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41.
Humblot V Lorenzo MO Baddeley CJ Haq S Raval R 《Journal of the American Chemical Society》2004,126(20):6460-6469
A detailed comparison of tartaric acid (HOOC-CHOH-CHOH-COOH) and succinic acid (HOOC-CH(2)-CH(2)-COOH) molecules on a Cu(110) surface is presented with a view to elucidate how the two-dimensional chirality exhibited by such robust, chemisorbed systems is affected when both OH groups of the former molecule are replaced with H groups, a stereochemical change that leaves the metal-bonding functionalities of the molecule untouched but destroys both chiral centers. It is found that this change does not significantly affect the thermodynamically preferred chemical forms that are adopted, namely the doubly deprotonated bicarboxylate at low coverages (theta = (1)/(6) ML) and the singly deprotonated monocarboxylate at higher coverage. However, the kinetics of phase formation are significantly affected so that the conditions required for self-assembling pertinent two-dimensional chiral phases alter substantially. For both molecules, two-dimensional assembly is found to depend strongly on the nature of the local adsorption motif created, with each motif essentially acting as a "synthon" for the supramolecular assembly. In this respect, it seems that molecule-metal bonding interactions define the general self-assembly structure. The presence/absence of the OH groups, instead, cause a subtler, second-order effect on the finer details of the self-assembled structure. Finally, the creation of chirality in the achiral succinate system is shown to arise from adsorption-induced asymmetrization, inducing point chirality via molecular distortion and/or metal reconstruction of the local adsorption unit. This chiral adsorption unit is then responsible for creating chiral supramolecular through-space and through-metal interactions that propagate a chiral organization. However, the achirality of the succinate ensures that nucleation points of either chirality are equally created, producing a racemic conglomerate of coexisting mirror domains. It is in this aspect that the uniquely aligned OH groups of the rigid bitartrate system wield the greatest effect, by favoring one distortion/reconstruction for the (R,R)-bitartrate and its mirror image distortion/reconstruction for the (S,S)-enantiomer, creating surfaces that are globally chiral on the macroscopic scale. So overall, the OH groups do not dictate the general nature of the assembly but are critical as chiral propagators, breaking the degeneracy and thus promoting asymmetry to chirality. 相似文献
42.
Donzello MP De Mori G Viola E Ercolani C Bodo E Mannina L Capitani D Rizzoli C Gontrani L Aquilanti G Kadish KM D'Angelo P 《Inorganic chemistry》2011,50(23):12116-12125
The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)(2)Th(2)Pyz], and its Pd(II) derivative, [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, formed by reaction of [(CN)(2)Th(2)Pyz] with [(C(6)H(5)CN)(2)PdCl(2)] were characterized by X-ray, UV-visible, (1)H and (13)C NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)(2)Th(2)Pyz] shows the presence of one thienyl ring positioned orthogonal to the rest of the molecule, with the two vicinal thienyl rings lying orthogonal to each other in a rare arrangement. NMR studies of [(CN)(2)Th(2)Pyz] in the solid state and in solutions of dimethylformamide or dimethyl sulfoxide confirm a nonequivalence of the thienyl rings in the solid state and also in solution. EXAFS results indicate that two distinct Pd(II) coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, with identical Pd-N(pyz) (2.03(3) ?) and Pd-Cl (2.36(3) ?) bond lengths but with different Pd-S1 (2.25(4) ?) and Pd-S2 (3.21(5) ?) bond distances in an overall asymmetric molecular framework. Density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical studies also provide information about the structure and spectral behavior of the precursor and its metalated Pd(II) derivative. (1)H/(13)C NMR and UV-visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl(2) with [Th(8)TPyzPzM] where Th(8)TPyzPz = tetrakis-2,3-[5,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg(II)(H(2)O) or Zn(II). Spectroscopic data on the newly synthesized [(PdCl(2))(4)Th(8)TPyzPzM] compounds suggest that the binding of PdCl(2) involves coordination sites of the type S(2(th))PdCl(2) with the two thienyl rings of each di(2-thienyl)pyrazino fragment bound to Pd(II) in an equivalent manner ("th-th" coordination). This is similar to what was found for the corresponding octapyridinated analogues ("py-py" coordination). 相似文献
43.
Mg-based materials are promising for orthopedic, dental, and cardiovascular applications but their high degradation rate in vivo (release of Mg ions and debris particles) is cause of great concern. Protective treatments involving fluoride conversion coatings have been proposed in order to reduce corrosion rates. The aim of this study was to evaluate Mg debris biodegradation and its possible cytotoxic effects on osteoblastic cells in situ. Neutral Red dying and Acridine Orange staining techniques were used as endpoints to analyse the cytotoxic effects at 100-2000 μg/mL concentration range. Results showed a marked variation of Mg ion concentration in the culture medium after different exposure periods (1, 2, or 24h). Interestingly, the release rate of magnesium ions was dependent on the presence or absence fluoride treatment. Adverse effects induced by ≥1000 μg/mL MP doses and Mg ion concentrations higher than 480 μg/mL were observed on cells. Results showed significant differences between the concentration of Mg ions in the presence and absence of cells. This fact reveals a dynamic equilibrium mediated by Mg ion input and output in the cells that leads to the change in MP corrosion rates. Fluoride release from conversion coatings did not show cytotoxic effects. 相似文献
44.
Wei D Grande L Chundi V White R Bower C Andrew P Ryhänen T 《Chemical communications (Cambridge, England)》2012,48(9):1239-1241
Graphite was electrochemically exfoliated in mixtures of room temperature ionic liquids and deionized water containing lithium salts to produce functionalized graphenes and such an electrochemical exfoliation technique can be directly used in making primary battery electrodes with significantly enhanced specific energy capacity. 相似文献
45.
Laurent De Windt Detlef W. M. Hofman Lorenzo Pisani Enrico Clementi 《International journal of quantum chemistry》1995,53(2):131-147
The Coulomb–Hole–Hartree–Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E. Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The present formulation makes use of either one or more determinants in order to ensure proper dissociation products, following the early proposal of G.C. Lie and E. Clementi in the context of density functional computations for molecular systems. The new formulation is tested against the dissociation energies of a large number of molecules and it is found satisfactory. © 1995 John Wiley & Sons, Inc. 相似文献
46.
A complete potentiometric and NMR relaxometric solution study on the heptadentate 2,2',2″,2'″-[(6-piperidinyl-1,3,5-triazine-2,4-diyl)dihydrazin-2-yl-1-ylidene]tetraacetic acid (PTDITA) ligand has been carried out. This ligand is based on the 1,3,5-triazine ring with two hydrazine-N,N-diacetate groups in positions 2 and 4 and a piperidine moiety in position 6. The introduction of the triazine ring into the ligand backbone is expected to modify its flexibility and then to affect the stability of the corresponding complexes with transition-metal and lanthanide ions. Thermodynamic stabilities have been determined by pH potentiometry, UV spectrophotometry, and (1)H NMR spectroscopy for formation of the complexes with Mg(2+), Ca(2+), Cu(2+), Zn(2+), La(3+), Gd(3+), and Lu(3+) ions. PTDITA shows a good binding affinity for Gd(3+) (logK = 18.49, pGd = 18.6) and an optimal selectivity for Gd(3+) over the endogenous Ca(2+), Zn(2+), and Cu(2+) (K(sel) = 6.78 × 10(7)), which is 3 orders of magnitude higher that that reported for Gd(DTPA) (K(sel) = 2.85 × 10(4)). This is mainly due to the lower stability of the Cu(II)- and Zn(II)(PTDITA) complexes compared to the corresponding DTPA complexes, which suggests an important role of the triazine ring on the selectivity for the Gd(3+) ion. The relaxometric properties of Gd(PTDITA) have been investigated in aqueous solution by measuring the (1)H relaxivity as a function of the pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion profile). Variable-temperature (17)O NMR data have provided direct information on the kinetic parameters for exchange of the coordinated water molecules. A simultaneous fit of the data suggests that the high relaxivity value (r(1) = 10.2 mM(-1) s(-1)) is a result of the presence of two inner-sphere water molecules along with the occurrence of relatively slow rotation and electronic relaxation. The water residence lifetime, (298)τ(M) = 299 ns, is quite comparable to that of clinically approved magnetic resonance imaging contrast agents. The displacement of the inner-sphere water molecules by bidentate endogeneous anions (citrate, phosphate, and carbonate) has also been evaluated by (1)H relaxometry. In general, the binding interaction is markedly weak, and only in the case of citrate, a ca. 35% decrease in relaxivity was observed in the presence of 60 equiv of the anion. Phosphate and carbonate also interact with the paramagnetic ion, likely as monodentate ligands, but formation of the ternary complex is accompanied by a modest increase of r(1) due to the contribution of second-sphere water molecules. 相似文献
47.
Development of a stereoselective Ugi reaction starting from an oxanorbornene β-amino acid derivative
Banfi L Basso A Chiappe C De Moliner F Riva R Sonaglia L 《Organic & biomolecular chemistry》2012,10(19):3819-3829
We have synthesised a novel oxanorbornene β-aminoacid derivative and employed it in a stereoselective Ugi reaction. Hypothesis regarding the mechanism taking place during the reaction have been made and validated through the determination of the relative and absolute configuration of the Ugi adducts. Use of the correct choice of solvents can increase stereoselection. The resulting bicyclic peptidomimetics can be used as a novel class of pluripotent substrates to be elaborated according to the synthetic strategies previously elaborated in our laboratories. 相似文献
48.
Journal of Computer-Aided Molecular Design - We systematically tested the Autodock4 docking program for absolute binding free energy predictions using the host-guest systems from the recent SAMPL6,... 相似文献
49.
Marco Bellagamba Luigi Bencivenni Lorenzo Gontrani Leonardo Guidoni Claudia Sadun 《Structural chemistry》2013,24(3):933-943
In this work, we report a multitechnique (energy-dispersive X-ray diffraction, computational methods, and FTIR spectroscopy) study of the tautomeric equilibrium of 1,2,3-triazole, one of the few small nitrogen-containing eterocycles liquid at room temperature. The T-2H form (C 2v symmetry) is found to be strongly favored in gas and solid phases, whereas the neat liquid gives diffraction patterns that can be interpreted satisfactorily with the structure functions calculated from some molecular dynamics results for both T-2H and T-1H tautomers, although the T-2H form gives a slightly better agreement. 相似文献
50.