1,721,115 research outputs found
EASILY REVERSIBLE IONIC FRAGMENTATION OF A TETRAHEDRAL UNSATURATED RHENIUM CLUSTER, A REAPPRAISAL OF THE REACTION OF {[RE-4(MU(3)-H)(4)(CO)(12)] WITH METHANOL}
No full textThe unsaturated tetrahedral cluster {[Re-4(mu(3)-H)(4)(CO)(12)] in the
presence of weak donor molecules L, like methanol or acetone, reversibly
fragments to unsaturated triangular cluster anions and to mononuclear
cations, probably via spiked-triangular [Re-4(mu-H)(4)(CO)(12)L(4)]
intermediates.
Analisi di membrane e miscele lipidiche di membrana mediante 31P-NMR
No full textPer mezzo di una manipolazione dietetica sono stati ottenuti dei campioni di membrana di globulo rosso bovino e di miscele lipidiche di membrana differenziatesi per contenuto proteine-lipidi (colesterolo, fosfolipidi, glicolipidi), per classi fosfolipidiche e per lunghezza delle catene di acidi grassi. Scopo della presente ricerca è stata l’indagine, mediante spettroscopia di risonanza magnetica nucleare del 31P, sulla natura e strutturazione dei fosfolipidi presenti in questi campioni già dettagliatamente caratterizzati nella composizione e studiati mediante calorimetria differenziale a scansione. Sono state studiate tre serie di campioni contenenti le membrane, le frazioni lipidiche totali e le frazioni fosfolipidiche disperse in tampone TRIS-EDTA, pH 7,4. In tutti i campioni è presente una risonanza anisotropa la cui asimmetria è indicativa di una struttura tipo “bilayer”. La mobilità dei fosfolipidi appare più limitata nei campioni di membrana rispetto a quelli delle altre due serie. Nei campioni lipidici si osserva inoltre una riduzione della mobilità che rispecchia le modificazioni indotte con il trattamento dietetico. I campioni di membrana e dei soli fosfolipidi sono stati trattati con colato sodico (20 mg/ml) per identificare le classi di fosfolipidi presenti. I segnali isotropi ottenuti sono stati attribuiti per confronto con soluzioni standard di fosfolipidi puri dispersi nello stesso tampone in presenza di colato. A parte la presenza di fosfato inorganico nei campioni di membrane esiste una completa riproducibilità qualitativa e quantitativa tra le due serie di spettri, confermando la possibilità di eseguire l’analisi dei fosfolipidi direttamente sulle membrane. I rapporti quantitativi tra PC, PE e SM sono in accordo con i dati delle analisi mediante TLC. Meno soddisfacenti sono i dati per PS e PI che appaiono sottostimati, probabilmente per problemi di rilassamento dovuti alla diversa carica elettrica. Le indicazioni sulla strutturazione completano i risultati sulla variazione di fluidità determinata con microcalorimetria differenziale suggerendo possibili significative alterazioni funzionali indotte da manipolazioni dietetiche
AN UNUSUAL SPECIES FROM THE REACTION OF THE UNSATURATED ANION {[RE3(MU-H)3(CO)9(MU-3-O ... H ... NME3)]-, AND CRYSTAL AND MOLECULAR-STRUCTURE OF ITS TETRAETHYLAMMONIUM SALT}
No full textThe reactions of the unsaturated cluster anion [Re3(μ-H)4(CO)10]- with Me3NO, in a solvent of low donor ability, such as acetone or tetrahydrofuran, give three products, in a ratio which depends on the reaction conditions: two of them, the unsaturated [Re3(μ-H)4(CO)9(NMe3)]- and [Re3(μ-H)4(CO)9(ONMe3)]- species, have been characterized spectroscopically. The third has been isolated and characterized by X-ray single crystal analysis as an adduct formed by hydrogen bonding between Me3N and the triangular cluster anion [Re3(μ-H)3(CO)9(μ3-OH)]-. The crystals are monoclinic, space group P21/m, with a 8.576(3), b 20.442(4), c 8.748(3) Å, β 108.08(4)°; Z = 2. The structure was solved by Patterson and Fourier methods by use of 2085 unique reflections and refined to a final R value of 0.026. The mean ReRe and ReO(OH) bond lengths are 2.979 and 2.126 Å, respectively. The O · H · N hydrogen bond is short and almost linear (O ⋯ N 2.502(10) Å). Various experiments have demonstrated that the triply bridging OH group can, according to the reaction conditions, come either from a water molecule (present in the commercial amine oxide) or by attack of a hydridic ligand on the oxygen of a coordinated amine oxide molecule. The same adduct has been obtained starting from the dianion [Re3(μ-H)3(CO)9(μ3-O)]2-, directly by reaction with Me3NHCl or in a two step sequence of protonation to [Re3(μ-H)3(CO)9(μ3-OH)]- and subsequent treatment with Me3N
Oxygen chemisorption on γ-Al2O3·phthalocyaninato·-cobalt(II): Coordination of O2 by complexed cobalt centers; Tumbling of molecular oxygen on surface cobalt centers
No full textThe system phthalocyaninatocobalt(II)·Al2O3, ([Co(pc)]·γ-Al2O3), if contacted with air at high temperatures (70 - 150 °C), has been found to be active to oxygen chemisorption. ESR measurements revealed that the chemisorption is a reversible process, leading to the formation of [Co(pc)O2]·γ-Al2O3 as a paramagnetic intermediate. ESR spectroscopy has also been used to investigate the rate of angular reorientation of O2 absorbed on the [Co(pc)]·γ-Al2O3 surface at a variety of temperatures. Computer simulations of the ESR spectra were compared with spectra obtained experimentally, and correlation times for reorientation and anisotropic g values were calculated. The fitting of the anisotropic g values to a model based on second-order perturbation theory suggests that the ground state of [Co(pc)O2]·γ-Al2O3 is (x2 - y2)2(z2)2(xz or yz)2(yz or xz), with the unpaired electron localized mainly on cobalt. According to this finding, oxygen interacts with cobalt mainly through a π overlap. © 1982
C-13 NUCLEAR MAGNETIC-RESONANCE EVIDENCE OF A RELAXATION PROCESS DOMINATED BY SCALAR COUPLING WITH A QUADRUPOLAR NUCLEUS IN {[RE3(MU-H)4(CO)10]-}
No full textCarbon-13 n.m.r. spin-lattice relaxation times of the anion [Re3(μ-H)4(CO)10]- have been determined at various temperatures and field strengths in order to elucidate the different components of the relaxation mechanism. Scalar coupling with the quadrupolar Re isotopes (185Re, 187Re; I = 52) proved to be the main relaxation pathway at room temperature in the range 20-68 MHz. Analysis of the data allowed the estimation of the quadrupolar relaxation time of Re and of the coupling constant 1J(187Re-C). The latter has been determined for the first time
C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE PROPERTIES OF THE UNSATURATED ANION {[RE3(MU-H)4(CO)9(NCME)]- AT LOW-TEMPERATURE}
No full textThe carbonyl resonances of [Re3(μ-H)4(CO)9(NCMe)]- have been assigned and the 13C spin-lattice relaxation times measured at different field strengths (20.15,50.31, and 75.43 MHz) at 238 K. The results indicate that scalar coupling of the second kind with rhenium isotopes is the dominant relaxation mechanism at this temperature. Values of T1Q (Re) and 1J(187Re-C) were calculated. The presence on the H2Re(CO)3(NCMe) moiety of a poor π-acceptor ligand doubles 1J(187Re-C) with respect to the unsubstituted H2Re(CO)4 moiety in the precursor [Re3(μ-H)4(CO)10]-
Electron spin resonance of some paramagnetic compounds of iridium(II)
No full textThis paper deals with some electronic problems suggested by Malatesta. Electron spin resonance with [IrBr3(NO)(PPh3)2] shows that the electronic configuration of the paramagnetic compound is described by that of the Ir-NO moiety. Specifically the three values of g tensor components demonstrate that the unpaired electron is located in a largely comprised π* (NO) orbital, so that the electronic state of NO is similar to that of gaseous trapped NO. The overlap between Ir and NO orbitals is through the dπ (dxz or dyz) metal orbitals, with consequent bent coordination of NO ligand. In conclusion the stability of d7 configuration of Ir in this monomeric compound is due to the easy electron transfer between metal and ligand, in agreement with that described in other monomeric Ir(II) compounds. © 1982
The crystal structure of 10-bromoanthrone
No full textCrystals of 10-bromoanthrone are triclinic, space group PT, with a=7"304 (1), b=7-768 (1), c=
11.358 (1)~, e=90.33 (1), ,8=104.58 (1), y= 116.83 (1) °, Z=2. Approximately 2300 intensities were
collected on a Pailred diffractometer from a crystal mounted along a. The structure was derived by direct
phasing methods and refined by full-matrix least-squares calculations. Final refinement led to an R of
0.061 and standard deviations in the range 0.003-0.006 ,~ in the positions of the heavy atoms. The molecule,
whose symmetry is very close to m, exhibits a 'butterfly' shape: the planes of the two outer rings
make a dihedral angle of 164 °. The most salient details of the molecular geometry are the long C(10)-Br
distance of 2.032 (4) ,~, and the boatlike configuration of the central ring, the atoms C(10) and
Spectroscopic titration of {[Re-4(mu(3)-H)(4)(CO)(12)] with chloride ions. An electron-precise tetranuclear cluster as intermediate in the transformation of a tetranuclear unsaturated cluster into trinuclear unsaturated anions}
No full textThe unsaturated tetrahedral cluster {[Re-4(mu(3)-H)(4)(CO)(12)] easily
added halide ions giving [Re(4)H(4)X(CO)(12)](-) anions, which in the
presence of even poor donor species underwent fragmentation to
unsaturated triangular cluster anions.
MUTUAL EXCHANGE AND ISOMERIZATION PROCESSES IN THE 3 ISOMERS OF THE MIXED-METAL TRIANGULAR CLUSTER COMPLEX [RE2PT(MU-H)2(CO)8(PPH3)2]
No full textThe dynamic processes operative in the three isomers of the triangular cluster [Re2Pt(mu-H)2(CO)8(PPh3)2] (1) have been studied. H-1 and P-31 variable-temperature NMR spectra of the isomer 1a (in which both the phosphines are bound to Pt) indicate the presence of two mutual exchange processes: one, operative at lower temperature, exchanges only the phosphines, while the second one, active above 238 K, exchanges the hydrides. Computer simulations of the spectra allowed the activation parameters to be calculated (E(a) = 49.2 +/- 1.4 and 60.2 +/- 1.3 kJ/mol, respectively). The isomer 1a at T > 273 K irreversibly rearranges, by the exchange of a phosphine and a carbonyl between Pt and Re, giving the equilibrium mixture of the two isomers 1b and 1c (differing in the location of the phosphine bound to Re): this slow first-order reaction has been studied at three temperatures in a conventional kinetic experiment (284-294 K, E(a) = 80 +/- 7 kJ/mol). 2D EXSY experiments on P-31 spectra (295-316 K) and computer simulations of H-1{P-31} spectra (33-363 K) have been employed to measure the rate of interconversion between the isomers 1b and 1c, and the activation parameters (E(a) = 74.0 +/- 1.7 kJ/mol) have been estimated accordingly. C-13 variable-temperature NMR spectra suggest a localized scrambling of the carbonyls in the Re(CO)3PPh3 moiety of 1b
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