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Model Study (model + study)
Selected AbstractsThe Role of Axial Ligation in Nitrate Reductase: A Model Study by DFT Calculations on the Mechanism of Nitrate ReductionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2008Kuntal Pal Abstract The reactivity differences of the model anionic complexes [Mo(mnt)2(X)(PPh3)], [mnt2, = 1,2-dicyanoethylenedithiolate; X = SPh (1a), SEt (1b), Cl (1c), Br (1b)] towards oxygen atom transfer from nitrate, which is a key step performed by nitrate reductase, has been investigated by density functional theory calculations. Unlike complexes 1a and 1b, complexes 1c and 1d do not react with nitrate. Thermodynamically, all these complexes have a similar ability to generate the pentacoordinate active state [Mo(mnt)2(X)], by dissociation of PPh3, although the inaccessibility of the dxy orbital in 1c,d and the instability of the corresponding nitrate-bound enzyme substrate (ES) type complex contributes to their failure to reduce nitrate. The nature of the ES complex for 1a,b is described. The variation in the experimental data due to the change of axial ligation from SPh to SEt on the catalytic pathway has also been addressed. The gas-phase and solvent-corrected potential energy surface for the reaction of 1a,b with nitrate are established with fully optimized minima and transition states.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Stress Development Due to Capillary Condensation in Powder Compacts: A Two-Dimensional Model StudyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2000Stefan Lampenscherf A model experiment is presented to investigate the relationship between the humidity-dependent liquid distribution and the macroscopic stress in a partially wet powder compact. Therefore, films of monosized spherical particles were cast on silicon substrates. Using environmental SEM the geometry of the liquid necks trapped between particles was imaged as a function of relative humidity. Simultaneously the macroscopic stress in the substrate adhered particle film was measured by capacitive deflection measurement. The experimentally found humidity dependence of the liquid neck size and the macroscopic film stress are compared with model predictions. The circle,circle approximation is used to predict the size of the liquid necks between touching particles as a function of the capillary pressure. Using the modified Kelvin relation between capillary pressure and relative humidity, we consider the effect of an additional solute which may be present in the capillary liquid. The results of the stress measurement are compared with the model predictions for a film of touching particles in hexagonal symmetry. The contribution of the capillary interaction to the adhesion force between neighboring particles is calculated using the integrated Laplace equation. The resulting film stress can be approximated relating this capillary force to an effective cross section per particle. The experimentally found humidity dependence of the liquid neck size is in good agreement with the model predictions for finite solute concentration. The film stress corresponds to the model predictions only for large relative humidities and shows an unexpected increase at small values. As is shown with an atomic force microscope, the real structure of the particle,particle contact area changes during the wet/dry cycle. A solution/reprecipitation process causes surface heterogeneities and solid bridging between the particles. It is claimed that the existence of a finite contact zone between the particles gives rise to the unexpected increase of the stress at small relative humidities. [source] Ab Initio Quantum Chemical Investigation of the First Steps of the Photocycle of Phototropin: A Model Study,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2003Christian Neiß ABSTRACT Phototropin is a blue light,activated photoreceptor that plays a dominant role in the phototropism of plants. The protein contains two subunits that bind flavin mononucleotide (FMN), which are responsible for the initial steps of the light-induced reaction. It has been proposed that the photoexcited flavin molecule adds a cysteine residue of the protein backbone, thus activating autophosphorylation of the enzyme. In this study, the electronic properties of several FMN-related compounds in different charge and spin states are characterized by means of ab initio quantum mechanical calculations. The model compounds serve as idealized model chromophores for phototropism. Reaction energies are estimated for simple model reactions, roughly representing the addition of a cysteine residue to the flavin molecule. Excitation energies were calculated with the help of time-dependent density functional theory. On the basis of these calculations we propose the following mechanism for the addition reaction: (1) after photoexcitation of FMN out of the singlet ground state S0, excited singlet state(s) are populated; these relax to the lowest excited singlet state S1, and subsequently by intersystem crossing FMN in the lowest triplet state, T1 is formed; (2) the triplet easily removes the neutral hydrogen atom from the H,S group of the cysteine residue; and (3) the resulting thio radical is added. [source] Model study of time-dependent muscle response to pulsed electrical stimulationBIOELECTROMAGNETICS, Issue 5 2010Ravindra P. Joshi Abstract A systems-level model analysis of neuromuscular response to external electrical stimulation is presented. Action potential (AP) generation, dynamics of voltage-based calcium release at the motor endplates controlled by the arrival of APs, and muscle force production are all comprehensively included. Numerical predictions exhibit trends that are qualitatively similar to measurements of muscle response in rats from a burst of cortical stimulation and a nanosecond impulse. Modulation of neural membrane conductances (including possible electroporation) that alters the neural impulse generation frequency is hypothesized as a possible mechanism leading to observed changes in muscle force production. Other possibilities such as calcium release at nerve end endings also exist. It is also proposed that multipulsing strategies and changing the electric field direction by using multielectrode systems would be useful. Bioelectromagnetics 31:361,370, 2010. © 2010 Wiley-Liss, Inc. [source] Synthesis and Properties of Fluorescent cycloSal Nucleotides Based on the Pyrimidine Nucleoside m5K and Its 2,,3,-Dideoxy Analog dm5KEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2006Henning J. Jessen Abstract The synthesis of the fluorescent thymidine nucleoside analog 5-methyl-pyrimidin-2-one nucleoside 1 (m5K) and that of its 2,,3,-dideoxy derivative 2 (dm5K) are described. Moreover, the conversion of 1 and 2 into the corresponding 3-methyl- cycloSal-phosphate triesters 11 and 12 or an enzyme-cleavable prodrug 5-acetoxymethylpropionate- cycloSal-phosphate triester 13, and the fluorescence and hydrolysis properties of these new lipophilic triesters are reported. Finally, the suitability of these cycloSal pronucleotides as probes is demonstrated in a cell-extract hydrolysis experiment and a model study for cell uptake. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Converting Core Compounds into Building Blocks: The Concept of Regiochemically Exhaustive FunctionalizationEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2005Elena Marzi Abstract In a model study, 3-fluorophenol and 3-fluoropyridine were converted into the each time four possible carboxylic acids by passing through the corresponding organometallic intermediates. As an attempt to generalize the findings reveals, a restricted set of principles and methods suffices to cope with all standard scenarios. The most valuable and versatile tools for the regiochemically exhaustive functionalization of a great variety of substrate patterns are the optionally site-selective metalation (either by reagent/substrate matching or by peripheral coordination control), the use of activating or congesting protective groups and the basicity gradient-driven heavy halogen migration (followed by halogen/metal permutation). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Seismic reflection coefficients of faults at low frequencies: a model studyGEOPHYSICAL PROSPECTING, Issue 3 2008Joost Van Der Neut ABSTRACT We use linear slip theory to evaluate seismic reflections at non-welded interfaces, such as faults or fractures, sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces, whereas the imaginary parts can be related directly to the interface compliances. The imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances, which can be related to fault properties upon using a fault model. At normal incidence the expressions uncouple and the complex-valued P-wave reflection coefficient can be related linearly to the normal compliance. As the normal compliance is highly sensitive to the infill of the interface, it can be used for gas/fluid identification in the fault plane. Alternatively, the tangential compliance of a fault can be estimated from the complex-valued S-wave reflection coefficient. The tangential compliance can provide information on the crack density in a fault zone. Coupling compliances can be identified and quantified by the observation of PS conversion at normal incidence, with a comparable linear relationship. [source] Carbon emission and sequestration by agricultural land use: a model study for EuropeGLOBAL CHANGE BIOLOGY, Issue 6 2002L. M. Vleeshouwers Abstract A model was developed to calculate carbon fluxes from agricultural soils. The model includes the effects of crop (species, yield and rotation), climate (temperature, rainfall and evapotranspiration) and soil (carbon content and water retention capacity) on the carbon budget of agricultural land. The changes in quality of crop residues and organic material as a result of changes in CO2 concentration and changed management were not considered in this model. The model was parameterized for several arable crops and grassland. Data from agricultural, meteorological, soil, and land use databases were input to the model, and the model was used to evaluate the effects of different carbon dioxide mitigation measures on soil organic carbon in agricultural areas in Europe. Average carbon fluxes under the business as usual scenario in the 2008,2012 commitment period were estimated at 0.52 tC ha,1 y,1 in grassland and ,0.84 tC ha,1 y,1 in arable land. Conversion of arable land to grassland yielded a flux of 1.44 tC ha,1 y,1. Farm management related activities aiming at carbon sequestration ranged from 0.15 tC ha,1 y,1 for the incorporating of straw to 1.50 tC ha,1 y,1 for the application of farmyard manure. Reduced tillage yields a positive flux of 0.25 tC ha,1 y,1. The indirect effect associated with climate was an order of magnitude lower. A temperature rise of 1 °C resulted in a ,0.05 tC ha,1 y,1 change whereas the rising CO2 concentrations gave a 0.01 tC ha,1 y,1 change. Estimates are rendered on a 0.5 × 0.5° grid for the commitment period 2008,2012. The study reveals considerable regional differences in the effectiveness of carbon dioxide abatement measures, resulting from the interaction between crop, soil and climate. Besides, there are substantial differences between the spatial patterns of carbon fluxes that result from different measures. [source] Geomorphic characteristics of the Minjiang drainage basin (eastern Tibetan Plateau) and its tectonic implications: New insights from a digital elevation model studyISLAND ARC, Issue 2 2006Hui-Ping Zhang Abstract The Minshan Mountain and adjacent region are the major continental escarpments along the eastern Tibetan Plateau. The Minjiang drainage basin is located within the plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digital elevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), we know that the Minjiang drainage basin has distinct geomorphic characteristics. The regular increasing of local topographic relief from north to south is a result of the Quaternary sediment deposition within the plateau and the holistic uplift of the eastern margin of the Tibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drainage sub-basins and channel profiles show that the tributaries on the opposite sides are asymmetric. Lower perimeter and area of drainage sub-basins, total channel length and bifurcation ratio within eastern flank along the Minjiang mainstream are the result of the Quaternary differential uplift of the Minshan Mountain region. Shorter stream lengths and lower bifurcation ratio might be the indications of the undergrowth and newborn features of these eastern streams, which are also representative for the eastern uplift of the Minshan Mountain. [source] Interaction of heparin with Ca2+: A model study with a synthetic heparin-like hexasaccharideISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2000Jesús Angulo The binding of Ca2+ to synthetic hexasaccharide 1, containing the structural motifs of the regular region of heparin, has been investigated using NMR spectroscopy and molecular modeling. The NMR data of the calcium salt of 1 indicate the existence of specific Ca2+ binding, and molecular modeling results predict three different types of binding sites with different negative potential and preorganized geometry. The presence of Ca2+ does not seem to affect the overall helical structure of hexasaccharide 1, although it seems to have a marked influence on the flexibility of the oligosaccharide backbone. [source] Antiplaque and antigingivitis effectiveness of a hexetidine mouthwashJOURNAL OF CLINICAL PERIODONTOLOGY, Issue 7 2003N. C. Sharma Abstract Objective: To assess the antiplaque/antigingivitis efficacy of a hexetidine-containing mouthwash. Methods: This examiner-blind, parallel group, controlled clinical study examined the effectiveness of a hexetidine (0.1%) mouthwash both in inhibiting the development of supragingival plaque and in reducing gingivitis. One hundred and thirty-four adult subjects completed the 2-week experimental gingivitis model study. Following baseline examinations, which included plaque index, modified gingival index and gingival bleeding index, subjects received a full dental prophylaxis. Subjects were randomly assigned to one of three mouthwashes (hexetidine 0.1%, chlorhexidine 0.12% (positive control) or a 5% hydroalcohol negative control) and commenced three times daily supervised rinsing as their sole method of oral hygiene. All indices were rescored after 2 weeks. Results: Compared to the negative control group, the hexetidine group demonstrated a statistically significant inhibition and reduction of supragingival plaque and gingival inflammation with reductions of 6.3%, 33.5% and 56% for gingivitis, plaque and gingival bleeding, respectively. The results of the chlorhexidine group were used to validate the study. Conclusion: The study confirms the efficacy of a hexetidine rinse in reducing supragingival plaque and gingival inflammation. Zusammenfassung Zielsetzung: Untersuchung der Antiplaque- und Antigingivitiseffektivität einer Hexetidin-Mundspüllösung. Methoden: Diese kontrollierte klinische Studie mit verblindetem Untersucher im Parallelarm-Design untersuchte die Effektivität einer Hexitidin-Mundspüllösung (0,1%) sowohl für die Hemmung supragingivaler Plaquebildung als auch zur Reduktion der Gingivitis. 134 erwachsene Probanden beendeten die 2 Wochen dauernde Studie mit experimenteller Gingivitis. Nach der Erstuntersuchung, die die Erhebung des Plaque Index, des Modifizierten Gingival Index und des Gingivalen Blutungs Index umfasste, erhielten die Probanden eine professionelle Zahnreinigung. Den Probanden wurden randomisiert 3 Spüllösungen zugewiesen (Hexitidin 0,1%, Chlorhexidin 0,12% [positive Kontrolle] oder ein 5%iger Hydroalkohol [negative Kontrolle]) und begannen damit als alleinige Mundhygienemaßnahme 3 mal täglich unter Aufsicht zu spülen. Nach 2 Wochen wurden die Indizes erneut erhoben. Ergebnisse: Im Vergleich zur negativen Kontrolle zeigte die Hexitidin-Gruppe eine statistisch signifikante Hemmung und Reduktion der supragingivalen Plaque und gingivalen Entzündung mit Reduktionen von 6,3%, 33,5% bzw. 56% für Gingivitis, Plaque bzw. gingivale Blutung. Die Ergebnisse der Chlorhexidin-Gruppe dienten zur Validierung der Studie. Schlussfolgerung: Diese Studie bestätigt die Wirksamkeit von Hexitidin zur Reduktion supragingivaler Plaque und gingivaler Entzündung. Résumé Cette étude clinique contrôlée par groupe parallèle avec examinateur aveugle a estimé l'efficacité d'un bain de bouche à 0,1% d'héxatidine tant à inhiber le développement de la plaque sus-gingivale qu'à réduire la gingivite. Cent trente-quatre adultes ont achevé un gingivite expérimentale de deux semaines. A la suite de l'examen de base comprenant l'indice de plaque, l'indice gingival modifié et l'indice de saignement gingival, les sujets ont reçu une prophylaxie dentaire complète. Ils ont ensuite été répartis de manière randomisée pour utiliser un des trois bains de bouche suivants : héxatidine 0,1%, chlorhexidine 0,12% (contrôle positif) ou l'hydroalcool 5% (contrôle négatif), et ont commencé a effectuer un rinçage supervisé trois fois par jour comme unique méthode d'hygiène buccale. Tous les indices ont été relevés après deux semaines. Comparé au groupe négatif le groupe héxatidine montrait une inhibition et une réduction significatives de la plaque sus-gingivale et de l'inflammation gingivale avec des réductions respectives de 6,3, 33,5 et 56% pour la gingivite, la plaque dentaire et le saignement gingival. Les résultats du groupe chlorhexidine ont été utilisés pour valider cette étude. Celle-ci confirme l'efficacité de l'héxatidine à réduire la plaque dentaire sus-gingivale et l'inflammation gingivale. [source] In vitro differences of stress concentrations for internal and external hex implant,abutment connections: a short communicationJOURNAL OF ORAL REHABILITATION, Issue 1 2006Y. MAEDA summary, The aim of this study was to clarify the difference in the stress distribution patterns between implants with external-hex or internal-hex connection systems using in vitro models. Three 13 mm fixtures with external-hex and internal-hex connections were installed into an acrylic bone analogue. One piece abutments of 7 mm height was connected. Strain gauges were attached to the abutment surface, and the cervical and fixture tip areas of the bone analogue surface. Vertical and horizontal load applied was 30 N. Data were normalized for each model by obtaining values relative to the sum of the three values. Almost the same force distribution pattern was found under vertical load in both systems. Fixtures with external-hex showed an increase in strain at the cervical area under horizontal load, while in internal-hex fixtures the strain was at the fixture tip area. Within limitations of our model study, it was suggested that fixtures with internal-hex showed widely spread force distribution down to the fixture tip compared with external hex ones. [source] Effect of magnetic attachment with stress breaker on lateral stress to abutment tooth under overdenture,JOURNAL OF ORAL REHABILITATION, Issue 10 2004T. Gonda summary, Recently, a newly developed magnetic attachment with stress breaker was used in retentive components in overdentures. Excessive lateral stress has a more harmful effect on natural teeth than axial stress, and the magnetic attachment with stress breaker is expected to reduce lateral forces on abutment teeth and protect it teeth from excessive stress. However, the properties of this retainer have not yet been determined experimentally. This study compares the lateral forces on abutment teeth for three retainers under loading on the denture base in a model study. A mandibular simulation model is constructed to measure lateral stress. Three types of retentive devices are attached to the canine root. These devices include the conventional root coping, the conventional magnetic attachment and the new magnetic attachment with stress breaker. For each retentive device, load is generated on the occlusal table of the model overdenture, and the lateral stress on the canine root and the displacement of the overdenture measured. The magnetic attachment with stress breaker does not displace the denture and exhibits lower lateral stress in the canine root than conventional root coping and magnetic attachments. [source] Extracting charge density distributions from diffraction data: a model study on ureaACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2000R. Y. De Vries The quality of the extraction of electron density distributions by means of a multipole refinement method is investigated. Structure factors of the urea crystal have been obtained from an electron density distribution (EDD) resulting from a density function calculation with the CRYSTAL95 package. To account for the thermal motion of the atoms, the stockholder-partioned densities of the atoms have been convoluted with thermal smearing functions, which were obtained from a neutron diffraction experiment. A POP multipole refinement yielded a good fit, R = 0.6%. This disagreement factor is based on magnitudes only. Comparison with the original structure factors gave a disagreement of 0.8% owing to differences in magnitude and phase. The fitted EDD still showed all the characteristics of the interaction density. After random errors corresponding to the experimental situation were added to the structure factors, the refinement was repeated. The fit was R = 1.1%. This time the resulting interaction density was heavily deformed. Repetition with another set of random errors from the same distribution yielded a widely different interaction density distribution. The conclusion is that interaction densities cannot be obtained from X-ray diffraction data on non-centrosymmetric crystals. [source] Health monitoring of plants by their emitted volatiles: trichome damage and cell membrane damage are detectable at greenhouse scaleANNALS OF APPLIED BIOLOGY, Issue 3 2009R.M.C. Jansen Abstract Pathogen attack and herbivore infestation have a major impact on plant health. In a model study, these two plant health issues were simulated to study whether plant health can be monitored at greenhouse scale through the analysis of volatile organic compounds (VOCs) in greenhouse atmosphere. To simulate pathogen attack and herbivore infestation, we repeatedly stroked the stems of tomato plants (Lycopersicon esculentum) and repeatedly removed their side shoots. In addition, we studied the effect of fruit picking on the concentration of plant-emitted VOCs in greenhouse atmosphere. Analysis of air samples obtained before these treatments revealed up to 17 VOCs that are known to be released from tomato plants, of which the most dominant one was the monoterpene ,-phellandrene. When plants were 7 weeks old, the concentration of this VOC was approximately 0.06 ppbv before treatment. When plants were 12 weeks old, this concentration was raised to approximately 0.14 ppbv. Stroking of the stems, removing the side shoots and fruit picking resulted in an increase in the concentrations of all mono- and most sesquiterpenes up to 60-fold, which was expected because these VOCs are well-known constituents of trichomes. The treatments did not result in substantially increased concentrations of the stress-related compounds ,-copaene, methyl salicylate and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. In contrast to stroking and fruit picking, shoot removal resulted in the emission of the lipoxygenase-derived product (Z)-3-hexenol in greenhouse atmosphere expressing cell membrane degradation. The findings presented in this paper focus on the feasibility of monitoring plant health through the analysis of VOCs in greenhouse air, but findings might also be relevant for atmospheric chemistry. [source] Drainage patterns and tectonic forcing: a model study for the Swiss AlpsBASIN RESEARCH, Issue 2 2001A. Kühni ABSTRACT A linear surface process model is used to examine the effect of different patterns of rock uplift on the evolution of the drainage network of the Swiss Alps. An asymmetric pattern of tectonic forcing simulates a phase of rapid retrothrusting in the south of the Swiss Alps (,Lepontine'-type uplift). A domal pattern of tectonic forcing in the north of the model orogen simulates the phase of the formation of the ,Aar massif', an external basement uplift in the frontal part of the orogenic wedge (,Aar'-type uplift). Model runs using the ,Lepontine'-type uplift pattern result in a model mountain chain with a water divide in the zone of maximum uplift and orogen-normal rivers. Model runs examining the effect of ,Lepontine'-type uplift followed by ,Aar'-type uplift show that the initially formed orogen-normal river system and the water divide are both very stable and hardly affected by the additional uplift. This indifference to changes in tectonic forcing is mainly due to the requirement of a high model erosion capacity for the river systems in order to reproduce the exhumation data (high-grade rocks in the south of the Swiss Alps point to removal of a wedge-shaped nappe stack with a maximum thickness of about 25 km). The model behaviour is in agreement with the ancestral drainage pattern of the Alps in Oligocene and Miocene times and with the modern pattern observed in the Coast Range of British Columbia; in both cases river incision occurred across a zone of rapid uplift in the lower course of the rivers. The model behaviour does not, however, explain the modern drainage pattern in the Alps with its orogen-parallel rivers. When the model system is forced to develop two locally independent main water divides (simultaneous ,Lepontine'- and ,Aar'-type uplift), a zone of reduced erosional potential forms between the two divides. As a consequence, the divides approach each other and eventually merge. The new water divide remains fixed in space independent of the two persisting uplift maxima. The model results suggest that spatial and temporal changes in tectonic forcing alone cannot produce the change from the orogen-normal drainage pattern of the Swiss Alps in Oligocene,Miocene times to the orogen-parallel drainage observed in the Swiss Alps today. [source] Mechanism of Reppe's Nickel-Catalyzed Ethyne Tetramerization to Cyclooctatetraene: A DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2004Bernd F. Straub Dr. Abstract In this B3,LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C2H2)3 yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the NiC , bond. A highly strained monoethyne trans -nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis -structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(,2 -C2H2)(,2 -C8H8), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, CC bond formation steps at Ni2(,-cot) fragments have been predicted to feature prohibitively high overall reaction barriers. In dieser B3,LYP-Studie werden homoleptische Nickel(0)-Ethinkomplexe als Katalysatorruhezustand von Reppes Cyclooctatetraensynthese vorhergesagt. Kupplung zweier Ethinliganden in Ni(C2H2)3 ergibt Monoethin-Nickelacyclopentadien im geschwindigkeitsbestimmenden Schritt. Der Koordination von Ethin folgt die Insertion eines Ethinliganden in die NiC ,-Bindung. Ein hoch gespanntes Monoethin- trans -Nickelacycloheptatrien wird gebildet. Dieses trans -Intermediat ist nicht in der Lage, Benzol reduktiv zu eliminieren, ohne vorher in eine cis -Struktur zu isomerisieren. Stattdessen kollabiert es schnell zu Nickelacyclononatetraen. Koordination von Ethin induziert eine barrierefreie reduktive Eliminierung zum Cyclooctatetraen Komplex Ni(,2 -C2H2)(,2 -C8H8), gefolgt von einem einfach verlaufenden Ligandenaustausch. Andere Ethinkupplungspfade wurden als weniger begünstigt berechnet. Der COT-Ligand bindet sowohl für mononukleare als auch für dinukleare Spezies deutlich schwächer an Nickel(0) als Ethin. Aus diesem Grund werden für CC-Bindungsbildungsschritte am Ni2(,-cot)-Fragment prohibitiv hohe freie Aktivierungsenthalpien vorhergesagt. [source] Overcoming Kinetic Limitations of Electron Injection in the Dye Solar Cell via Coadsorption and FRETCHEMPHYSCHEM, Issue 5 2008Conrad Siegers Abstract A new, extremely simple concept for the use of energy transfer as a means to the enhancement of light absorption and current generation in the dye solar cell (DSC) is presented. This model study is based upon a carboxy-functionalized 4-aminonaphthalimide dye (carboxy-fluorol) as donor, and (NBu4)2[Ru(dcbpy)2(NCS)2] (N719) as acceptor chromophores. A set of three different devices is assembled containing either exclusively carboxy-fluorol or N719, or a mixture of both. This set of transparent devices is characterized via IV-measurements under AM1.5G and monochromatic illumination and their light-harvesting and external quantum efficiencies (LHE and EQE, respectively) are determined as well. It is shown that the device containing only the donor chromophore has a marginal power conversion efficiency, thus indicating that carboxy-fluorol is a poor sensitizer for the DSC. Cyclovoltametric measurements show that the poor sensitization ability arises from the kinetic inhibition of electron injection into the TiO2 conduction band. Comparing the spectral properties of the DSCs assembled presently, however, demonstrates that light absorbed by carboxy-fluorol is almost quantitatively contributing to the photocurrent if N719 is present as an additional sensitizer. In this case, N719 acts as a catalyst for the sensitization of TiO2 by carboxy-fluorol in addition to being a photosensitizer. Evaluation of the maximum output power under blue illumination shows that the introduction of an energy-donor moiety via coadsorption, leads to a significant increase in the monochromatic maximum output power. This result demonstrates that energy transfer between coadsorbed chromophores could be useful for the generation of current in dye-sensitized solar cells. [source] | |||||||||||||||||||