Intermediate Bridging Research Articles

Solvent-dependent modulation of metal–metal electronic interactions in a dinuclear cyanoruthenate complex: a detailed electrochemical, spectroscopic and computational study

The dinuclear complex [{Ru(CN)(4)}(2)(μ-bppz)](4-) shows a strongly solvent-dependent metal-metal electronic interaction which allows the mixed-valence state to be switched from class 2 to class 3 by changing solvent from water to CH(2)Cl(2). In CH(2)Cl(2) the separation between the successive Ru(II)/Ru(III) redox couples is 350 mV and the IVCT band (from the UV/Vis/NIR spectroelectrochemistry) is characteristic of a borderline class II/III or class III mixed valence state. In water, the redox separation is only 110 mV and the much broader IVCT transition is characteristic of a class II mixed-valence state. This is consistent with the observation that raising and lowering the energy of the d(π) orbitals in CH(2)Cl(2) or water, respectively, will decrease or increase the energy gap to the LUMO of the bppz bridging ligand, which provides the delocalisation pathway via electron-transfer. IR spectroelectrochemistry could only be carried out successfully in CH(2)Cl(2) and revealed class III mixed-valence behaviour on the fast IR timescale. In contrast to this, time-resolved IR spectroscopy showed that the MLCT excited state, which is formulated as Ru(III)(bppz(˙-))Ru(II) and can therefore be considered as a mixed-valence Ru(II)/Ru(III) complex with an intermediate bridging radical anion ligand, is localised on the IR timescale with spectroscopically distinct Ru(II) and Ru(III) termini. This is because the necessary electron-transfer via the bppz ligand is more difficult because of the additional electron on bppz(˙-) which raises the orbital through which electron exchange occurs in energy. DFT calculations reproduce the electronic spectra of the complex in all three Ru(II)/Ru(II), Ru(II)/Ru(III) and Ru(III)/Ru(III) calculations in both water and CH(2)Cl(2) well as long as an explicit allowance is made for the presence of water molecules hydrogen-bonded to the cyanides in the model used. They also reproduce the excited-state IR spectra of both [Ru(CN)(4)(μ-bppz)](2-) and [{Ru(CN)(4)}(2)(μ-bppz)](4-) very well in both solvents. The reorganization of the water solvent shell indicates a possible dynamical reason for the longer life time of the triplet state in water compared to CH(2)Cl(2).

A formula for the wall-amplified added mass coefficient for a solid sphere in normal approach to a wall and its application for such motion at low Reynolds number

This work re-examines the potential flow theory for a sphere in normal approach to a wall, based on the classical results derived by Lamb [Hydrodynamics (Dover, New York, 1932)] and Milne-Thomson [Theoretical Hydrodynamics, 5th ed. (Dover, New York, 1968)]. These authors generated an expression in which the kinetic energy for a sphere in an unbounded fluid is augmented by a wall correction function in terms of an infinite series that depends on the scaled center-to-wall distance, h∗=h/a, with a denoting the sphere radius. By truncating the series at the order of h∗−3, the resulting one-term correction function, 3/8h∗−3, is widely employed to approximate the wall-amplified added mass coefficient, CAM(h∗), in multiphase flow research. Nonetheless, this work shows that this one-term correction deviates greatly from corrections including higher order terms when the interstitial gap drops below the half sphere radius. Thus, an explicit formula is developed, for all h∗, using a near-wall Padé approximation, an intermediate bridging function, and a far-field approximation. This proposed formula provides an efficient and reasonable approximation to the infinite series and thus may serve as an improved wall correction function as compared to the one-term formula. The developed formula is applied to compute the unsteady approach of a nonrotating sphere toward a wall in a viscous fluid at low Reynolds number condition. In addition to Brenner’s wall correction on the quasisteady viscous force [H. Brenner, “The slow motion of a sphere through a viscous fluid towards a plane surface,” Chem. Eng. Sci. 16, 242 (1961)], the current formula is employed to modify both the added mass coefficient, from 1/2, and the history force. This latter force is modified by integrating the wall-modified potential flow theory with the boundary layer theory. If the one-term correction is used in the equation of motion, underestimation of the sphere motion and the force magnitudes are observed. Lastly, the limiting value of the infinite series derived by Lamb and Milne-Thomson as h→a is analytically evaluated, leading to a result in terms of the generalized zeta function, ζ(3,1). When this limiting value is compared to the one-term correction at h=a, a 38% deficiency of the wall-augmented kinetic energy is revealed, resulting in an underestimated CAM(h∗) that plays a crucial role in presenting the near-wall normal approach.

In vitro Toxicity Testing of Nanoparticles in 3D Cell Culture

Common 2D cell cultures do not adequately represent the functions of 3D tissues that have extensive cell-cell and cell-matrix interactions, as well as markedly different diffusion/transport conditions. Hence, testing cytotoxicity in 2D cultures may not accurately reflect the actual toxicity of nanoparticles (NPs) and other nanostructures in the body. To obtain more adequate and detailed information about NP-tissue interactions, we here introduce a 3D-spheroid-culture-based NP toxicology testing system. Hydrogel inverted colloidal crystal (ICC) scaffolds are used to create a physiologically relevant and standardized 3D liver tissue spheroid model for in vitro assay application. Toxicity of CdTe and Au NPs are tested in both 2D and 3D spheroid cultures. The results reveal that NP toxic effects are significantly reduced in the spheroid culture when compared to the 2D culture data. Tissue-like morphology and phenotypic change are identified to be the major factors in diminishing toxicity. Acting as an intermediate stage bridging in vitro 2D and in vivo, our in vitro 3D cell-culture model would extend current cellular level cytotoxicity to the tissue level, thereby improving the predictive power of in vitro NP toxicology.

Shining Light on Dinitrogen Cleavage: Structural Features, Redox Chemistry, and Photochemistry of the Key Intermediate Bridging Dinitrogen Complex

The key intermediate in dinitrogen cleavage by Mo(N[t-Bu]Ar)3, 1 (Ar = 3,5-C6H3Me2), has been characterized by a pair of single crystal X-ray structures. For the first time, the X-ray crystal structure of (mu-N2)[Mo(N[t-Bu]Ar)3]2, 2, and the product of homolytic fragmentation of the NN bond, NMo(N[t-Bu]Ar)3, are reported. The structural features of 2 are compared with previously reported EXAFS data. Moreover, contrasts are drawn between theoretical predictions concerning the structural and magnetic properties of 2 and those reported herein. In particular, it is shown that 2 exists as a triplet (S = 1) at 20 degrees C. Further insight into the bonding across the MoNNMo core of the molecule is obtained by the synthesis and structural characterization of the one- and two-electron oxidized congeners, (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4], 2[B(Ar(F))4] (Ar(F) = 3,5-C6H3(CF3)2) and (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4]2, 2[B(Ar(F))4]2, respectively. Bonding in these three molecules is discussed in view of X-ray crystallography, Raman spectroscopy, electronic absorption spectroscopy, and density functional theory. Combining X-ray crystallography data with Raman spectroscopy studies allows the NN bond polarization energy and NN internuclear distance to be correlated in three states of charge across the MoNNMo core. For 2[B(Ar(F))4], bonding is symmetric about the mu-N2 ligand and the NN polarization is Raman active; therefore, 2[B(Ar(F))4] meets the criteria of a Robin-Day class III mixed-valent compound. The redox couples that interrelate 2, 2(+), and 2(2+) are studied by cyclic voltammetry and spectroelectrochemistry. Insights into the electronic structure of 2 led to the discovery of a photochemical reaction that forms NMo(N[t-Bu]Ar)3 and Mo(N[t-Bu]Ar)3 through competing NN bond cleavage and N2 extrusion reaction pathways. The primary quantum yield was determined to be Phi(p) = 0.05, and transient absorption experiments show that the photochemical reaction is complete in less than 10 ns.

Structure of a NHEJ Polymerase-Mediated DNA Synaptic Complex

Nonhomologous end joining (NHEJ) is a critical DNA double-strand break (DSB) repair pathway required to maintain genome stability. Many prokaryotes possess a minimalist NHEJ apparatus required to repair DSBs during stationary phase, composed of two conserved core proteins, Ku and ligase D (LigD). The crystal structure of Mycobacterium tuberculosis polymerase domain of LigD mediating the synapsis of two noncomplementary DNA ends revealed a variety of interactions, including microhomology base pairing, mismatched and flipped-out bases, and 3' termini forming hairpin-like ends. Biochemical and biophysical studies confirmed that polymerase-induced end synapsis also occurs in solution. We propose that this DNA synaptic structure reflects an intermediate bridging stage of the NHEJ process, before end processing and ligation, with both the polymerase and the DNA sequence playing pivotal roles in determining the sequential order of synapsis and remodeling before end joining.

N2O Decomposition on TiO2 (110) from Dynamic First-Principles Calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.

Humoral immune response to citrullinated collagen type II determinants in early rheumatoid arthritis

Collagen type II (CII) is a relevant joint-specific autoantigen in the pathogenesis of rheumatoid arthritis (RA). Whereas the reasons for the breakage of self tolerance to this major cartilage component are still enigmatic, T cell responses to glycosylated CII determinants in RA patients indicate that post-translational modifications play a role. Since the conversion of arginine into citrulline by peptidylarginine deiminases (PAD) in some non-joint-specific antigens such as filaggrin or fibrin has been shown to give rise to RA-specific humoral immune responses, we investigated whether PAD modification of cartilage-specific CII might affect its recognition by circulating autoantibodies in early RA. In vitro treatment with purified PAD led to arginine deimination of native CII or of synthetic CII peptides as evidenced by amino acid analysis. The citrullination resulted in modified recognition of the immunodominant CII epitope C1(III) (amino acid residues 359-369) by murine and human antibodies. In a cohort of early RA patients (n=286), IgG antibodies directed toward a synthetic citrullinated C1(III) peptide (citC1(III)-P) were detectable with a prevalence of 40.4%. The partial autoantibody cross-reactivity between citC1(III)-P and citrullinated peptides mimicking epitopes of the cytoskeletal autoantigen filaggrin suggests that autoimmunity to cartilage-specific modified self might be a critical intermediate bridging recognition of PAD-modified extra-articular autoantigens with the disruption of tolerance to native cartilage constituents.

Optically induced deexcitation of rare-Earth ions in a semiconductor matrix.

We report on verification of the proposed energy transfer mechanism responsible for photoluminescence of rare earth (RE) ions in semiconductors. Using two-color spectroscopy in the visible and the midinfrared regions (with a free-electron laser) we demonstrate reversal of the most important step in the excitation process. In that way, formation of the intermediate state bridging atomic states of the RE ion core and extended orbitals of a semiconducting host is explicitly confirmed and its characteristic energy spectroscopically determined. The study is performed for InP:Yb. It is argued, however, that the conclusions are valid for all semiconductor:RE systems, including the notorious Si:Er.

Bridge mediated ultrafast heterogeneous electron transfer

Bridge mediated photoinduced ultrafast heterogeneous electron transfer (ET) from a molecularly anchored chromophore to a semiconductor surface is modelled theoretically. The continuum levels of the semiconductor substrate are taken into account in the numerical calculations via a polynomial expansion. Electron transfer for the direct injection case in the strong coupling limit is studied and compared with cases where intermediate bridging states are successively introduced to weaken the effective electronic coupling. The role of vibronic coherences in the strong electronic coupling limit as well as in off-resonant bridge mediated electron transfer is also discussed.

BIFEST

This paper presents BIFEST, an ATPG system that employs the built-in intermediate voltage test technique in an efficient ATPG process to deal with CMOS bridging faults. Fast and accurate calculations of the intermediate bridging voltages and the variant threshold tolerance margins on a resistive bridging fault model are presented. A PODEM-like, PPSFP-based ATPG process is developed to generate test patterns for faults that are conventionally logic-testable. The remaining faults are then dealt with by special circuits, called built-in intermediate voltage sensors (BIVSs). By this methodology, almost the same fault coverage as that employingIDDQtesting can be achieved with only logic monitoring required.

Charge transfer via interfaces, especially of nanoscale materials

Nanoscale materials and their properties are mainly defined by the interface material and its properties. The charge transfer in those materials, i.e., across and along interfaces is complex. In contrast to classical charge transfer via direct tunneling dying away quickly in 0.5 nm, most charges are transferred at interfaces via resonant tunneling and higher-order intermediate state tunneling bridging 2-25 nm. The basic concepts of this incoherent charge transfer are presented and known intermediate states are elucidated and model systems are discussed.

Synthesis and properties of polynuclear complexes containing {Mo(NO)[HB(dmpz)3]Cl} metal centres axially bound to a Ru(tpp) core (dmpz = 3,5-dimethylpyrazol-1-yl; tpp =meso-5,10,15,20-tetraphenylporphyrinate)

A series of mononuclear complexes [Mo(NO){HB(dmpz)3}Cl(L)][dmpz = 3,5-dimethylpyrazol-1-yl; L = a potentially bridging ligand of which one terminus (pyridyl or phenolate) is attached to the Mo and the second terminus (pyridyl) is pendant] were attached to the axial positions of a Ru(tpp) core (tpp =meso-5,10,15,20-tetraphenylporphyrinate)via the pendant pyridyl groups. By using [Ru(tpp)(CO)(EtOH)], of which only the axial EtOH ligand is substitution labile, binuclear complexes [{(OC)(tpp)Ru}(µ-L){Mo(NO)[HB(dmpz)3]Cl}] were prepared. With [Ru(tpp)(thf)2](thf = tetrahydrofuran), in which both thf ligands are labile, the trinuclear complexes [{Cl[HB(dmpz)3](ON)Mo}(µ-L){Ru(tpp)}(µ-L){Mo(NO)[HB(dmpz)3]Cl}] were prepared. The new complexes have been thoroughly characterised by 1H NMR, fast atom bombardment mass spectrometry, IR, UV/VIS and EPR spectroscopy and electrochemistry as appropriate. Weak electrochemical interactions between the molybdenum groups and the Ru(tpp) core are apparent. In the trinuclear complexes with two axial paramagnetic molybdenum groups (17 valence-electron configuration) a weak spin-exchange interaction between the two remote molybdenum centres can be detected when the intermediate bridging ligands L are 4,4′-bipyridine but not when L is 3,3′-dimethyl-4,4′-bipyridine, possibly due to an increased dihedral twist of the two bridging ligands in the latter case.

Using bridging analogies and anchoring intuitions to deal with students' preconceptions in physics

AbstractLessons were designed to deal with students' alternative conceptions in three areas of mechanics: static normal forces, frictional forces, and Newton's third law for moving objects. Instructional techniques such as class discussions of the validity of an analogy between a target problem and an intuitive anchoring example, and forming a structured chain of intermediate bridging analogies were used. There were large differences in pre–posttest gains in favor of the experimental group. In formulating a model of learning processes that can explain these results, it is argued that (a) the lessons have a more complex structure than a simple model of analogy use; (b) rational methods using analogy and other plausible reasoning processes that are neither proof based nor directly empirical can play a very important role in science instruction; (c) much more effort than is usually allocated should be focused on helping students to make sense of an analogy; and (d) researchers and curriculum developers should be focusing at least as much attention on students' useful prior knowledge as they are on students' alternative conceptions.

Atrioventricular septal defects: cross-sectional echocardiographic and morphologic comparisons

We examined the cross-sectional echocardiographic findings of 171 patients with atrioventricular septal defects. The echocardiographic findings were confirmed by angiography, surgery and/or autopsy. The echocardiographic findings determined whether the common atrioventricular junction was guarded by a common valve or separate right and left valves. In addition, we were able to judge whether the bridging leaflets were related to the septal structures so as to permit both interatrial and interventricular communications [127 cases] or whether the interatrial communication (“ostium primum atrial septal defect”) [43 cases] or an interventricular communication [1 case] existed in isolation. Defects existing with a common atrioventricular valve could be further classified as having minimal bridging of the antero-superior leaflet (Rastelli Type A [113 cases]); intermediate bridging (Rastelli Type B [3 cases]); or extreme bridging (Rastelli Type C [11 cases]). Of the patients with Down's syndrome, 9 had separate right and left valves while 66 had a common valve, all the latter existing in the setting of minimal bridging of the antero-superior leaflet. In the overall group, there were 9 cases having an unbalanced ventricular mass, 5 with right ventricular dominance and 4 with left dominance. Other associated defects were common. The echocardiographic findings were supplemented by pulsed Doppler examination. Atrioventricular valve insufficiency, when mild, was frequently demonstrated only in the right atrium just above the leaflets of the atrioventricular valve. When there was more severe valve insufficiency the regurgitant jet could be detected for greater distances behind the atrioventricular valve and in either or both atria, but more frequently in the left atrium. As might be anticipated in view of the complexity of the lesion, a combined imaging approach yielded the most accurate results.