Experimental and four‐component relativistic DFT studies of tungsten carbonyl complexes

We present a theoretical and experimental study of the structure and nuclear magnetic resonance (NMR) parameters of the pentacarbonyltungsten complexes of η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine, η²‐norbornene, and imidazolidine‐2‐thione. The three complexes have a pseudo‐octahedral molecular structure with the six ligands bonded to the tungsten atom. The η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine‐pentacarbonyl tungsten complex was synthesized for the first time. For all compounds, we present four‐component relativistic calculations of the NMR parameters at the Dirac–Kohn–Sham density functional level of theory using hybrid functionals. These large‐scale relativistic calculations of NMR chemical shifts and spin–spin coupling constants were compared with available experimental data, either taken from the literature or measured in this work. The inclusion of solvent effects modeled using a conductor‐like screening model was found to improve agreement between the calculated and experimental NMR parameters, and our best estimates for the NMR parameters are generally in good agreement with available experimental results. The present work demonstrates that four‐component relativistic theory has reached a level of maturity that makes it a convenient and accurate tool for modeling and understanding chemical shifts and indirect spin–spin coupling constants of organometallic compounds containing heavy elements, for which conventional non‐relativistic theory breaks down. Copyright © 2015 John Wiley & Sons, Ltd.     

Sedimentation rate of erythrocyte from physics prospective

The European Physical Journal E - A two-dimensional multiphase cellular automaton (CA) model is proposed for the prediction of growth kinetics and microstructural evolution during peritectic...     

Irreversible Brownian Heat Engine

We model a Brownian heat engine as a Brownian particle that hops in a periodic ratchet potential where the ratchet potential is coupled with a linearly decreasing background temperature. We show that the efficiency of such Brownian heat engine approaches the efficiency of endoreversible engine \(\eta =1-\sqrt{{T_{c}/T_{h}}}\) [23]. On the other hand, the maximum power efficiency of the engine approaches \(\eta ^{MAX}=1-({T_{c}/T_{h}})^{1\over 4}\). It is shown that the optimized efficiency always lies between the efficiency at quasistatic limit and the efficiency at maximum power while the efficiency at maximum power is always less than the optimized efficiency since the fast motion of the particle comes at the expense of the energy cost. If the heat exchange at the boundary of the heat baths is included, we show that such a Brownian heat engine has a higher performance when acting as a refrigerator than when operating as a device subjected to a piecewise constant temperature. The role of time on the performance of the motor is also explored via numerical simulations. Our numerical results depict that the time t and the external load dictate the direction of the particle velocity. Moreover, the performance of the heat engine improves with time. At large t (steady state), the velocity, the efficiency and the coefficient of performance of the refrigerator attain their maximum value. Furthermore, we study the effect of temperature by considering a viscous friction that decreases exponentially as the background temperature increases. Our result depicts that the Brownian particle exhibits a fast unidirectional motion when the viscous friction is temperature dependent than that of constant viscous friction. Moreover, the efficiency of this motor is considerably enhanced when the viscous friction is temperature dependent. On the hand, the motor exhibits a higher performance of the refrigerator when the viscous friction is taken to be constant.     

Extraction and characterization of cellulose single fiber from native Ethiopian Serte (Dracaena steudneri Egler) plant leaf

Biomaterials are renewable sources which are widely distributed, locally accessible, high possibility of recycling and biodegradation behavior. This investigation deals with the extraction and characterizing of new fiber obtained from Serte plant leaf that is found in Ethiopia. The physical, chemical and mechanical characters of the fiber had been tested and comparison with other plant fibers was done for the first time. Optimization of fiber extraction process has been done by varying the extraction variables like NaOH concentration, temperature and extraction time. Fibers possessing 56% cellulose content along with significant amount of hemicellulose, lignin and ash with tensile strength of 330?MPa were obtained. FTIR as well as X-ray diffraction analysis were also done to further analyze the fiber. This new plant leaf fiber can be another alternative resource in place of synthetic fibers depending on their application such as reinforcing polymer matrices.     

Structure and NMR spectra of cyclophanes with unsaturated bridges (cyclophenes)

The calculated structures of several known and hypothetical cyclophanes with ethylene bridges (cyclophenes) are reported together with experimental and calculated values of their NMR parameters. Of the exchange‐correlation functionals and basis sets used in this work, only the ωB97X‐D/6‐311++G(2d,2p) and ωB97X‐D/cc‐pVQZ yielded values of the C_sp3–C_sp3 bond length close to the experimental data, although significant differences still remain. As far as the NMR parameters are concerned, except for close‐lying signals, chemical shifts and coupling constants calculated at the ωB97X‐D/cc‐pVQZ level reproduce in most cases the experimental trends. Contrary to the calculations of geometries, an agreement between the values of the NMR parameters obtained at ωB97X‐D/cc‐pVQZ level and the experimental ones is the poorest compared with that of the ωB97X‐D/6‐311++G(2d,2p) one. Taking into account that the results of the different calculations show the same qualitative trends in most cases, we believe that they correctly describe the structure and properties of the hypothetical molecules studied here. Copyright © 2012 John Wiley & Sons, Ltd.     

Mercury Methylation by Cobalt Corrinoids: Relativistic Effects Dictate the Reaction Mechanism

The methylation of Hg^II(SCH₃)₂ by corrinoid‐based methyl donors proceeds in a concerted manner through a single transition state by transfer of a methyl radical, in contrast to previously proposed reaction mechanisms. This reaction mechanism is a consequence of relativistic effects that lower the energies of the mercury 6p_1/2 and 6p_3/2 orbitals, making them energetically accessible for chemical bonding. In the absence of spin–orbit coupling, the predicted reaction mechanism is qualitatively different. This is the first example of relativity being decisive for the nature of an observed enzymatic reaction mechanism.     

Gold nanoparticle fluorescent molecular beacon for low-resolution DQ2 gene HLA typing

Coeliac disease is an inflammation of the small intestine triggered by gluten ingestion. We present a fluorescent genosensor, exploiting molecular-beacon-functionalized gold nanoparticles, for the identification of human leukocyte antigen (HLA) DQ2 gene, a key genetic factor in coeliac disease. Optimization of sensor performance was achieved by tuning the composition of the oligonucleotide monolayer immobilized on the gold nanoparticle and the molecular beacon design. Co-immobilization of the molecular beacon with a spacing oligonucleotide (thiolated ten-thymine oligonucleotide) in the presence of ten-adenine oligonucleotides resulted in a significant increase of the sensor response owing to improved spacing of the molecular beacons and extension of the distance from the nanoparticle surface, which renders them more available for recognition. Further increase in the response (approximately 40%) was shown to be achievable when the recognition sequence of the molecular beacon was incorporated in the stem. Improvement of the specificity of the molecular beacons was also achieved by the incorporation within their recognition sequence of a one-base mismatch. Finally, gold nanoparticles functionalized with two molecular beacons targeting the DQA1*05* and DQB1*02* alleles allowed the low-resolution typing of the DQ2 gene at the nanomolar level.     

Synergistic effect of ZnS outer layers and electrolyte methanol content on efficiency in TiO(2)/CdS/CdSe sensitized solar cells

The effect of methanol content in water based polysulfide electrolytes in TiO(2)/CdS/CdSe quantum dot sensitized solar cells (QDSSCs) prepared by the SILAR method was studied. In addition, the effect of coating the mesoporous QD sensitized films with ZnS outer layers was investigated. Charge recombination reactions were measured using time resolved spectroscopic measurements. These studies reveal a synergistically beneficial effect from using ZnS layers and methanol in the polysulfide electrolyte on the control of charge transfer processes within these devices and ultimately on overall cell performance.     

Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane

Extending our earlier findings for [3.3]paracyclophane, NMR line shape studies of the conformational dynamics in [3.2] and [4.3]paracyclophanes are reported, of which the former is conformationally homogeneous and the latter occurs in two enantiomeric forms. For [3.2]paracyclophane, the Arrhenius activation energy E_a = 11.6 ± 0.1 kcal/mol and preexponential factor log (A/s^?1) = 12.92 ± 0.07 were found. In [4.3]paracyclophane, the conformational dynamics are quite complicated because, apart from interconversions of each enantiomer into itself proceeding via inversion of the propano bridge with rate constant k₁, the enantiomers mutually rearrange with rate constant k₂ due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic ¹H spectra of the aromatic protons for these two conformational processes (E_a = 11.2 ± 0.5 kcal/mol and log (A/s^?1) = 13.6 ± 0.5 for the former, and E_a = 9.7 ± 0.4 kcal/mol and log (A/s^?1) = 13.2 ± 0.4 for the latter) is the highlight of this work. In the investigated temperature range, in [4.3]paracyclophane, the occurrence of other conformational processes beyond those mentioned above can be excluded, because they would produce different line shape patterns than those actually observed. Copyright © 2013 John Wiley & Sons, Ltd.