Magnetic resonance imaging of velocity fields, the void fraction and gas dynamics in a cavitating liquid

In acoustic cavitation, the relationship between the bubble dynamics on the microscale and the flow properties on the macroscale is critical in determining sonochemical reaction kinetics. A new technique was developed to measure the void fraction and estimate water mobility in the vicinity of cavitating bubbles using phase-encoded magnetic resonance imaging with short characteristic measurement timescales (0.1–1 ms). The exponential behavior of the NMR signal decay indicated the fast diffusion regime, with the relationship between local mechanical dispersion D _mix and the average bubble radius R, D_mix >> \frac2R²10^-4s, D_{\rm mix}\gg \frac{2R^2}{10^{-4}\hbox{s}}, resulting in dispersion of orders of magnitude greater than diffusion in quiescent water. For two different samples (water and a surfactant solution), the independent measurements of three-dimensional void fraction and velocity fields permitted the calculation of compressibility, divergence and vorticity of the cavitating medium. The measured dynamics of the dissolved gas, compared with that of the surrounding liquid, reflected the difference in the bubble coalescence and lifetimes and correlated with the macroscopic flow parameters.     

Mössbauer and XRD study of hot dip galvanized alloy

Mössbauer spectroscopy has been used to investigate the nature of the Zinc-Iron alloys present within the Hot Dip Galvanized (HDG) layers of steel with a silicon content of 0.35 %. The investigation also studied the impact of the powder coating pretreatment on the nature of the alloy layers. The acid etching process within the pretreatment process in particular would be expected to have a significant impact on the HDG layer. This study utilized ⁵⁷Fe Mössbauer spectroscopy to examine identically processed samples prior to and post pre treatment. XRD and ⁵⁷Fe CEMS measurements were performed on hot galvanized S355J2 + N samples, forming sandwiched structure. Both XRD and CEMS reveal the presence of dominant steel phase in accordance with its estimated occurrence on the surface of the sandwiched samples. Minor Γ-Fe3Zn10, ζ-FeZn15 and solid solution Fe-Zn as well as minor Fe-Si phases could also be identified.     

Electrical and thermal analyses of solar PV module by considering realistic working conditions

Journal of Thermal Analysis and Calorimetry - Photovoltaic (PV) cells are used as clean energy technologies for generating electricity from solar irradiance. In designing and modeling of PV-based...     

Highly Concise Synthesis of 3'-"Up"-ethynyl-5'-methylbicyclo-[3.1.0]-hexyl Purine and Pyrimidine Nucleoside Derivatives Using Rhodium(II) Carbenoid Cycloaddition and Highly Diastereoselective Grignard Reaction

Synthesis of north‐5'‐methylbicyclo[3.1.0]hexyl purine and pyrimidine nucleosides with an ethynyl group at C‐3' position has been successfully accomplished by a facile method. Methylbicyclo[3.1.0]hexanone (±)‐ 5 having three contiguous chiral centers was remarkably simply constructed only by four steps containing a carbenoid insertion reaction in the presence of rhodium(II) acetate dimer and CuSO₄, giving a correct relative stereochemistry of the generated three chiral centers. Upon Grignard reaction of (±)‐ 5 with ethynylmagnesium bromide, exclusive diastereoselectivity was observed. Condensation of glycosyl donor (±)‐ 9 with purine nucleobase afforded only the desired N⁹‐alkylated nucleoside, while condensation with pyrimidine, N³‐benzoylated uracil gave the desired N¹‐alkylated nucleoside (±)‐ 13 with the undesired O²‐alkylated nucleoside (±)‐ 14 . Probably, (±)‐ 14 would be formed due to steric hindrance caused upon approaching for N¹‐alkylation.     

Linear and non-linear vibration and frequency response analyses of microcantilevers subjected to tip-sample interaction

Despite their simple structure and design, microcantilevers are receiving increased attention due to their unique sensing and actuation features in many MEMS and NEMS. Along this line, a non-linear distributed-parameters modeling of a microcantilever beam under the influence of a nanoparticle sample is studied in this paper. A long-range Van der Waals force model is utilized to describe the microcantilever-particle interaction along with an inextensibility condition for the microcantilever in order to derive the equations of motion in terms of only one generalized coordinate. Both of these considerations impose strong nonlinearities on the resultant integro-partial equations of motion. In order to provide an understanding of non-linear characteristics of combined microcantilever-particle system, a geometrical function is wisely chosen in such a way that natural frequency of the linear model exactly equates with that of non-linear model. It is shown that both approaches are reasonably comparable for the system considered here. Linear and non-linear equations of motion are then investigated extensively in both frequency and time domains. The simulation results demonstrate that the particle attraction region can be obtained through studying natural frequency of the system consisting of microcantilever and particle. The frequency analysis also proves that the influence of nonlinearities is amplified inside the particle attraction region through bending or shifting the frequency response curves. This is accompanied by sudden changes in the vibration amplitude estimated very closely by the non-linear model, while it cannot be predicted by the best linear model at all.     

Asymmetric synthesis of novel thioiso dideoxynucleosides with exocyclic methylene as potential antiviral agents

Novel thioiso pyrimidine and purine nucleosides substituted with exocyclic methylene have been synthesized, starting from D-xylose. The glycosyl donor 14 was synthesized from D-xylose, using cyclization of dimesylate 10 with sodium sulfide as a key step. Cyclization proceeded in pure S(N)2 reaction without going through S(N)1 reaction in the presence of an allylic functional group at low reaction temperature (0 degrees C) in polar solvent (DMF), affording compound 12 as a major product. At higher temperatures, S(N)2' product 11 was almost exclusively obtained as a major product. On the other hand, glycosylation of 14 with 6-chloropurine under Mitsunobu conditions afforded the desired S(N)2 product 26, while palladium-catalyzed glycosylation resulted in the sole formation of S(N)2' product 34.     

Stereoselective synthesis of a novel apio analogue of neplanocin A as potential S-adenosylhomocysteine hydrolase inhibitor

A total synthesis of apio-neplanocin A, which combines properties of apio nucleoside and neplanocin A and is a potential inhibitor of S-adenosylhomocysteine hydrolase, was accomplished starting from D-ribose via stereoselective hydroxymethylation and RCM reaction. [reaction: see text]     

Stereoselective synthesis of 2'-C-methyl-cyclopropyl-fused carbanucleosides as potential anti-HCV agents

Stereoselective synthesis of 2'-C-methyl-cyclopropyl-fused carbanucleosides was accomplished via stereoselective cyclopropanation, regioselective cleavage of the isopropylidene group, stereoselective Grignard reaction, and cyclic sulfate chemistry. [reaction: see text]     

Stereoselective synthesis and conformational study of novel 2',3'-Didehydro-2',3'-dideoxy-4'-selenonucleosides

Stereoselective synthesis of novel 2',3'-didehydro-2',3'-dideoxy-4'-selenonucleosides (4'-seleno-d4Ns) 4a- c was accomplished via 4'-selenoribofuranosyl pyrimidines 11a- c, as key intermediates. 4'-Selenoribofuranosyl pyrimidines 11a- c were efficiently synthesized from d-ribose or d-gulonic gamma-lactone using a Pummerer-type condensation as a key step. Introduction of 2',3'-double bond was achieved by treating cyclic 2',3'-thiocarbonate with 1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine.     

The response of selected isomers of B80 buckyball toward NH3 adsorption: a density functional theory investigation

Density functional theory calculations at the B3LYP/6-31G(d) and B3LYP/6-31+G(d) levels were carried out for the adsorption of NH₃ on three symmetric isomers of B₈₀ {C ₁, T _h, I _h}. To investigate the binding features of B₈₀ isomers with NH₃, different studies including the structural and electronic parameters, the ¹⁴N electric field gradient tensors and the atoms in molecules (AIM) properties were considered. The calculated parameters by these investigations can be used as powerful tools to find out some of the unknown aspects of electronic structures of the boron buckyball and its isomers. According to previous studies, boron buckyball as an amphoteric and a hard molecule has two distinct reactive sites defined as cap and frame which act as an acid and a base, respectively. Regarding the obtained results in this study, all the isomers had the same exposure when NH₃ molecule reacted with the external wall of B₈₀. For instance, the stability of N–B bond in the cap site was significantly more than the stability of N–B bond in the frame. Moreover, the adsorption of NH₃ on frame site showed a considerable reduction in HOMO–LUMO energy gap. According to AIM theory, an electrostatic nature was observed for N–B interaction. Concerning the selected isomers of buckyball, the capability of the NH₃–B₈₀ complexes to localize electron at the N–B bond critical points depend on the reaction sites significantly. In general, ¹⁴N nuclear quadruple coupling constants and asymmetry parameter reveal a remarkable effect of NH₃ adsorption on electronic structure of the B₈₀.