Experimental investigation on the effectiveness of MHTHS using different metal oxide-based nanofluids

Journal of Thermal Analysis and Calorimetry - In recent times, the mini heat sink is suggested for cooling the miniature components employed in several engineering applications. In the present...     

ANTHEM: anatomically tailored hexagonal MRI

PURPOSE: This study aimed to investigate the use of anatomically tailored hexagonal sampling for scan-time and error reduction in MRI. MATERIALS AND METHODS: Anatomically tailored hexagonal MRI (ANTHEM), a method that combines hexagonal sampling with specific symmetry in anatomical geometry, is proposed. By using hexagonal sampling, aliasing artifacts are moved to regions where, due to the nature of the anatomy, aliasing is inconsequential. This can be used to either reduce scan time while maintaining spatial resolution or reduce residual errors in speedup techniques like UNFOLD and k-t BLAST/SENSE, which undersample k-space and unwrap fold-over artifacts during reconstruction. Computer simulations as well as phantom and volunteer studies were used to validate the theory. A simplified reconstruction algorithm for hexagonally sampled and subsampled k-space data was also used. RESULTS: A reduction in sampling density of 13.4% and 25% in each hexagonally sampled dimension was achieved for spherical and conical geometries without aliasing or reduction in spatial resolution. Optimal subsampling schemes that can be utilized by UNFOLD and k-t BLAST/SENSE were derived using hexagonal subsampling, which resulted in maximal, isotropic dispersal of the aliases. In combination with UNFOLD, ANTHEM was shown to move residual aliasing artifacts to the corners of the field of view, yielding reduced artifacts in CINE reconstructions. CONCLUSIONS: ANTHEM was successful in reducing acquisition time in conventional MRI and in reducing errors in UNFOLD imaging.     

Experimental investigation on the heat transfer performance of MHTHS using ethylene glycol-based nanofluids

Journal of Thermal Analysis and Calorimetry - Energy storage plays an important role in improving the applicability of a wide range of energy systems. Buildings could be considered as a desirable...     

Detection and identification of nature of mutual synchronization for low- and high-frequency non-premixed syngas combustion dynamics

Nonlinear Dynamics - Non-premixed syngas combustion anchored by a bluff body is investigated for its multimodal operation of low- and high-frequency instability corresponding to different Reynolds...     

Oscillation of solutions of impulsive vector hyperbolic differential equations with delays

In this article, we investigate a class of impulsive vector hyperbolic differential equation with delays. Several sufficient conditions are established for H-oscillation of solutions of such systems subject to the Neumann boundary condition by employing certain second-order impulsive differential inequality, where H is a unit vector in ? ^M . The main results are illustrated by two examples.     

On the origin of regio- and stereoselectivity in singlet oxygen addition to enecarbamates

The reactions of excited state singlet molecular oxygen ((1)Δ(g),(1)O(2)) continue to witness interesting new developments. In the most recent manifestation, (1)O(2) is tamed to react with enecarbamates in a stereoselective manner, which is remarkable, in view of its inherently high reactivity (Acc. Chem. Res. 2008, 41, 387). Herein, we employed the CAS-MP2(8,7)/6-31G* as well as the CAS-MP2(10,8)/6-31G* computations to unravel the origin of (i) diastereoselectivities in dioxetane or hydroperoxide formation and (ii) regioselectivity leading to a [2 + 2] cycloadduct or an ene product when (1)O(2) reacts with an oxazolidinone tethered 2-phenyl-1-propenyl system. The computed Gibbs free energy profiles for E- and Z-isomers when (1)O(2) approaches through the hindered and nonhindered diastereotopic faces (by virtue of chiral oxazolidinone) of the enecarbamates exhibit distinct differences. In the case of E-isomer, the relative energies of the transition structures responsible for hydroperoxide (ene product) are lower than that for dioxetane formation. On the other hand, the ene pathway is predicted to involve higher barriers as compared to the corresponding dioxetane pathway for Z-isomer. The energy difference between the rate-determining diastereomeric transition structures involved in the most favored ene reaction for E-enecarbamate suggests high diastereoselectivity. In contrast, the corresponding energy difference for Z-enecarbamate in the ene pathway is found to be diminishingly close, implying low diastereoselectivity. However, the dioxetane formation from Z-enecarbamate is predicted to exhibit high diastereoselectivity. The application of activation strain model as well as the differences in stereoelectronic effects in the stereocontrolling transition structures is found to be effective toward rationalizing the origin of selectivities reported herein. These predictions are found to be in excellent agreement with the experimental observations.     

Mechanistic insights on platinum- and palladium-pincer catalyzed coupling and cyclopropanation reactions between olefins

The mechanism of M(II)-PNP-pincer catalyzed reaction between (i) ethene, (ii) trans-butene with 2-methylbut-2-ene, 2,3-dimethylbut-2-ene and tert-butylbutene is examined by using density functional theory methods (where M = Pt or Pd). All key intermediates and transition states involved in the reaction are precisely located on the respective potential energy surfaces using the popular DFT functionals such as mPW1K, M06-2X, and B3LYP in conjunction with the 6-31+G** basis set. The reaction between these olefins can lead to a linear coupling product or a substituted cyclopropane. The energetic comparison between coupling as well as cyclopropanation pathways involving four pairs of olefins for both platinum (1-4) and palladium (5-8) catalyzed reactions is performed. The key events in the lower energy pathway in the mechanistic course involves (i) a C-C bond formation between the metal bound olefin (ethene or trans-butene) and a free olefin, and (ii) two successive [1,2] hydrogen migrations in the ensuing carbocationic intermediates (1c-4c, and 1d-4d), toward the formation of the coupling product. The computed barriers for these steps in the reaction of metal bound ethene to free tert-butylbutene (or other butenes) are found to be much lower than the corresponding steps when trans-butene is bound to the metal pincer. The Gibbs free energy differences between the transition states leading to the coupling product (TS(d-e)) and that responsible for cyclopropanated product (TS(d-g)) are found to be diminishingly closer in the case of the platinum pincer as compared to that in the palladium system. The computed energetics indicate that the coupled product prefers to remain as a metal olefin complex, consistent with the earlier experimental reports.     

Mechanism and electronic effects in nitrogen ylide-promoted asymmetric aziridination reaction

The mechanism and stereoselectivity of the aziridination reaction between guanidinium ylide and a series of para-substituted benzaldehydes have been studied by using density functional theory methods. The mechanistic details and analyses of the key elementary steps involved in (a) the addition of nitrogen ylide to benzaldehydes and (b) subsequent fragmentation of the resulting oxaspirocyclic intermediate are presented. The relative energies of important transition states and intermediates are found to be useful toward rationalizing reported diastereoselective product formation. The relative energies of the key transition states could be rationalized on the basis of the differences in steric, electrostatic, and other stabilizing weak interactions. The deformation analysis of the transition state geometries exhibited good correlation with the predicted activation barriers. The changes in cis/trans diastereoselectivity preferences upon changes in the electron donating/withdrawing abilities of the para substituents on benzaldehyde are identified as arising due to vital differences in the preferred pathways. The large value of reaction constant (ρ > 4.8) estimated from the slope of good linear Hammett plots indicated high sensitivity to the electronic nature of substituents on benzaldehyde. The formation of trans-aziridine in the case of strong electron donating groups and cis-aziridines with weakly electron donating/withdrawing group has been explained by the likely changes in the mechanistic course of the reaction. In general, the predicted trends are found to be in good agreement with the earlier experimental reports.     

Oriented (Local) Electric Fields Drive the Millionfold Enhancement of the H-Abstraction Catalysis Observed for Synthetic Metalloenzyme Analogues

This contribution follows the recent remarkable catalysis observed by Groves et al. in hydrogen-abstraction reactions by a) an oxoferryl porphyrin radical-cation complex [Por^⋅+Fe^IV(O)L_ax] and b) a hydroxoiron porphyrazine ferric complex [PyPzFe^III(OH)L_ax], both of which involve positively charged substituents on the outer circumference of the respective macrocyclic ligands. These charge-coronated complexes are analogues of the biologically important Compound I (Cpd I) and synthetic hydroxoferric species, respectively. We demonstrate that the observed enhancement of the H-abstraction catalysis for these systems is a purely electrostatic effect, elicited by the local charges embedded on the peripheries of the respective macrocyclic ligands. Our findings provide new insights into how electrostatics can be employed to tune the catalytic activity of metalloenzymes and can thus contribute to the future design of new and highly efficient hydrogen-abstraction catalysts.