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131.
We report computational investigations on the mechanism and the selectivity of Pd-catalyzed allylic alkylation of γ-valerolactone. Density functional calculations using the B3LYP functional are performed on the selectivity-determining nucleophilic addition step of this reaction. The B3LYP results of commonly assumed pathways fail to reproduce the observed selectivity of the reaction. Therefore, alternative pathways are considered for the nucleophilic addition step, to explain the experimentally established role of the additives LiCl and lithium diisopropyl amide (LDA) in the Pd-catalyzed reaction. These pathways involve different approaches of the enolate toward the η(3) -allylpalladium complex that are mainly guided by stabilizing Cl(δ-) ???Li(δ+) ???O(enolate) interactions in the transition state. In the calculations, the experimentally observed trans-product selectivity for the prototypical reaction with (S)-BINAP ligands is found only when assuming the addition of a "mixed" Li-enolate/LiCl adduct to the η(3) -allylpalladium complex. This mechanism provides a reasonable explanation for the experimental results and sheds light on the role of LiCl in the reaction. The analysis of the different transition-state models allows us to identify steric and electronic factors that stabilize or destabilize the relevant diastereomeric transition states. Calculations for different combinations of substrates (γ-valerolactone and δ-caprolactone) and catalysts (with (R)- and (S)-BINAP ligands) reproduce the experimentally observed selectivities well and thus provide further support for the proposed mechanism. 相似文献
132.
A pressure induced structural phase transition from NaCl-type (B1) to CsCl-type (B2) structure has been predicted in transition metal carbides, namely TiC, ZrC, NbC, HfC, and TaC by using an interionic potential theory with modified ionic charge (Zm ), which includes Coulomb screening effect due to d-electron. The phase transition pressure (PT ) relies on large volume discontinuity in pressure–volume relationship, and identifies the structural phase transition from B1 phase to B2 phase. The variation of second-order elastic constants with pressure follows a systematic trend identical to that observed in other compounds of NaCl-type structure. The Born criterion for stability is found to be valid in transition metal carbides. 相似文献
133.
We have predicted high pressure structural behavior and elastic properties of alkaline earth tellurides (AETe; AE = Ca, Sr,
Ba) by using two body interionic potential approach with modified ionic charge (Z
m
e). This method has been found quite satisfactory in case of the rare earth compounds. The equation of state curve, structural
phase transition pressure from NaCl (B1) to CsCl (B2) phase and associated volume collapse at transition pressure of alkaline
earth tellurides (AETe) obtained from this approach, so have been compared with experimentally measured data reveal good agreement.
We have also investigated bulk modulus, second and third order elastic constants and pressure derivatives of second order
elastic constants at ambient pressure which shows predominantly ionic nature of these compounds. First time, we have calculated
the Poisson ratio, Young and Shear modulus of these compounds.
相似文献
134.
Sanjit K. Debnath Mahendra P. Kothiyal Seung-Woo Kim 《Optics and Lasers in Engineering》2009,47(11):1125-1130
Spectral phase in a white-light interferogram contains information about the absolute optical path difference (OPD) in the interferometer. Evaluation of spectral phase is therefore important in applications such as profilometry with white light. In spectrally resolved white-light interferometry (SRWLI) the white-light interferogram is spectrally decomposed by a spectrometer in order to determine this phase. Several single-frame techniques in SRWLI have been proposed for the evaluation of the phase including Fourier transform, Hilbert transform, spatial phase shifting, windowed Fourier transform and wavelet transform. In this paper we present a comparative study of these techniques in this application in relation to the temporal phase-shifting technique which is a multi- frame method. Further, we also propose a modified method to remove the influence of source spectrum modulation in Hilbert transform procedure. 相似文献
135.
Anubha Goyal Ashavani Kumar Prabir K. Patra Shaily Mahendra Salomeh Tabatabaei Pedro J. J. Alvarez George John Pulickel M. Ajayan 《Macromolecular rapid communications》2009,30(13):1116-1122
We demonstrate a simple one‐step method for synthesizing noble metal nanoparticle embedded free standing polydimethylsiloxane (PDMS) composite films. The process involves preparing a homogenous mixture of metal salt (silver, gold and platinum), silicone elastomer and the curing agent (hardener) followed by curing. During the curing process, the hardener crosslinks the elastomer and simultaneously reduces the metal salt to form nanoparticles. This in situ method avoids the use of any external reducing agent/stabilizing agent and leads to a uniform distribution of nanoparticles in the PDMS matrix. The films were characterized using UV‐Vis spectroscopy, transmission electron microscopy and X‐ray photoemission spectroscopy. The nanoparticle‐PDMS films have a higher Young's modulus than pure PDMS films and also show enhanced antibacterial properties. The metal nanoparticle‐PDMS films could be used for a number of applications such as for catalysis, optical and biomedical devices and gas separation membranes.
136.
Mahendra K. Sharma Sebastian Blomeyer Beate Neumann Hans‐Georg Stammler Maurice van Gastel Alexander Hinz Rajendra S. Ghadwal 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(49):17763-17767
The divinyldiarsene radical cations [{(NHC)C(Ph)}As]2(GaCl4) (NHC=IPr: C{(NDipp)CH}2 3 ; SIPr: C{(NDipp)CH2}2 4 ; Dipp=2,6‐iPr2C6H3) and dications [{(NHC)C(Ph)}As]2(GaCl4)2 (NHC=IPr 5 ; SIPr 6 ) are readily accessible as crystalline solids on sequential one‐electron oxidation of the corresponding divinyldiarsenes [{(NHC)C(Ph)}As]2 (NHC=IPr 1 ; SIPr 2 ) with GaCl3. Compounds 3 – 6 have been characterized by X‐ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As?As π‐bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C?As bonds from 1 / 2 → 3 / 4 → 5 / 6 . The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO‐related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75As (I=3/2) nuclei. 相似文献
137.
Manu Mannattil Ambrish Pandey Mahendra K. Verma Sagar Chakraborty 《The European Physical Journal B - Condensed Matter and Complex Systems》2017,90(12):259
Constructing simpler models, either stochastic or deterministic, for exploring the phenomenon of flow reversals in fluid systems is in vogue across disciplines. Using direct numerical simulations and nonlinear time series analysis, we illustrate that the basic nature of flow reversals in convecting fluids can depend on the dimensionless parameters describing the system. Specifically, we find evidence of low-dimensional behavior in flow reversals occurring at zero Prandtl number, whereas we fail to find such signatures for reversals at infinite Prandtl number. Thus, even in a single system, as one varies the system parameters, one can encounter reversals that are fundamentally different in nature. Consequently, we conclude that a single general low-dimensional deterministic model cannot faithfully characterize flow reversals for every set of parameter values. 相似文献
138.
In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k?11/5 form at large scales, to a steeper approximate k?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux. 相似文献
139.
Mashiko H Gilbertson S Li C Khan SD Shakya MM Moon E Chang Z 《Physical review letters》2008,100(10):103906
We demonstrated a novel optical switch to control the high-order harmonic generation process so that single attosecond pulses can be generated with multiple-cycle pulses. The technique combines two powerful optical gating methods: polarization gating and two-color gating. An extreme ultraviolet supercontinuum supporting 130 as was generated with neon gas using 9 fs laser pulses. We discovered a unique dependence of the harmonic spectra on the carrier-envelope phase of the laser fields, which repeats every 2 pi radians. 相似文献
140.
Pankaj M. KoinkarSandip S. Patil Tae-Gyu KimDaisuke Yonekura Mahendra A. More Dilip S. JoagRi-ichi Murakami 《Applied Surface Science》2011,257(6):1854-1858
Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B2O3 concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B2O3 concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films. 相似文献