Search results for "molecule"
showing 10 items of 5162 documents
Extension of Walsh's rules to more general systems
1972
It is demonstrated that location of hydrogen atoms within bonding distance of a simple triatomic molecule can alter the order of energy levels relative to that of the parent system without significantly changing the behavior of the orbital energies under geometrical variation. As a result the ground state electronic configurations of H n AB2 systems often differ from those of triatomics with the same number of electrons and this fact is shown to be directly responsible for the existence of such distinctive geometrical structures as those possessed by cyclopropane, cyclopropyl and allyl cations and cyclopropene. Thus the familiar prescription of Walsh's rules which states that isoelectronic …
Static and dynamic screening effects in the electrostatic self-assembly of nano-particles.
2014
In the description of charge screening in the electrostatic self-assembly of nanoparticles (molecules) embedded into a polar solvent, the static screening effects (a contribution associated with the rapid spatial redistribution of small and highly mobile ions of a solvent) are traditionally treated phenomenologically, using the Yukawa short-range potential for describing the interaction between these particles. However, this model has a limited range of applicability being valid only for infinitely diluted systems and high salt concentrations. During a slow self-assembling process with nanoparticle formation, very dense structural elements (aggregates) are formed, in which the distances bet…
Photoinduced optical anisotropy in organic molecular films controlled by an electric field
1993
Abstract The photoinduced reorientation of dye molecules in molecular films on solid substrates has been controlled with external dc electric fields, leading to solid state structures, which are macroscopically polar, temporally stable at room temperature, and well-defined at the molecular level. A simulation based on the excitation-driven rotational diffusion of the molecules in the potential of their neighbors yielded a realistic model and indicated a collective character of the process. This shows that we are close to a quantitative comprehension of the molecular interactions within these films.
Photoconductivity in Discotic Liquid Crystals: A New Class of High-Mobility Materials
1994
Abstract Using a time-of-flight technique, different transport mechanisms, deep trapping, multiple shallow trapping and Gaussian transport, can be observed in the different temperature and phase regions of the liquidcrystalline (LC) photoconductor hexapentyloxytriphenylene (HPT). Transient photocurrents and carrier mobilities for various temperatures, electric fields, and sample histories were examined. The ideal intrinsic Gaussian transport, observed for holes in the mesophase, puts HPT into a new class of highmobility materials with both hole mobilities on the order of 1.10−3cm2/Vs and a steplike current decay. These features result from the fact that there is obviously neither a position…
Synthesis, crystal structure and magnetic properties of the first structurally characterized 1,2-dithiocroconato-containing Cu(II) complex, [Cu(bpca)…
1996
Abstract The first crystal and molecular structure of a transition metal complex containing 1,2-dithiocroconate (1,2-dtcr, dianion of 1,2-dimercaptocylopent-1-ene-3,4,5-trione), [Cu(bpca)(H2O)]2[Cu(1,2-dtcr)2]·2H2O (where bpca is the bis(2-pyrdidylcarbonyl)amide anion), has been determined by single crystal X-ray diffraction methods. The compound crystallizesin the monoclinic syste, space group P21/c, with a = 11.661(3), b = 20.255(6), c = 8.265(3) A , s = 107.26(2)° and Z = 2. The structure is formally built of [Cu(1,2-dtcr)2]2− and [Cu(bpca)(H2O)]+ ions and water of hydration. The copper atom of the anion is situated at a crystallographic inversion centre, bonded to four sulfur atoms in a…
Dihydrogen complexes of metalloporphyrins: characterization and catalytic hydrogen oxidation activity
1992
A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H{sub 2}) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by {sup 1}H NMR. The H-H bond length was found to increase when Os was replaced by Ru or when *Im was replaced by THF. The bond distances (as determined by T{sub 1}) range from 0.92 to 1.18 {angstrom}. The first example of a bimetallic bridging dihydrogen complex, Ru{sub 2}(DPB)(*IM){sub 2}(H{sub 2}), was also prepared. The H{sub 2} ligand is simultaneously bound to both Ru-metal centers. High-field {sup 1}H NMR experiments (620 MHz) revealed a -7.37 Hz dipolar splitting of the H{sub 2} ligand for this complex. Analysis of this splitting suggests that th…
Optical, Electrochemical, and Catalytic Properties of the Unsaturated Host Pd3(dppm)3(CO)2+and Pd4(dppm)4(H)2+2Clusters: An Overview
2004
This paper presents an overview of the optical, photophysical, and photochemical properties including UV-visible and luminescence spectra in solution at 298 and 77 K, along with electrochemical, and catalytic behavior under reduction conditions (for both thermally and electrochemically assisted systems) of the tri- and tetranuclear Pd3(dppm)3(CO)2+ and Pd4(dppm)4(H)2+ 2 clusters (dppm=bis(diphenylphosphino)methane). This review is also complemented with relevant information about their syntheses, molecular and electronic structures supported from computer modeling, EHMO and DFT calculations, and their host-guest behavior with anions and neutral molecules, in relation with their observed rea…
Quantitative Characterization of the Local Electrophilicity of Organic Molecules. Understanding the Regioselectivity on Diels−Alder Reactions
2002
Regional electrophilicity at the active sites of the reagents involved in polar Diels−Alder processes may be described on a quantitative basis using an extension of the global electrophilicity inde...
Activation of PPARβ/δ inhibits leukocyte recruitment, cell adhesion molecule expression, and chemokine release
2009
Abstract Activation of the nuclear receptor PPARb/d inhibits acute inflammatory responses in vitro with human primary cells and in vivo by targeting the endothelial cell-leukocyte interaction. The infiltration of PMNs into tissues is a prominent feature in inflammation. The mechanism underlying PMN recruitment depends on the release of chemotactic mediators and CAM expression on endothelial cells. The nuclear receptor PPARβ/δ is widely expressed in many tissues, including the vascular endothelium; however, its role in acute inflammation remains unclear. Using intravital microscopy in the mouse cremasteric microcirculation, we have shown that activation of PPARβ/δ by its selective ligand GW5…
The Chemokine Networks in Sponges: Potential Roles in Morphogenesis, Immunity and Stem Cell Formation
2003
Porifera (sponges) are now well accepted as the phylum which branched off first from the common ancestor of all metazoans, the Urmetazoa. The transition to the Metazoa became possible because during this phase, cell-cell as well as cell-matrix adhesion molecules evolved which allowed the formation of a colonial stage of animals. The next prerequisite for the evolution to the Urmetazoa was the establishment of an effective immune system which, flanked by apoptosis, allowed the formation of a first level of individuation.