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At least 235 records · Page 13Linked to original sources
PubMed2006

Assembly of triangular trimetallic complexes by triamidophosphine ligands: spin-frustrated Mn2+ plaquettes and diamagnetic Mg2+ analogues with a combined through-space, through-bond pathway for 31P-31P spin-spin coupling.

The reactions of 2 equiv of the ligand precursor P(CH2NHPh)3 or P[CH2NH-3,5-(CF3)2C6H3]3 with 3 equiv of Mn[N(SiMe3)2]2 provide high-yielding routes to the triangular trinuclear Mn(II) complexes [P(CH2NPh)3]2Mn3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mn3(THF)3. The solid-state structures of these paramagnetic complexes have approximate C3 symmetry. The magnetic moments from 300 to 1.8 K could be fit as a magnetic Jahn-Teller distorted isosceles triangle. These complexes exhibit spin frustration and possess an S = 1/2 ground state, as revealed by a plot of magnetization versus field at 1.8 K; at fields above 3.8 T, the occupation of an excited state with S = 3/2 becomes significant. The diamagnetic magnesium analogues were prepared by the reaction of the ligand precursor P(CH2NHPh)3, P[CH2NH-3,5-(CF3)2C6H3]3, or P(CH2NH-3,5-Me2C6H3)3 with nBu2Mg. The solid-state structures of [P(CH2NPh)3]2Mg3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mg3(THF)3 were determined. Solution 1H NMR spectroscopy was used to demonstrate that the solid-state structures are maintained in solution. The aryl group of the terminal amido donor exhibits slow rotation on the NMR time scale, and this was found to be an electronic effect. Solution 31P{1H} NMR spectroscopy revealed an unexpected 15 Hz coupling between phosphorus nuclei in these complexes. Calculations on a model complex using density functional theory demonstrates that this coupling occurs via a combined through-space, through-bond pathway.

Journal Article↗
PubMed2006

Rotational invariance and double frustration in the structures of gold clusters growing around the F(s)-defected MgO (100) surface.

The interaction of small gold clusters (Au(n), n = 1-4, 20) and a gold monolayer with the MgO (100) surface surrounding a neutral oxygen vacancy (F(s) center) is investigated using density-functional (DF) calculations. It is found that the presence of the defect modifies the interaction of gold not only with the vacancy itself, but also with the oxygen and magnesium atoms around it by increasing both the adhesion energy and the equilibrium bond distances. This is at variance with the interaction of metal atoms with the regular MgO (100) surface or the F(s) defect itself, in which an increase of the adhesion energy is associated with a shortening of the metal-surface distance. The resulting double frustration and cylindrical invariance of the metal-surface interaction cause small gold clusters growing around an F(s) nucleation center to be highly fluxional in terms both of rotational freedom and of multiple competing structural motifs. Fragmentation energies of the gold clusters are also discussed, finding that the lowest-energy pathway corresponds to the detachment of a dimer.

Journal Article↗
PubMed2006

Frustrated ostwald ripening in self-assembled monolayers of cruciform pi-systems.

This study details a scanning tunneling microscopy investigation into the mechanism of chiral grain growth in highly ordered, self-assembled monolayer films composed of cruciform pi-systems. Although the molecules themselves are achiral, when they adsorb from solution onto graphite, they adopt a gear-like conformation that, by virtue of the surface, is chiral. These handed subunits arrange themselves into enantiomeric two-dimensional domains. The unique finding from this study is that Ostwald ripening is frustrated between domain boundaries that are of opposite chirality because direct interconversion between the chiral units on the surface is energetically inhibited.

Journal Article↗
PubMed1996

Hostility and hearing protection behavior: the mediating role of personal beliefs and low frustration tolerance.

The authors examined whether hostility would negatively be associated with occupational health behavior, namely, the use of hearing protection devices (HPDs). Also examined as possible mediators were the protection motivation theory (PMT) components and low frustration tolerance (LFT). Participants were 226 male industrial workers, all exposed to potentially hearing-damaging noise. Hostility was negatively related to HPD use. It moderately correlated with the PMT components: negatively with perceived susceptibility, severity, effectiveness, and self-efficacy and positively with perceived barriers. Hostility correlated highly with LFT. Regression analyses confirmed the mediating role of perceived barriers, low self-efficacy, and LFT in the negative relationship between hostility and the use of HPDs. Thus, intrapsychic characteristics of hostile people may be significant for hearing protection behavior.

Adolescent↗
PubMed2005

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4.

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe(2)O(4). Using resonant X-ray scattering measurements, we determine the ordering of the Fe(2+) and Fe(3+) ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.

Journal Article↗
PubMed2004

Classical spin and quantum-mechanical descriptions of geometric spin frustration.

Geometric spin frustration (GSF) in isolated plaquettes with local spin s, i.e., an equilateral-triangle spin trimer and a regular-tetrahedron spin tetramer, was examined on the basis of classical spin and quantum-mechanical descriptions to clarify their differences and similarities. An analytical proof was given for how the state degeneracy and the total spin S of their ground states depend on the local spin s. The quantum-mechanical conditions for the occurrence of GSF in isolated plaquettes were clarified, and their implications were explored. Corner sharing between plaquettes and how it affects GSF in the resulting spin systems was examined.

Journal Article↗
PubMed2005

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal.

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.

Journal Article↗
PubMed2006

Switch interactions control energy frustration and multiple flagellar filament structures.

Bacterial flagellar filament is a macromolecular assembly consisting of a single protein, flagellin. Bacterial swimming is controlled by the conformational transitions of this filament between left- and right-handed supercoils induced by the flagellar motor torque. We present a massive molecular dynamics simulation that was successful in constructing the atomic-level supercoil structures consistent with various experimental data and further in elucidating the detailed underlying molecular mechanisms of the polymorphic supercoiling. We have found that the following three types of interactions are keys to understanding the supercoiling mechanism. "Permanent" interactions are always maintained between subunits in the various supercoil structures. "Sliding" interactions are formed between variable hydrophilic or hydrophobic residue pairs, allowing intersubunit shear without large change in energy. The formation and breakage of "switch" interactions stabilize inter- and intrasubunit interactions, respectively. We conclude that polymorphic supercoiling is due to the energy frustration between them. The transition between supercoils is achieved by a "transform and relax" mechanism: the filament structure is geometrically transformed rapidly and then slowly relaxes to energetically metastable states by rearranging interactions.

Computer Simulation↗
PubMed1991

The bed-wetting child. Current management of a frustrating problem.

Bed-wetting is a frustrating problem experienced by a significant number of children. It is the role of the physician to exclude serious underlying problems and, at the same time, educate the family concerning treatment options that may be best suited to their child. Once parents and child have a better understanding of the problem and realize that the outlook for nighttime bladder control is excellent, it is easier to start long-term care.

Child, Preschool↗
PubMed1987

Cytosolic free calcium increases before and oscillates during frustrated phagocytosis in macrophages.

When macrophages and neutrophils are allowed to settle onto an appropriate surface, they attach and spread in a frustrated attempt to phagocytose the substrate. Spreading is associated with extensive rearrangements of the actin cytoskeleton which resemble those occurring during phagocytosis. We have previously shown that spreading in human neutrophils is preceded by an increase in cytosolic-free calcium concentration [( Ca2+]i) (Kruskal, B. A., S. Shak, and F. R. Maxfield. 1986. Proc. Natl. Acad. Sci. USA. 83:2919-2923). To assess the generality of this signal, we measured [Ca2+]i in single thioglycollate-elicited mouse peritoneal macrophages as they spread on an immune complex-coated surface, using fura-2 microspectrofluorometry. A [Ca2+]i increase always precedes spreading. This increase can involve several (up to 8) [Ca2+]i spikes, with an average peak value of 387 +/- 227 nM (mean +/- SD, n = 92 peaks in 24 cells), before spreading is detected. Neither spreading nor the magnitude of these spikes is significantly altered by removal of extracellular calcium. Many of the spreading macrophages exhibit periodic [Ca2+]i increases before and during spreading. The proportion which does so varies among experiments from 0 to 90%, but it is frequently greater than 40%. The largest number of cells (approximately 25%) exhibited only a single peak. In 13 cells that showed more than 10 peaks, the median period was 29 s (range 19-69 s). The average peak [Ca2+]i was 385 +/- 266 nM (mean +/- SD, n = 208 peaks in 14 cells). The calcium producing these increases is derived from intracellular pools. The oscillations occur with spreading on either opsonized or nonopsonized surfaces. The function of these oscillations is not clear, but the large number of cells which exhibit them suggest that they may be important to macrophage function.

Animals↗
PubMed1995

Continual assembly of half-desmosomal structures in the absence of cell contacts and their frustrated endocytosis: a coordinated Sisyphus cycle.

It is widely assumed that the coordinate assembly of desmosomal cadherins and plaque proteins into desmosome-typical plaque-coated membrane domains, capable of anchoring intermediate-sized filaments (IF), requires cell-to-cell contacts and a critical extracellular Ca2+ concentration. To test this hypothesis we studied several cell lines grown for years in media with less than 0.1 mM Ca2+ to steady-state low Ca2+ medium (LCM) conditions, particularly the human keratinocyte line HaCaT devoid of any junctional cell contact (HaCaT-L cells). Using immunolocalization and vesicle fractionation techniques, we found that the transmembrane glycoprotein, desmoglein (Dsg), colocalized with the plaque proteins, desmoplakin and plakoglobin. The sites of coassembly of desmosomal molecules in HaCaT-L cells as well as in HaCaT cells directly brought into LCM were identified as asymmetric plaque-coated plasma membrane domains (half-desmosomes) or as special plaque-associated cytoplasmic vesicles, most of which had formed endocytotically. The surface exposure of Dsg in these half-desmosomes was demonstrated by the binding, in vivo, of antibodies specific for an extracellular Dsg segment which also could cross-bridge them into symmetric quasi-desmosomes. Otherwise, these half-desmosomes were shown in LCM to be taken up endocytotically. Half-desmosomal assemblies were also seen in uncoupled cells in normal Ca2+ medium. We conclude that, in the absence of intercellular contacts, assembly of desmosomal proteins at the cell surface takes place, resulting in transient half-desmosomes which then, in LCM and without a stable partner connection to the adjacent cell, can be endocytotically resumed. This frustrated cycle of synthesis and assembly maintains an ensemble of molecules characteristic of epithelial differentiation and the potential to form desmosomes, even when the final junctional structure cannot be formed. We propose that these half-desmosomal structures are general cell structures of epithelial and other desmosome-forming cells.

Calcium↗
PubMed1985

Effects of nutritional supplementation on children's responses to novel, frustrating, and competitive situations.

The effects of caloric supplementation for pregnant women and children from an endemically malnourished population on children's school age behavior were examined. The child's abilities to respond to routinely stressful problem-solving situations and to interact appropriately with peers were the dependent variables. Seventy-eight boys and 60 girls in rural Guatemala received different amounts of food supplementation, pre- and postnatally, as part of the INCAP Longitudinal Study. Children were classified as High or Low Supplementation, based on supplemental caloric intake prenatally and from birth to four years. When measured at 6 to 8 yr, High Supplementation children showed more interest in and exploration of a novel environment, more involvement in a competitive game, greater persistence on a frustrating task, better motor impulse control, and greater initiative across several group tasks than did Low Supplementation children. In free play with peers, the High Supplementation children showed more frequent happy affect, social involvement, and moderate activity level, and were less frequently timid or anxious. Results are consistent with findings from studies on the effects of early undernutrition on infant behavior, and suggest that behavior impairments associated with early undernutrition have long-term consequences for the child's development.

Body Height↗
PubMed1994

"Coping with frustration" classes: development and preliminary outcomes with women who care for relatives with dementia.

We describe a novel intervention program in which wives or daughters who are caregivers are taught several specific skills to enable them to manage their frustrations more effectively. These skills include learning to relax in very stressful situations and learning to be appropriately assertive with their frail elder and with other family members. Class content and structure are presented, along with preliminary data on class efficacy that is based on an N of 48 caregivers who have currently completed this particular class series. We close by noting that this psychoeducational intervention has been very well received by colleagues in other agencies serving family caregivers. In fact, this class series is now being offered to a broad range of caregivers in the San Francisco Bay area.

Adaptation, Psychological↗
PubMed1999

Off-equilibrium dynamics of the frustrated Ising lattice gas.

We study by means of Monte Carlo simulations the off-equilibrium properties of a model glass, the frustrated Ising lattice gas in three dimensions. We have computed typical two times quantities, such as density-density autocorrelations and the autocorrelation of internal degrees of freedom. We find an aging scenario particularly interesting in the case of the density autocorrelations in real space that is very reminiscent of spin glass phenomenology. While this model captures the essential features of structural glass dynamics, its analogy with spin glasses may make possible its complete description using the tools developed in spin glass theory.

Journal Article↗