Molecular Dynamics of the Muonium-C60 Radical in Solid C60

The molecular dynamics and electronic structure of the μ+-C60 radical in crystalline C60 have been studied using muon spin rotation and relaxation. At room temperature μ+-C60 appears to be in a state of quasifree rotation. At the critical temperature TS=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on C60, suggesting that the molecular dynamics of μ+-C60 are strongly coupled to those of its C60 neighbors

Adsorption geometry of pyridine on the single domain Si(100)-2 x 1 surface: Fully polarization resolved NEXAFS

International audienceThe adsorption of pyridine at room temperature on single-domain Si(100)-2 x 1 surfaces at saturation coverage has been investigated by means of fully polarization resolved near-edge X-ray adsorption fine-structure spectroscopy (NEXAFS) experiments. The major product of pyridine chemisorption has been identified as the 1,4,5,6-tetra-sigma-bonded species that is grafted on two successive dimers keeping a C-2=C-3 bond parallel to the dirtier axis

Pyridine on Si(001)-(2 x 1): Density functional theory simulations compared with spectroscopic measurements

International audienceThe adsorption of pyridine at 200 and 300 K on single-domain Si(100)-2 x 1 surfaces at saturation coverage was investigated by means of x-ray photoelectron spectroscopy and angular-dependent near-edge x-ray adsorption fine-structure spectroscopy (NEXAFS). Spectroscopic results are compared with density functional theory calculations. The N 1(s) NEXAFS spectroscopic signatures of three chemical moieties (dative bond, C-C=N-C and Si-N sp(3)) are identified. At 200 K all surface products present Si-N bonds (dative bond and Si-N sp(3)), while at 300 K, C-C= N-C moiety is also detected probing minor species without a Si-N bond. At both temperatures the coexistence of these various species are dominated by 1,4-di-sigma butterflies (Si-N sp(3))

Two isostructural oxalato-bridged dimetallic heptanuclear [Ba^II₃M^III₄] complexes (M = Cr; Fe) associated with 3-aminopyridinium cations: Synthesis, crystal structure and magnetic properties

Two heterometallic heptanuclear oxalato-bridged [Ba II 3M III 4] complexes, (Org-H) 6[Ba 3(H 2O) 5.1Cr 4(C 2O 4) 12]·5H 2O (1) and (Org-H) 6[Ba 3(H 2O) 5.3Fe 4(C 2O 4) 12]·5H 2O (2) (Org-H = C 5H 7N 2 +: 3-aminopyridinium cation), have been synthesized through an ion-exchange reaction strategy by combining {Ba 6(H 2O) 17[M III(C 2O 4) 3] 4}·7H 2O (M = Cr; Fe) with (C 5H 7N 2) 2C 2O 4 in a 1:3 M ratio. They have been characterized by elemental and thermal analyses, IR spectroscopy, single-crystal X-ray diffraction and variable temperature magnetic susceptibility measurements. The hybrid salts 1 and 2 are isostructural and they crystallize in the monoclinic space group C2/c. Their structures consist of [Ba 3(H 2O) 5M 4(C 2O 4) 12] 6− dimetallic heptanuclear units (M = Cr, Fe), six 3-aminopyridinium cations and five crystallization water molecules. The d-metal atom is located in a distorted (2 + 2 + 2) octahedral environment of six O atoms from three chelating oxalato(2−) ligands. In the crystal, intermolecular N[sbnd]H⋯O and O[sbnd]H⋯O hydrogen bonds link the anions and 3-aminopyridinium cations and lattice water molecules into a three-dimensional framework. In addition, π-π stacking interactions [centroid-centroid distances of 3.680 to 3.938 Å] between the pyridine rings contribute to the stabilization of the framework. The magnetic properties of the two salts have been investigated and they revealed weak antiferromagnetic coupling between d-metal atoms. </p

Tris(oxalato)chromate(III) hybrid salts templated by pyridinium and mixed pyridinium-ammonium cations: synthesis, structures and magnetism

By modifying the stoichiometric ratio of starting materials, two tris(oxalato)chromate(III) salts, (C7H11N2)3[Cr(C2O4)3] (1) and (C5H8N3)2(NH4)[Cr(C2O4)3]·2H2O (2) {(C7H11N2)+ = 2-amino-4,6-dimethylpyridinium, (C5H8N3)+ = 2,6-diaminopyridinium}, were synthesized and characterized by elemental and thermal analyses, single-crystal X-ray diffraction, IR and UV − Vis spectroscopies, EPR and SQUID measurements. Salt 1 exhibits a 3-D supramolecular framework based on [Cr(C2O4)3]3- and 2-amino-4,6-dimethylpyridinim cations, (C7H11N2)+, via N–H···O hydrogen bonds. Interestingly, π–π stacking interactions between pyridine rings contribute to the stabilization of the crystal packing. In contrast to salt 1, no π–π stacking interactions are observed in the mixed-cation salt 2 and its crystal packing is consolidated by N–H···O and O − H···O hydrogen bonds. EPR spectra of 1 and 2 are consistent with the oxidation state +3 of the chromium center in an octahedral environment. Temperature-dependence of the magnetic susceptibility data investigated from 2 to 300 K revealed the existence of zero-field splitting effects (ZFS) for Cr(III) ions in both compounds.</p

Electronic properties of gold catalyst silicon nanowires by photoluminescence: the role of the surface.

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Mechanism of Benzene Monolayer Formation on Si(100)-2×1 Studied by Surface Differential Reflectance Spectroscopy

International audienceThe kinetics of benzene adsorption on a Si(100)-2x1 surface has been monitored using real-time surface differential reflectance spectroscopy. A model with two adsorption modes is confirmed where benzene binds to the silicon surface in tight-bridge (TB) and butterfly (BF) configurations. For the first time, the balance between these modes is probed continually as the adsorption progressed and the resulting kinetics is discussed. In a first stage, benzene adsorbs in the TB configuration until it reaches 17% of a monolayer. In the second stage, additional molecules adsorb in the BF configuration and TB gradually converts into BF. At saturation, about 50% of the dimers are occupied by BF. Moreover, a Monte Carlo simulation provides a proper description of the adsorption kinetics