Magnetic interactions in transition metal clusters embedded in matrices prepared by LECBD technique

The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix have been realized by the co-deposition of both beams arriving at the same time on a 45°-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to adjust continuously the atomic concentration of clusters in the matrix from 3 to 70 %. The typical size distribution of embedded clusters determined from HRTEM observations on diluted samples revealed nanocrystallizedgrains with a very narrow size dispersion. Moreover, magnetoresistance (MR) and magnetization measurements versus temperature and concentration were performed. The results show clearly a magnetic threshold around 25 % Co-clusters content in matrix, corresponding to the transition from superparamagnetic to magnetically ordered state. This threshold is identical to the 3D-percolation threshold for connected particles. A maximum of MR ratio of 12 % at 4.2 K was obtained for Co_25%Ag_75% granular film at the onset of non negligible magnetic interactions between Co-particles. The magnetic properties of our systems, strongly correlated to the nucleation processes, allowed us to determine the variation of the cluster radius as a function of the concentration and of the matrix nature.     

Electron transfer in environmental systems: a frontier for theoretical chemistry

The advances in understanding the kinetic behavior of certain environmental electron transfer (ET) systems are presented. Emphasis is placed on the homogeneous ET chemistry of transition metals, particularly the Fe^II/III system, in various relevant forms. In the context of modern ET theory, we examine the utility of computational chemistry methods for the calculation of ET quantities such as the reorganization energy and electronic coupling matrix element. We discuss successful application of the methods to topics of homogeneous oxidation of dissolved metal ions by molecular oxygen in aqueous solution, as well as the prediction of electron mobility in solid phase iron oxide crystals. The examples illustrate the significant potential for many more advances in understanding environmental ET systems through the combination of ET theory and computational chemistry.     

Is ferromagnetism an intrinsic property of the CuII/GdIII couple? 1. Structures and magnetic properties of two novel dinuclear complexes with a mu-phenolato-mu-oximato (Cu,Gd) core

Two original dinuclear (Cu(II),Gd(III)) complexes (1 and 2) deriving from polydentate nonsymmetrical Schiff base ligands LiH2 have been prepared. Formally they differ by the length of the diamino chain. They crystallize in the orthorhombic Pbca (No. 61) (1) and in the monoclinic P2(1/n) (No. 14) (2) space groups. The cell parameters are a = 12.6295(7) A, b = 20.7894(9) A, c = 18.3301(13) A, and Z = 8 for 1 and a = 12.7246(16) A, b = 13.5691(17) A, c = 14.5310(19) A, beta = 94.629(16) degrees , and Z = 4 for 2. These structural studies show that in both complexes the CuII and GdIII ions are doubly bridged by a phenolato oxygen atom and an oximato (N-O) pair. The bridging network is not planar. The more important distortions are observed for the complex having the larger diamino chain. Unexpectedly the latter complex presents an antiferromagnetic interaction, but the related J value is small (J approximately equal to -0.49 cm(-1)). In the former complex the interaction is ferromagnetic (J approximately equal to 3.5 cm(-1)) as it is for complexes containing (CuO2Gd) bridging cores which yield J values varying from 1.4 to 10.1 cm(-1).     

Molecular symmetry. IV. The coupled perturbed Hartree–Fock method

Symmetry methods employed in the ab initio polyatomic program HONDO are extended to the coupled perturbed Hartree–Fock (CPHF) formalism, a key step in the analytical computation of energy first derivatives for configuration interaction (CI) wavefunctions, and energy second derivatives for Hartree–Fock (HF) wavefunctions. One possible computational strategy is to construct Fock-like matrices for each nuclear coordinate in which the one- and two-electron integrals of the usual Fock matrix are replaced by the integral first derivatives. “Skeleton” matrices are constructed from the unique blocks of electron-repulsion integral derivatives. The correct matrices are generated by applying a symmetrization operator. The analysis is valid for many wavefunctions, including closed- or open-shell spin-restricted and spin-unrestricted HF wavefunctions. To illustrate the method, we compare the computer time required for setting up the coupled perturbed HF equations for eclipsed ethane using D_3h symmetry point group and various subgroups of D_3h. Computational times are roughly inversely proportional to the order of the point group.     

An information theoretical evaluation of the applicability of the ASTM infrared data base for retrieval purposes

Summary The information content of the identification by retrieval with binary coded infrared spectra (ASTM infrared datafile), taking into account the correlations between the speaks and the errors in the coded spectra, amounts to about 21 bits. This amount of information can be obtained when use is made of not more than 93 wavelength intervals, selected from the total number of 140 intervals that are available for retrieval. The information content can be raised to about 29 bits when for the retrieval procedure a window of 0.3 is introduced, thereby requiring only 72 intervals. The influence of the composition of the dataset upon the information content is considerable; for hydrocarbons an amount of information of only 15 bits is obtained with a search using 79 intervals.The probability of the match between the spectrum of the unknown compound and the right reference spectrum being superior to all other possible matches will increase from virtually zero to about 88% when a window of 0.3 is used.

---

Eine informationstheoretische Auswertung der Anwendung der ASTM-Sammlung — Infrarotspektren für die Bibliothekssuche

Zusammenfassung Der Informationsgehalt für die Identifizierung durch Vergleich binÄr codierter Infrarotspektren (ASTM-Sammlung Infrarotspektren) betrÄgt unter Berücksichtigung der Ungenauigkeiten und der Korrelation zwischen den Banden etwa 21 Bits. Diese Informationsmenge kann erhalten werden aus dem Vergleich von nur 93 WellenlÄngen-Intervallen, die aus einer Gesamtzahl von 140 verfügbaren selektiert wurden.Der Informationsgehalt für die Bibliothekssuche kann bis zu 29 Bits zunehmen, wenn für die Suche ein Fenster von 0,3 angewendet wird; die Zahl der benötigten Intervalle betrÄgt dann nur 72. Aus verschiedenen Spektrensammlungen erhÄlt man verschiedene Informationsmengen; zum Beispiel betrÄgt der Informationsgehalt für Kohlenwasserstoffe nur 15 Bits unter Benutzung von 79 Intervallen. Die Wahrscheinlichkeit, da\ der Vergleich des Spektrums einer unbekannten Verbindung mit dem richtigen Vergleichsspektrum eine bessere übereinstimmung liefert als der Vergleich mit allen anderen Vergleichsspektren, nimmt von ungefÄhr null bis 88% zu, wenn ein Fenster von 0,3 angewendet wird.

     

Influence of anionic ligands (X) on the nature and magnetic properties of dinuclear LCuDgX3.nH2O complexes (LH2 standing for tetradentate Schiff base ligands deriving from 2-hydroxy-3-methoxybenzaldehyde and X being Cl, N3C2, and CF3COO)

The monometallic precursor L1Cu (L1H2 standing for 1,3-bis((3-methoxysalicylidene)amino)-2,2'-dimethylpropane) reacts with GdC13 x 6H2O to afford a dinuclear complex which crystallizes in the orthorhombic space group Pca2(1) (No. 29) in a cell having the dimensions a = 9.0246(11) A, b = 16.5198(14) A, c = 20.286(2) A, and Z = 4. Analysis of the structural data shows that it may be formulated as [L1CuCl2Gd(H2O)4]Cl x 2H2O. The cationic dinuclear unit possesses a CuO2Gd bridging core which is almost planar. The complex displays a ferromagnetic interaction (10.1 cm(-1) which is the largest yet reported for a structurally characterized dinuclear (Cu-Gd) complex. Lower magnetic interactions are observed for neutral L1CuGdX3 x H2O complexes (X = N3C2, CF3COO). Consideration of the magnetic and structural data obtained for various dinuclear (Cu-Gd) complexes leads to a correlation between the magnitude of the magnetic interaction and the exponential of the dihedral angle between the two halves of the CuO2Gd bridging core.     

Le problème de la volatilité de l'anhydride molybdique

Résumé A l'aide de la thermobalance deChevenard, l'auteur a étudié la volatilité de l'anhydride molybdique et construit les isothermes de 300, 400, 500, 600 et 700° C. L'anhydride seul ne se sublime qu'au-dessus de 782° et les conclusions dePavelka etZucchelli ⁵ sur sa volatilité ne se vérifient pas par la méthode de la pesée continue.En vue de l'analyse chimique, le poids théorique d'anhydride s'obtient d'autant plus vite que la température est plus élevée.L'auteur interprète la courbe de thermolyse du paramolybdate d'ammonium, de deux acides molybdiques, l'un jaune, l'autre blanc, étudie les précipités servant à doser pondéralement le molybdène, avec température de départ de l'anhydride (voir Tableau).Puis, elle étudie les hétérocomplexes avec le silicium, le phosphore, le germanium, l'arsenic et donne les conditions de température requises pour les pesées; elle fait une mention spéciale pour les phosphomolybdates d'ammonium et d'oxinium, donne une nouvelle méthode gravimétrique du germanium avec la dibromo-oxine.L'anhydride molybdique, accompagné d'un autre anhydride provenant de la destruction d'un hétérocomplexe, montre une volatilité plus faible que lorsqu'il est seul ou associé à l'ammoniac, à des corps organiques ou mélangé à la silice dans le rapport SiO₂/12 MoO₃.

---

Summary With the aid of theChevenard thermobalance, the author has studied the volatility of molybdic anhydride and constructed the isotherms for 300, 400, 500, 600 and 700° C. When alone the anhydride does not sublime below 782°, and the conclusions ofPavelka andZucchelli ⁵ concerning the volatility are not confirmed by the method of continuous wriggling. From the analytical standpoint, the theoretical weight of the anhydride is obtained more quickly the more the temperature is raised.The author interprets the curve of the thermolysis of ammonium paramolybdate, of two molybdic acids, one yellow, the other colorless, and she has studied the precipitates used in the gravimetric determination of molybdenum with the temperature at which the anhydride is lost (see Table). She then studied the heterocomplexes with silicon, phosphorus, germanium, arsenic, and stated the temperature conditions required for the weighings. She made special mention for the phosphomolybdates of ammonium and oxine, and reported a new gravimetric method for germanium with dibromo oxine.Molybdic anhydride, accompanied by another anhydride coming from the destruction of a heterocomplex has a lower tendency to volatilize than when it is alone or associated with ammonia, organic compounds, or mixed with silica in the ratio SiO₂ : 12 MoO₃.

---

Zusammenfassung Mit Hilfe der Thermowaage vonChevenard wurde die Flüchtigkeit von Molybdänsäureanhydrid untersucht und die Isothermen bei 300, 400, 500, 600 und 700° festgestellt. Das Anhydrid allein ist unterhalb von 782° nicht zu verflüchtigen. Die Angaben vonPavelka undZucchelli ⁵ über dessen Flüchtigkeit ließen sich durch fortlaufende Gewichtskontrolle nicht bestätigen.Bei der chemischen Analyse erhält man das theoretische Gewicht des Anhydrids um so schneller, je rascher man die Temperatur steigert.Die Thermolysekurven des Ammonium-para-Molybdats, der gelben und der weißen Modifikation der Molybdänsäure werden erläutert. Die zur gewichtsanalytischen Bestimmung des Molybdäns dienenden Niederschläge wurden hinsichtlich der Temperatur der Verflüchtigung des Anhydrids untersucht (siehe Tabelle).Weiterhin wurden die Heterokomplexe mit Silizium, Phosphor, Germanium und Arsen untersucht und die für deren Gewichtsanalyse erforderlichen Temperaturen festgelegt; besonders in Betracht gezogen wird das Phosphormolybdat des Ammoniums und des Oxiniums. Eine neue gravimetrische Methode zur Bestimmung von Germanium mit Dibromoxin wird angegeben.Wenn Molybdänsäureanhydrid in Verbindung mit anderen, aus der Zerstörung von Heterokomplexen stammenden Anhydriden vorliegt, zeigt es wesentlich geringere Flüchtigkeit, als wenn es allein, an Ammonium oder an organische Substanzen gebunden, oder im Gemenge mit Silizium im Verhältnis SiO₂ : 12 MoO₃ zur Untersuchung gelangt.

---

---

Avec 29 figures.