetry (Wyckoff site 6c).

Synthesis and characterization of Eu3+, Ti4+ @ ZnO organosols and nanocrystalline c-ZnTiO3 thin films aiming at high transparency and luminescence.

By exploiting colloidal properties, such as transparency, rheology and versatile chemistry, we propose to synthesize new photonic nanomaterials based on colloidal solutions and thin films. This contribution highlights our efforts to elaborate and to characterize nanostructures based on the ZnO-TiO2 system. Using a recently developed sol-gel route to synthesize new Ti4+@ZnO organosols, we were able to prepare, at relatively low temperature (400 °C) and short annealing time (15 min), highly transparent, luminescent, nanocrystalline Eu3+ doped c-ZnTiO3 thin films. The organosols and thin films were characterized with UV-visible-near infrared absorption, ellipsometry, photoluminescence spectroscopy, dynamic light scattering, x-ray diffraction and scanning electron microscopy.

Aza-beta3-cyclopeptides: a new way of controlling nitrogen chirality.

Sixteen and 24 membered aza-beta(3)-peptidic macrocycles containing a alpha-hydrazinoacid or a beta(3)-aminoacid were synthesized. The conformation of these pseudopeptides was determined by using NH chemical shift analysis, NH extinction, VT-NMR experiments, and X-ray diffraction. The study shows that a stable conformation is retained between 223 and 413 K. The latter is characterized by an uninterrupted internal H-bond network and a syndiotactic arrangement of the asymmetric centers. It means that the presence of the optically pure residue acts as a conformational lock to select a single enantiomer through the cyclization by controlling the absolute configuration of all the nitrogen atoms. To our knowledge, this represents the first example of a dynamic enantioselection process involving several centers prone to pyramidal inversion. These results give a new impulsion to the control of nitrogen chirality, which remained limited to small cycles for 60 years.

Synthesis, electronic and crystal structures, and physical studies of the superconductor Cs(~1)Mo12S14, final step of the condensation of the Mo6L8(i)L6(a) unit.

The ternary reduced molybdenum sulphide Cs(~1)Mo12S14 has been synthesized by solid-state reaction at 1400 degrees C for 96 h in sealed molybdenum crucibles. The compound crystallizes in the trigonal space group P31c with the following lattice parameters: a = 9.9793 (2) A, c = 6.3730 (2) A, Z = 1. Its crystal structure was determined from single crystal X-ray diffraction data and consists of interconnected Mo6S8(i)S6(a) units forming an original three-dimensional framework in which large tunnels are occupied randomly by the Cs+ ions. 133Cs static NMR studies are in favor of a static cesium disorder. Unlike Ba4Mo12S18 where some Mo6S8(i)S6(a) units are also connected through S(i-i) ligands, this connection mode does not lead to significant interactions in the title compound. Single-crystal resistivity measurements show that Cs(~1)Mo12S14 presents a metallic behavior with a superconducting transition at 7.7 K as confirmed by magnetic measurements.

The Chevrel phase HgMo(6)S(8).

The crystal structure of HgMo(6)S(8), mercury(II) hexa-molybdenum octa-sulfide, is based on (Mo(6)S(8))S(6) cluster units ( symmetry) inter-connected through inter-unit Mo-S bonds. The Hg(2+) cations occupy large voids between the different cluster units and are covalently bonded to two S atoms. The Hg atoms and one S atom lie on sites with crystallographic and 3 symmetry, respectively. Refinement of the occupancy factor of the Hg atom led to the composition Hg(0.973 (3))Mo(6)S(8).

Aza-beta3-cyclotetrapeptides.

The cyclization of aza-beta(3)-tetrapeptides gives access to new CTP (cyclotetrapeptide) analogues. These stereocontrolled templates are assembled without any asymmetric synthesis. X-ray crystallographic structure and NMR analysis show that the macrocyclic scaffold is characterized by a fully cooperative intramolecular H-bond network, in sharp contrast with the nanotubular assemblies observed for beta(3)-cyclotetrapeptides. This folding property reduces considerably the polarity of aza-beta(3)-tetrapeptides and should be useful in addressing intracellular targets.

Postsynthetic modification of C3-symmetric aza-beta3-cyclohexapeptides.

We have synthesized a series of C3-symmetric aza-beta3-cyclohexapeptides with functionally diverse side chains carrying a good functional diversity. The very simple chemical sequence that we used (debenzylation/acylation) makes it certain that the series synthesized could be easily expanded, leading to a wide family of C3-symmetric cyclohexapeptides analogues. The macrocyclic backbone of the aza-beta3-cyclohexapeptides shows a highly ordered conformation that is sustained by a dense intramolecular H-bond network where all endocyclic NHs are hydrogen bonded, the side chains being projected in equatorial position around the macrocycle. The resulting internal secondary structure relies on the cooperative alternation of two slightly different C8-bifidic pseudocycles, which differ mainly by the hybridization of the Nalpha nitrogen atom (N-Nsp3-turn and N-Nsp2-turn). In both cases, the nitrogen lone pair participates to stabilize the pseudocycle. This has been established by NMR experiments and X-ray diffraction analysis. As in the precursors, the nitrogen stereocenters are characterized by a strikingly slow rate of pyramidal inversion, considering the size of the macrocycle.

Mo(15)S(20): first evidence of a new molybdenum cluster type in a metastable solid-state compound.

The Mo(15)S(20) compound was obtained by thermal decomposition of the metastable binary Mo(15)S(19) in sealed silica tube at temperatures above 500 degrees C. Its crystal structure was solved and refined from a two-component composite crystal by X-ray diffraction in the hexagonal space group P6(3)/m and consists of an equal mixture of the original Mo(9)S(27) cluster unit and the classical one Mo(6)S(8)S(6) interconnected through Mo--S bonds. The Mo core of the Mo(9)S(27) unit is totally new and formed a tricapped trigonal prism. Quantum chemical calculations carried out in order to understand these trends as well as magnetic susceptibility measurements are also reported. The title compound becomes superconducting below 5 K.

Aza-beta3-cyclohexapeptides: pseudopeptidic macrocycles with interesting conformational and configurational properties slow pyramidal nitrogen inversion in 24-membered rings!

Among pseudopeptidic foldamers, aza-beta3-peptides have the unique property to possess nitrogen stereocenters instead of carbon stereocenters. As the result of pyramidal inversion at N(alpha)-atoms along the backbone, they behave as a set of C8-based secondary structures in equilibrium. This structural modulation is exploited here to prepare 24-membered macrocycles with great efficiency. Both crystal structures and spectroscopic data establish that aza-beta3-cyclohexapeptides adopt a highly organized conformation where the relative configuration of chiral nitrogen atoms is alternated. This makes them an interesting scaffold as the stereocontrol occurs spontaneously through the cyclization. These compounds reveal an unprecedented slow pyramidal nitrogen inversion in macrocycles. Pyramidal ground state stabilization, hindered rotation, steric crowding, and H-bond cooperativity are proposed to participate in this striking phenomenon. The equilibrium between invertomers of aza-beta3-cyclohexapeptides is reminiscent of the interchange between the two chair forms of cyclohexane.

Synthesis, structural evolution, and electrical properties of the novel Mo12 cluster compounds K(1+x)Mo12S14 (x = 0, 1.1, 1.3, and 1.6) with a tunnel structure.

The new structural type (1) K(2.3)Mo12S14 was prepared by solid-state reaction at 1500 degrees C in a sealed molybdenum crucible. The compound crystallizes in the trigonal space group P1c, Z = 2, (1) a = 9.1720(7) Angstroms, c = 16.403(4) Angstroms. Its crystal structure was determined from single-crystal X-ray diffraction data and consists of interconnected Mo12S14 units that form an original and unprecedented three-dimensional framework in which large tunnels are occupied randomly by a part of the K+ ions. The remaining K+ ions are localized between two consecutive Mo(12)S(14) units along the c axis. By carrying out topotactic oxydo-reduction reactions at low temperature (<100 degrees C), we were able to remove or insert K+ ions in the channels and thus form isostructural phases K(1+x)Mo12S14 (0 < or = x < or = 1.6). Thus, we have solved the crystal structures for the following three compositions: (2) K(2.1)Mo12S14, (3) KMo12S14, and (4) K(2.6)Mo12S14 ((2) a = 9.1476(4) A, c = 16.421(1) Angstroms; (3) a = 9.0797(9) Angstroms, c = 16.412(6) Angstroms; and (4) a = 9.1990(4) Angstroms, c = 16.426(4) Angstroms). Electrical resistivity measurements carried out on single crystals of K(2.3)Mo12S14 and KMo12S14 indicate that the former is semiconducting, whereas the latter is metallic. The evolution of the Mo-Mo distances with respect to the stoichiometry in potassium is discussed.

Conformation of N(alpha)-substituted hydrazino acetamides in CDCl3, the precious help of the analysis of Deltadelta between amidic hydrogens, and correlation to the conformation of aza-beta3-peptides.

[structures: see text] We studied the conformation of a series of primary amides in a solution of chloroform. Classical NMR tools such as dilution experiments, influence of DMSO, and 2D-NOESY, together with X-ray diffraction, were combined with an analysis of the difference of the chemical shift Deltadelta between the geminal amidic protons. This study was addressed in order to understand the conformation adopted by hydrazino acetamides 1a and 1b as model compounds for aza-beta3-peptides. In this manner, it was possible to show that the amidic group of these compounds acts as a H-bond donor and interacts with two different H-bond acceptors. We concluded that the hydrazinoturn, a specific bifurcated H-bond system observed in the solid state, is also the preferred conformation of hydrazino acetamides 1a and 1b in solution. Our results show that the short-range interaction with the N(alpha)-nitrogen lone pair not only stabilizes the C8 pseudocycle but could also contribute to the folding process of aza-beta3-peptides. In light of this, it could explain why aza-beta3-peptides develop a different H-bond network in comparison to their isosteric beta3-peptides analogues. Our work is in keeping with the recent interest of hydrazino peptides as an extension of the beta-peptide concept.

Crystal structures of aza-beta3-peptides, a new class of foldamers relying on a framework of hydrazinoturns.

Crystals of aza-beta3-peptides have been obtained. This gives the first opportunity for hydrazino peptides, in the sense of oligomers built exclusively with alpha-hydrazinoacetic units, to be observed in the solid state. The structures make it clear that the H-bond network developed by aza-beta3-peptides differs radically from those of the corresponding beta3-peptides but strongly resembles that of the alpha-aminoxy peptides. Our study contributes to the current interest in hydrazino peptides as an extension of the beta-peptide concept.

Syntheses and structural, physical, and theoretical studies of the novel isostructural Mo9 cluster compounds Ag(2.6)CsMo9Se11, Ag(4.1)ClMo9Se11, and h-Mo9Se11 with tunnel structures.

The new isostructural compounds Ag(2.6)CsMo9Se11 (1) and Ag(4.1)ClMo9Se11 (2) were prepared by solid-state reaction in evacuated sealed silica tubes at 1200 degrees C and 860 degrees C, respectively. By topotactic reduction-oxidation reaction of Ag(4.1)ClMo9Se11 with I2, we also obtained the metastable compound h-Mo9Se11 (3). The three compounds crystallize in the hexagonal space group P6(3)/m, Z = 2, (1) a = 10.0472(2) A, c = 11.9548(2), (2) a = 10.0321(2) A, c = 11.8734(2) A, and (3) a = 9.4204(2) A, c = 12.1226(2) A. Their crystal structures were determined from single-crystal X-ray diffraction data and consist of interconnected Mo9Se11 units forming an original and unprecedented three-dimensional framework in which large tunnels are occupied randomly by a part of the Ag+ and the Cl- ions in 2 and the Cs+ ions in 1, the remaining Ag+ in 1 being localized in mirror planes around the 3-fold axis. First-principle calculations allow the understanding of the variation of the atomic distances. Electrical resistivity measurements carried out on single crystals of Ag(2.6)CsMo9Se11 and Ag(4.1)ClMo9Se11 in which the number of electrons per Mo9 cluster is different indicate that the former is semiconducting whereas the latter is semimetallic.