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971.
Aza-Michael adducts of chalcones and diethyl N-arylphosphoramidates undergo reductive cyclization with sodium borohydride followed by sodium hydride to afford 1,2,4-trisubstituted azetidines diastereoselectively in a one-pot procedure and excellent yields. 相似文献
972.
The self-focusing of a laser pulse through a tunnel ionizing gas (helium) has been studied in both non-relativistic and relativistic
regimes, relaxing the near-axis approximation. In the non-relativistic regime, the laser pulse produces multiple ionization
of the gas and faces strong defocusing due to the steep radial density gradient caused by the same. The uneven defocusing
of paraxial and marginal rays leads to a beam acquiring a ring shaped intensity distribution. In the relativistic regime,
the laser pulse produces fully ionized plasma within a few wave periods, subsequently the relativistic mass effect and the
ponderomotive force induced electron cavitation cause periodic self-focusing.
PACS 52.38.Hb; 42.65.Jx 相似文献
973.
The t-year event is a commonly used characteristic to describe the extreme flood peak in hydrological designs. The annual maximum series (AMS) and the partial duration series (PDS) are two basic approaches in flood analyses. In this paper, we first derive the distribution of the maximum extreme or the joint distribution of two or more maximum extremes from historical records based on a stochastic model, and then estimate statistical characteristics, including the t-year event, from the distribution. In addition to the two classical approaches (AMS and PDS), two additional approaches are proposed for estimating the unknown parameters in this paper. The first one uses two or more annual maximums (MAMS) as the sample to estimate the distribution of the maximum extremes. The second one uses multi-variate shock model to estimate the distribution of the maximum extremes for a multi-modal streamflow. The distribution of the extreme streamflow and the associated characteristics in the Bird Creek in Avant, Oklahoma, in the St. Johns River in Deland, Florida, and in the West Walker River in Coleville, California are estimated by using the stochastic model. To investigate further the performance of the estimation, the stochastic models based on AMS, MAMS and PDS related are also applied to the simulated data. The results show that the stochastic model and the related methods are reliable. 相似文献
974.
A highly practical procedure for the preparation of novel classes of chalcogen bearing diamines [{H2N(CH2)n}2E] and diols [{HO(CH2)n}2E] (n = 2 or 3 and E = Se or Te) by the reaction of disodiumchalcogenide and haloalkylamines or haloalcohols is presented. 相似文献
975.
Salih H.A. Tripathi V.K. Pandey B.K. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2003,31(3):324-328
The second-harmonic generation of an intense self-guided right circularly polarized laser beam in a magnetized plasma is investigated. The laser imparts oscillatory velocity to electrons and exerts a radial ponderomotive force on them to create a depleted density channel. The critical power for self-focusing shows huge reduction as electron cyclotron frequency approaches the laser frequency (/spl omega//sub c/ /spl rarr/ /spl omega/). In the presence of the self-created radial density gradient, the laser drives a density perturbation at the fundamental frequency. The density perturbation beats with the oscillatory velocity to produce a second harmonic current density, driving second harmonic radiation copropagating with the laser. The second harmonic, however, is azimuthally asymmetric with /spl theta/-variation as exp(i/spl theta/). Its amplitude shows resonant enhancement as /spl omega//sub c/ /spl rarr/ /spl omega/. 相似文献
976.
The fabrication and characterization of refractive index profiles of multimode mode polymer optical waveguides is reported. Effect of annealing temperature on guided losses of polymer (polystyrene) thin film optical waveguide is also presented. Efficient coupling over a 110 nm spectral bandwidth was experimentally demonstrated with a prism-coupler designed to match the dispersion of a particular waveguide. 相似文献
977.
S. J. Sartandel S. K. Jha S. V. Bara R. M. Tripathi 《Journal of Radioanalytical and Nuclear Chemistry》2014,300(2):873-877
An investigation on the distribution of 226Ra and 228Ra activity concentration in coastal surface sea water from Okha in Gujarat to Ratnagiri in Maharashtra state along the west coast of India was carried out. In-situ pre-concentration technique was used to measure radium isotopes by passing 1,000 L of seawater through MnO2 impregnated polypropylene filter cartridges at all the locations. 226Ra was estimated using gamma ray peak of its daughter radionuclides 214Bi and 214Pb. 228Ra was estimated from its daughter 228Ac. In the coastal waters, 226Ra and 228Ra activity concentration were observed to be in the range of 1.5–2.9 and 2.5–8.6 Bq m?3 with a mean of 2.2 and 4.9 Bq m?3 respectively. The activity of 228Ra was observed to be more than 226Ra in all the locations. The variation in spatial distribution of the radium isotopes activity concentration and its ratio with respect to location is discussed in the paper. The radioactive database obtained represents reference values for coastal environment of India. 相似文献
978.
S. J. Sartandel S. V. Bara S. Chinnaesakki S. K. Jha R. M. Tripathi 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(3):1441-1447
Phosphate deposits are generally characterized by high levels of natural radionuclide concentrations. Natural radionuclides from the uranium and thorium series were measured, using high-resolution gamma-spectrometry in phosphate rock and phosphogypsum samples from the phosphate fertilizer industry in India. Equilibrium was found to be disrupted during the chemical processing of phosphate rock with 83 % of the 226Ra and only 5 % of 238U fractionating to phosphogypsum. Activity concentrations of 238U and 226Ra in phosphogypsum produced from various fertilizer industries of India showed levels < 1,000 Bq kg?1 and pose no restriction for use in building/construction material. 相似文献
979.
Ravi Kumar Thakur Akansha Mishra K.K.G. Ramakrishna Rohit Mahar Sanjeev K. Shukla A.K. Srivastava Rama P. Tripathi 《Tetrahedron》2014
A series of novel nucleoside analogues having dual bases (pyrimidine and triazole) and sugars have been synthesized by CuAAC reaction of azido sugars with propynylated pyrimidines. These compounds were evaluated for their in vitro α-glucosidase inhibitory activity. In this series, compounds 4b, 4d, 8a, 8b, 8c, 8e, 8g, 8h, and 8i exhibited very good inhibition of α-glucosidase enzyme. Nucleoside analogues 8a, 4b, 8h, and 8c displayed 47.4%, 41.8%, 39.4%, and 34.6% inhibition, respectively, comparable to the standard drug acarbose (53.4%). 相似文献
980.
An efficient one-pot, convenient catalysis for the synthesis of 5-substituted-1H-tetrazoles is reported. The [3+2] cycloaddition involves various nitriles, sodium azide in refluxing DMF and AgNO3 as catalyst to give corresponding 5-substituted-1H-tetrazoles in good to excellent yields. It is expected that the reaction proceeds via in situ formation of a silver azide species, which participates in coordination of nitrile moiety followed by cycloaddition of azide ion to give tetrazole. 相似文献