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排序方式: 共有67条查询结果,搜索用时 15 毫秒
1.
We present a method for computing classical Newtonian trajectories that minimize the path length or transit time from reactant
to product. Our approach is based on a generalization of the fast-marching method, which allows us to construct the solution
of the Hamilton-Jacobi equation for the action that optimizes the desired quantity. The resulting “reactive paths” can be
interpreted as reaction coordinates but, unlike more conventional choices, they contain dynamical information about the chemical
system of interest. 相似文献
2.
Jung JC Kache R Vines KK Zheng YS Bijoy P Valluri M Avery MA 《The Journal of organic chemistry》2004,69(26):9269-9284
A convergent, total synthesis of epothilones B (2) and D (4) is described. The key steps are Normant coupling to establish the desired (Z)-stereochemistry at C12-C13, Wadsworth-Emmons olefination of methyl ketone 28 with the phosphonate ester 8, diastereoselective aldol condensation of aldehyde 5 with the enolate of keto acid derivatives to form the C6-C7 bond, selective deprotection of acid 52, and macrolactonization. 相似文献
3.
Carsten Lenczyk Dr. Dipak Kumar Roy Bijoy Ghosh Johannes Schwarzmann Prof. Ashwini K. Phukan Prof. Dr. Holger Braunschweig 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(36):8585-8589
A series of bis(σ)-borane complexes of Group 6 transition metals were prepared by direct dihydroborane coordination to the metal center. Reaction of [M(CO)3(PCy3)2] and two dihydroboranes [DurBH2] and [(Me3Si)2NBH2] (Dur=2,3,5,6-Me4C6H) yielded bis(σ)-borane complexes fac-[M(CO)3(PCy3){η2-(H2BR)}] (R=Dur; 1 : M=Cr, 2 : M=W; R=N(SiMe3)2; 3 : M=Cr, 4 : M=W). In the case of molybdenum, we have isolated an arene complex ( 5 ) with [DurBH2] in which the Dur group acts as a η6-bound ligand, and with [(Me3Si)2NBH2] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO)2(PCy3)2{η2-(H2BN(SiMe3)2}] ( 6 ), with a different pattern of auxiliary ligands. The complexes were investigated by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and computational methods. Quantum theory of atoms in molecules (QTAIM) calculations demonstrated that the borane complexes may be described as pure bis(σ)-borane complexes rather than elongated or stretched examples given that the calculations do not show the presence of a ring-critical point (RCP) at the ring formed by the interactions of the B−H with metal center. 相似文献
4.
OBJECTIVE: This study aimed to prospectively compare measurement precision of calf intramyocellular lipid (IMCL) quantification at 3.0 and 1.5 T using (1)H magnetic resonance spectroscopy ((1)H-MRS). MATERIALS AND METHODS: We examined the soleus and tibialis anterior (TA) muscles of 15 male adults [21-48 years of age, body mass index (BMI)=21.9-38.0 kg/m(2)]. Each subject underwent 3.0- and 1.5-T single-voxel, short-echo-time, point-resolved (1)H-MRS both at baseline and at 31-day follow-up. The IMCL methylene peak (1.3 ppm) was scaled to unsuppressed water peak (4.7 ppm) using the LCModel routine. Full width at half maximum (FWHM) and signal-to-noise ratios (SNRs) of unsuppressed water peak were measured using jMRUI software. Measurement precision was tested by comparing interexamination coefficients of variation (CV) between different field strengths using Wilcoxon matched pairs signed rank test in all subjects. Overweight subjects (BMI>25 kg/m(2)) were analyzed separately to examine the benefits of 3.0-T acquisitions in subjects with increased adiposity. RESULTS: No significant difference between 3.0 and 1.5 T was noted in CVs for IMCL of soleus (P=.5). CVs of TA were significantly higher at 3.0 T (P=.02). SNR was significantly increased at 3.0 T for soleus (64%, P<.001) and TA (62%, P<.001) but was lower than the expected improvement of 100%. FWHM at 3.0 T was significantly increased for soleus (19%, P<.001) and TA (7%, P<.01). Separate analysis of overweight subjects showed no significant difference between 3.0- and 1.5-T CVs for IMCL of soleus (P=.8) and TA (P=.4). CONCLUSION: Using current technology, (1)H-MRS for IMCL at 3.0 T did not improve measurement precision, as compared with 1.5 T. 相似文献
5.
A canonical planar beta-hairpin peptide, stereochemically reengineered into a semicircular bracelet type motif by L-to-D stereochemical inversion in two pairs of its cross-strand neighbor residues, displays protein like ordering including two-state behavior in H2O, which is unusual for a small peptide of this size. 相似文献
6.
Bijoy Kundu Hsiang-nan Li Jim Samuelsson Pankaj Jain 《The European Physical Journal C - Particles and Fields》1999,8(4):637-642
We recalculate the proton Dirac form factor based on the perturbative QCD factorization theorem, which includes Sudakov suppression.
The evolution scale of the proton wave functions and the infrared cutoffs for the Sudakov re-summation are carefully chosen
such that the soft divergences from large coupling constants are diminished and perturbative QCD predictions are stabilized.
We find that the King–Sachrajda model for the proton wave function leads to results which are in better agreement with experimental
data than those from the Chernyak–Zhitnitsky wave function.
Received: 27 November 1998 / Published online: 7 April 1999 相似文献
7.
Bijoy K. Dey 《Journal of mathematical chemistry》2011,49(9):2032-2052
The optimal creation of a reduced space that effectively captures the long timescale dynamics of a non-linear molecular system
over a range of frequencies is described. The technique builds on a previously developed subspace method based on linear constant
projective transformation of the original full space. The present work attempts to propose transformation that are spatially
dependent thereby leading to an effective subspace for better representing the dynamics of interests. The algorithm seeks
out an optimal transformation consistent with desired low frequency motion in a rather general way. The method is demonstrated
for a six-dimensional nonlinear system reduced to two-dimensions. Superior performance is found in evaluating ensemble-averaged
classical dynamical properties. 相似文献
8.
Sudhir K. SharmaSahaj Gupta Mohammad SaifuddinAnil K. Mandadapu Piyush K. AgarwalHarsh M. Gauniyal Bijoy Kundu 《Tetrahedron letters》2011,52(1):65-68
Two simple, highly efficient three component tandem reactions for the synthesis of diversified NaNb di-carbamate-4,9-dihydro-3-iodo-α-carbolines and Na-carbamate-3-iodo-α-carbolines have been described. The strategy involves one-pot condensation of bis-carbamate protected 2-amino indoles with disubstituted propargyl alcohols and I2/ICl. The salient feature of the reaction involves iodocyclo-elimination of Nb-linked carbamate under mild condition in the final step. 相似文献
9.
Using the Landau theory of phase transitions it has been shown that for a second order phase transition Ω/kBTc ? 0.01. and its isomorphs 4Ω/J1 ? 1 and for a first order transition Ω/kBTc ? 0.01. 相似文献
10.
Sabina Yasmin Z. Siti Rozaila Bijoy Sonker Barua Faruque-Uz-Zaman Chowdhury Md. Abdur Rashid 《辐射效应与固体损伤》2018,173(7-8):657-672
One sign of a vibrant Bangladeshi economy has been the move away from the use of more traditional housing materials towards a preference for modern constructional media. Glass, one such example, used both decoratively and in a structural context, offers various advantageous properties and facets including a protective feature against radiation that has not previously been considered. Current interest examines the dosimetric possibilities offered by the commercial glass as a secondary shield and also in retrospective ionising radiation exposure analysis. Four popular brands of window glass are investigated, all available within the local market (PHP-Bangladesh, Usmania-Bangladesh, Nasir-Bangladesh and Rider-China), all with the same thickness and colour, varying in terms of elemental weight fractions as evaluated by energy dispersive X-ray analysis. As potential attenuators of transmitted radiation thereby forming secondary barriers against radiation exposure from penetrating radiations, the four brands of glass have been studied using photon energies from 59 up to 1332 keV, a range of values representative of that potentially encountered in incidents. Use has been made of a well-shielded high-purity germanium γ-ray spectrometer and associated electronics, providing for evaluation of the characteristic barrier parameters of half-value layer, radiation protection efficiency and effective atomic number (Zeff). Of the four brands investigated, Rider provides superior secondary shielding performance. Concerning potential retrospective dosimetry the effective atomic number of the glass samples are comparable with that of the commercial thermoluminescence (TL) dosimeter TLD-200. At high doses, the TL yields are sufficient to provide for retrospective accident dosimetry. 相似文献