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At least 685 records · Page 38Linked to original sources
PubMed2002

Unsymmetrical linear pentanuclear nickel string complexes: [Ni(5)(tpda)(4)(H(2)O)(BF(4))](BF(4))(2) and [Ni(5)(tpda)(4)(SO(3)CF(3))(2)](SO(3)CF(3)).

The one-electron oxidized linear pentanuclear nickel complexes [Ni(5)(tpda)(4)(H(2)O)(BF(4))](BF(4))(2) (1) and [Ni(5)(tpda)(4)(SO(3)CF(3))(2)](SO(3)CF(3)) (2) have been synthesized by reacting the neutral compound [Ni(5)(tpda)(4)Cl(2)] with the corresponding silver salts. These compounds have been characterized by various spectroscopic techniques. Compound 1 crystallizes in the monoclinic space group P2(1)/n with a = 15.3022(1) A, b = 31.0705(3) A, c = 15.8109(2) A, beta = 92.2425(4) degrees, V = 7511.49(13) A(3), Z = 4, and compound 2 crystallizes in the monoclinic space group C2/c with a = 42.1894(7) A, b = 17.0770(3) A, c = 21.2117(4) A, beta = 102.5688(8) degrees, V = 14916.1(5) A(3), Z = 8. X-ray structural studies reveal an unsymmetrical Ni(5) unit for both compounds 1 and 2. Compounds 1 and 2 show stronger Ni-Ni interactions as compared to those of the neutral compounds.

Journal Article↗
PubMed2006

Even-numbered metal chain complexes: synthesis, characterization, and DFT analysis of [Ni4(mu4-Tsdpda)4(H2O)2] (Tsdpda2- = N-(p-toluenesulfonyl)dipyridyldiamido), [Ni4(mu4-Tsdpda)4]+, and related Ni4 string complexes.

The synthesis and the X-ray structure of two complexes exhibiting a linear chain of four nickel atoms is reported, following Ni4(mu4-phdpda)4 (1), which had been characterized previously. [Ni4(mu4-Tsdpda)4(H2O)2], where H2Tsdpda is N-(p-toluenesulfonyl)dipyridyldiamine (2), is axially coordinated to two water molecules, at variance with 1. One-electron oxidation of 2 resulted in the loss of the axial ligands, yielding [Ni4(mu4-Tsdpda)4]+, [3]+, which was also structurally characterized. Finally, we report the structure of Ni4(mu4-DAniDANy)4 (4), a complex synthesized starting from the new ligand N,N'-bis-p-anisyl-2,7-diamino-1,8-naphthyridine. Magnetic measurements concluded that 4 is diamagnetic, like 1, whereas 2 is antiferromagnetic (-2J(14) = 80 cm(-)(1), using the Heisenberg Hamiltonian H = -2J(14) S(1).S(4)), as are other axially coordinated chains with an odd number of nickel atoms. DFT calculations are reported on these complexes in order to rationalize their electronic structure and their magnetic behavior. The magnetic properties of the [Ni4]8+ complexes are governed by the electronic state of the Ni(II) atoms, which may be either low-spin (S = 0), or high-spin (S = 1). DFT calculations show that the promotion to high spin of two Ni atoms in the chain, either external or internal, depends on the interplay between axial and equatorial coordination. The synergy between axial coordination and the presence of electron-withdrawing toluenesulfonyl substituents in 2 favors the promotion to the high-spin state of the terminal Ni atoms, thus yielding an antiferromagnetic ground state for the complex. This is at variance with complexes 1 and 4, for which the lowest quintet state results from the promotion to high spin of the internal nickel atoms, together with an important ligand participation, and is destabilized by 9 to 16 kcal mol(-1) with respect to the diamagnetic ground state.

Journal Article↗
PubMed2007

Influence of electron-attractor substituents on the magnetic properties of Ni(II) string complexes.

Density functional theory calculations carried out on the anionic Ni(II) chain complexes {trinickeltetrakis[2,6-bis(amido)pyridine]}2- and {trinickeltetrakis[N,N'-bis(sulfonyl)pyridyldiamido]}2- show that the electron-attractor character of the sulfonyl substituent modifies the basicity of the amido ligand ends so as to raise the electronic state of the underlying metal atom from low spin (S = 0) to high spin (S = 1). The computed elongation of the outermost Ni-N bonds with the grafting of SO2H substituents is in agreement with this interpretation.

Journal Article↗
PubMed1999

Molybdenum(V) on an Oxide String. Synthesis and Structure of the Novel Linear Trinuclear Complex {[MoO(TPP)][O-Mo(TPP)-O][MoO(TPP)]}ClO(4).

The syntheses and crystallographic characterization of the linear trinuclear complex [Mo(3)O(4)(TPP)(3)]ClO(4) and the monomeric precursors [MoO(TPP)(OClO(3))] and [MoO(TPP)(OH(2))]ClO(4) are described. The crystallographic analyses of [MoO(TPP)(OClO(3))] and [MoO(TPP)(OH(2))]ClO(4) show that these similar complexes are typical oxomolybdenum(V)-porphyrin complexes whose only significant structural differences are related to their respective axial ligands. [Mo(3)O(4)(TPP)(3)]ClO(4) is obtained by the controlled hydrolysis of [MoO(TPP)(OClO(3))] or by the direct reaction of [MoO(TPP)(OClO(3))] with {[MoO(TPP)](2)O}. This hydrolysis product contains the novel [Mo(3)O(4)(TPP)(3)](+) ion, which has an unprecedented linear [O=Mo-O-Mo-O-Mo=O](7+) unit and three nearly parallel 24-atom porphyrin mean planes. The central porphyrin is found to be rotated 29.3 degrees and 29.7 degrees with respect to the two terminal porphyrin ligands, which are nearly eclipsed (twist angle 0.44 degrees ). The formation of [Mo(3)O(4)(TPP)(3)]ClO(4) instead of the &mgr;-hydroxo-bridged binuclear complexes obtained from the hydrolysis of analogous mononuclear metalloporphyrin complexes is rationalized on the basis of the greater pi-acceptor ability of Mo(V). Crystal data for [MoO(TPP)(OClO(3)).0.9CH(2)Cl(2): a = 11.437(2) Å, b = 13.567(6) Å, c = 13.764(4) Å, alpha = 89.15(1) degrees, beta = 108.86(1) degrees, gamma = 103.96(2) degrees, triclinic, space group P&onemacr;, Z = 2, T = 293(2) K. Crystal data for [MoO(TPP)(OH(2))ClO(4).1.5C(7)H(8): a = 13.2138(7) Å, b = 13.5959(12) Å, c = 14.6676(11) Å, alpha = 74.292(8) degrees, beta = 69.114(8) degrees, gamma = 69.030(5) degrees, triclinic, space group P&onemacr;, Z = 2, T = 127(2) K. Crystal data for [Mo(3)O(4)(TPP)(3)]ClO(4).C(6)H(1)(4).3.4CH(2)Cl(2).4H(2)O: a = 26.428(6) Å, b = 18.683(6) Å, c = 28.692(4) Å, beta = 113.98(2) degrees, monoclinic, space group P2(1)/c, Z = 4, T = 127(2) K.

Journal Article↗
PubMed2000

Molybdenum(V) on an oxide string. Formation and solution stability of the linear complex (mu-trans-dioxo(tetraphenylporphyrinato)molybdenato(V))- bis(oxo(tetraphenylporphyrinato)molybdenum(V)) perchlorate.

The unusual linear trinuclear complex [Mo3O4(TPP)3]+ is formed in solution upon the reaction of [MoO(TPP)-(OClO3)] with [[MoO(TPP)]2O], and an equilibrium between [Mo3O4(TPP)3]+ and its constituent species is rapidly established. Spectrophotometric experiments suggest that [Mo3O4(TPP)3]+ is the predominant species found in solutions resulting from the mixture of [MoO(TPP)(OClO3)] and [[MoO(TPP)]2O], and its formation is strongly favored (log K = 5.5 +/- 0.5 M-1). No evidence of higher oligomers has been observed. A mechanism for the formation of [Mo3O4(TPP)3]+ by the controlled hydrolysis of [MoO(TPP)(OClO3)] is proposed.

Catalysis↗
PubMed2005

Plasmonic nanoparticle chains via a morphological, sphere-to-string transition.

Au nanoparticles encapsulated within polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelles assemble into regular, one-dimensional arrays when they are exposed to solvent conditions that relax interfacial curvature in the micellar shell. Nanoparticle chaining was induced by adding salt, acid, or cationic carbodiimide to the suspension of purified encapsulated Au nanoparticles (Au@PS-b-PAA). The resulting assemblies were characterized by scanning and transmission electron microscopies, by dark-field optical microscopy, and by visible absorption spectroscopy. The length of the chains was modulated by varying the concentration of additive. More importantly, the spacing between Au nanoparticles was dictated entirely by the shell thickness of the Au@PS-b-PAA starting material. Far-field polarization microspectroscopy demonstrated directional surface plasmon coupling in a straightened nanoparticle chain, which is a basic requirement for the use of these assemblies as plasmon waveguides.

Acrylic Resins↗
PubMed2005

Finite temperature string method for the study of rare events.

A method is presented for the study of rare events such as conformational changes arising in activated processes whose reaction coordinate is not known beforehand and for which the assumptions of transition state theory are invalid. The method samples the energy landscape adaptively and determines the isoprobability surfaces for the transition: by definition the trajectories initiated anywhere on one of these surfaces has equal probability to reach first one metastable set rather than the other. Upon weighting these surfaces by the equilibrium probability distribution, one obtains an effective transition pathway, i.e., a tube in configuration space inside which conformational changes occur with high probability, and the associated rate. The method is first validated on a simple two-dimensional example; then it is applied to a model of solid-solid transformation of a condensed system.

Journal Article↗
PubMed1976

Visual processing of nonlinguistic strings: redundancy effects and reading ability.

Visual redundancy effects were investigated in two experiments using nonalphabetic material. Experiment 1 had adult subjects search for the presence or absence of a predetermined target symbol through single six-symbol linear arrays. Reaction times were significantly faster in both a distributional redundancy and a spatial redundancy condition than in a no-redundancy condition. Experiment 2 had good and poor sixth-grade readers serve in the no-redundancy and spatial redundancy conditions of Experment 1. Poor readers were equivalent to good readers in the no-redundancy condition but were significantly slower in the spatial redundancy condition. Thus, spatial redundancy was shown to be operative in single symbol search time in a paradigm that avoided confounding by higher order linguistic variables. The present experiments, in conjunction with prior findings that good and poor readers differ in letter search time by their ability to use spatial redundancy, suggest that processes involved in the component skill of letter identification cannot be considered trivial to the reading process.

Adult↗
PubMed2004

Effects of model-based and memory-based processing on speed and accuracy of grammar string generation.

Learners are able to use 2 different types of knowledge to perform a skill. One type is a conscious mental model, and the other is based on memories of instances. The authors conducted 3 experiments that manipulated training conditions designed to affect the availability of 1 or both types of knowledge about an artificial grammar. Participants were tested for both speed and accuracy of their ability to generate letter sequences. Results indicate that model-based training leads to slow accurate responding. Memory-based training leads to fast, less accurate responding and highest achievement when perfect accuracy was not required. Evidence supports participants' preference for using the memory-based mode when exposed to both types of training. Finally, the accuracy contributed by model-based training declined over a retention interval.

Achievement↗