全文获取类型
收费全文 | 694篇 |
免费 | 18篇 |
国内免费 | 6篇 |
专业分类
化学 | 431篇 |
晶体学 | 17篇 |
力学 | 8篇 |
数学 | 46篇 |
物理学 | 216篇 |
出版年
2023年 | 6篇 |
2022年 | 34篇 |
2021年 | 27篇 |
2020年 | 21篇 |
2019年 | 26篇 |
2018年 | 26篇 |
2017年 | 21篇 |
2016年 | 23篇 |
2015年 | 16篇 |
2014年 | 34篇 |
2013年 | 60篇 |
2012年 | 46篇 |
2011年 | 56篇 |
2010年 | 41篇 |
2009年 | 19篇 |
2008年 | 20篇 |
2007年 | 34篇 |
2006年 | 34篇 |
2005年 | 18篇 |
2004年 | 15篇 |
2003年 | 5篇 |
2002年 | 13篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 6篇 |
1994年 | 9篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 4篇 |
1965年 | 1篇 |
1944年 | 1篇 |
排序方式: 共有718条查询结果,搜索用时 234 毫秒
1.
Varsha J. Thombare Craig A. Hutton 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(15):5052-5056
Peptide macrocyclization is often a slow process, plagued by epimerization and cyclodimerization. Herein, we describe a new method for peptide macrocyclization employing the AgI‐promoted transformation of peptide thioamides. The AgI has a dual function: chemoselectively activating the thioamide and tethering the N‐terminal thioamide to the C‐terminal carboxylate. Extrusion of Ag2S generates an isoimide intermediate, which undergoes acyl transfer to generate the native cyclic peptide, resulting in a rapid, traceless macrocylization process. Cyclic peptides are furnished in high yields within 1 hour, free of epimerization and cyclodimerization. 相似文献
2.
M Lepère R Gobeille V.Malathy Devi M.A.H Smith B Aoaeh A.W Mantz 《Journal of Molecular Spectroscopy》2004,224(1):7-12
We have analyzed the methyl fluoride RQ(J,0)Q branch lines located near 1475 cm−1 using a simultaneous multi-spectrum fitting technique. In this analysis we have used previously recorded diode-laser data in which we collected many data points covering only one or two Q branch lines in a particular run. The analysis consists of simultaneously fitting 57 spectra collected with numerous pressure and path length conditions for all absorption lines. The data are concatenated to create one continuous spectrum of the Q branch. We have determined the intensity and self-broadened widths at 296 K for 23 RQ(J,0) lines. 相似文献
3.
Archana Devi 《European Polymer Journal》2007,43(6):2422-2432
Cardanol-based novolac-type phenolic resins were synthesized with different mole ratios of cardanol-to-formaldehyde, viz., 1:0.6, 1:0.7, and 1:0.8. These novolac resins were epoxidized with molar excess of epichlorohydrin at 120 °C in basic medium. The epoxidized novolac resins were, separately, blended with different weight ratios of carboxyl-terminated polybutadiene liquid rubber ranging between 0-25 wt% with an interval of 5 wt%. All the blends were cured at 150 °C with 40 wt% polyamide. The formation of various products during the curing of blend samples has been studied by Fourier-transform infra-red spectroscopic analysis. The tensile strength and elongation-at-break of the cured samples increased up to 15 wt% in the blend and decreased thereafter. This blend sample was also found to be most thermally stable system. The blend morphology, studied by scanning electron microscopy analysis, was finally correlated with the structural and property changes in the blends. 相似文献
4.
Protonation constant of an unsymmetrical Schiff base, salicylidene(N-benzoyl)glycyl hydrazone (SalBzGH), and formation constants
of its complexes have been determined potentiometrically at different temperatures in aqueous dioxane medium. Complexes of
SalBzGH with VO(IV), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) have been prepared. Elemental analyses, pH-metric,
molar conductance, magnetic susceptibility, electronic, IR, ESR, XRD (powder) and NMR studies have been carried out to study
the coordination behaviour of SalBzGH toward these metal ions. pH-metric and 1H NMR studies show the presence of two dissociable
protons in the ligand. IR and NMR spectra suggest the tridentate nature of the ligand, coordinating as a uninegative species
in the Mn(II) complex and as a dinegative species in all the other complexes. Presence of two different conformers of the
ligand at room temperature and stabilization of a single conformer upon complex formation have been established from1H NMR spectra of the metal-free ligand, Zn(II) and Hg(II) complexes recorded at 296 K. Electronic and ESR spectra indicate
highly distorted tetragonal geometry for VO(IV) and Cu(II) complexes. XRD powder patterns of the Zn(II) complexes are indexed
for an orthorhombic crystal system. 相似文献
5.
Heat transfer in unsteady MHD channel flow (of an incompressible viscous and electrically conducting fluid) under oscillatory pressure gradient when the channel surfaces are conducting and moving with time-dependent velocities has been analysed. The velocity, magnetic field and temperature distributions have been obtained and their numerical results are shown graphically.Symbols
u
velocity
-
H
o
applied magnetic field
-
H
x
induced magnetic field
-
T
temperature
-
T
1
*
,T
2
*
temperatures of the upper and lower planes
-
density
-
p
pressure
-
kinematic viscosity
-
magnetic diffusivity
-
electrical conductivity of the fluid
-
A
*
characteristic velocity
-
L
characteristic length
-
e
magnetic permeability of the fluid
-
C
p
specific heat
-
coefficient of viscosity
-
k
thermal conductivity
-
1,
2
permeabilities of the planes
-
1,
2
conductivities of the planes
-
1,
2
conductance ratios of the planes
-
Pr
Prandtl number (=C
p
/k)
-
E
Eckert number
-
M
Hartmann number (=
e
H
o
L/)
-
R
e
Reynolds number (=LA
*/)
-
R
m
magnetic Reynolds number (=
e
LA
*)
-
S
Pr.E(=S) 相似文献
6.
Ipsita Devi 《Tetrahedron letters》2004,45(47):8625-8627
Sodium bromide catalysed three-component cyclocondensation of aryl aldehydes, alkyl nitriles and dimedone proceeds under microwave irradiation in solvent free conditions to give highly functionalised tetrahydrobenzo[b]pyrans in excellent yields. 相似文献
7.
K. Ramulu B. M. Rao P. Madhavan M. Lalitha Devi M. K. Srinivasu K. B. Chandrasekhar 《Chromatographia》2007,65(3-4):249-252
A simple and new isocratic normal phase chiral HPLC method has been developed for the determination of enantiomeric purity
of pemetrexed disodium (l-enantiomer) in bulk drugs with a short run time of about 20 min. Chromatographic separation of l and d-enantiomers of pemetrexed disodium was achieved on an amylose based chiral stationary phase using a mobile phase consists
of hexane, ethanol and trifluoro acetic acid. The resolution between the enantiomers was found to be more than 2.0. The system
precision and method precision were found to be within 5% RSD for the distomer (d-enantiomer) at its specification level (i.e. not more than 1.0% w/w). The limit of detection and limit of quantification of distomer were 1.6 and 5 μg mL−1, respectively for 10 μL injection volume. The percentage recovery of distomer was ranged from 90.6 to 105.7 in bulk drug
samples. The test solution was found to be stable in the diluent for 48 h. The method was found to be specific for the enantiomers
of pemetrexed disodium and can be conveniently used for the quantification of undesired d-enantiomer present in the bulk drug samples of pemetrexed disodium. 相似文献
8.
M. Patri Varsha R. Hande Swati Phadnis B. Somaiah Suhasini Roychoudhury P. C. Deb 《先进技术聚合物》2004,15(5):270-274
A Novel solid polymer electrolyte (SPE) membrane containing both ? COOH and ? SO3H group has been prepared by simultaneous method of radiation grafting of acrylic acid onto FEP followed by sulfonation. The presence of weakly acidic acrylic acid controls the swelling in water while ? SO3H group provides conductivity due to its strongly ionic characteristic. FEP‐g‐acrylic acid and its sulfonated derivatives were characterized by their properties. While the mechanical properties decreased, other properties such as ion exchange capacity (IEC), water uptake and ionic conductivity increased with increase in graft content. These properties further changed on sulfonation. Acrylic acid being weakly acidic in nature, conductivity values of the grafted membrane were quite low. However, introduction of strong ? SO3H group resulted in conductivity closer to Nafion 117. Few sulfonated membranes have been tested with respect to H2/O2 fuel cell performance. Short‐term fuel cell test for 100 hr gave a stable performance. These membranes are less expensive compared to Nafion. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
9.
Furoin thiosemicarbazone (FTS) reacts with Ni(II), Pd(II) and Cu(II) in aqueous medium, giving yellow solutions at pH 9, 6 and 3 respectively. The complexes have absorption maxima at 360 nm for Ni(II) and Pd(II) and 355 nm for Cu(II). At these wavelengths the reagent absorbance is negligible. The molar absorptivities are 1.54 x 10(4) [Ni(FTS)(2)], 1.98 x 10(4) [Pd(FTS)(2)] and 1.45 x 10(4) 1.mole(-1).cm(-1) (CuFTS). Beer's law is valid up to 4, 5 and 3 ppm for Ni, Pd and Cu respectively. The apparent instability constant of the Ni-FTS system is found to be 6.5 x 10(-11), of the Cu-FTS system 7.1 x 10(-7) and of the Pd-FTS system 3 x 10(-12) at the recommended pH values. The effect of various ions is reported. 相似文献
10.
Four aluminophosphates, A3Al2P3O12 (A = Na, K (1), Rb (2), Tl (3)), have been synthesized by solid-state reactions and characterized by X-ray diffraction and NMR and IR spectroscopic techniques. Aluminum has trigonal bipyramidal coordination in the thallium compound and tetrahedral coordination in the others. Potassium, rubidium and thallium analogues have been structurally characterized by single-crystal X-ray diffraction and found to possess three-dimensional (Al2P3O12)3- anionic frameworks with channels occupied by A+ countercations. These frameworks are built from corner connections of PO4 tetrahedra with AlO4 tetrahedra in 1 and 2 and with AlO5 trigonal bipyramids in 3. Pertinent crystal data are as follows: for 1, orthorhombic space group Pna2(1), a = 8.685(2) A, b = 16.947(2) A, c = 8.458(3) A, Z = 4; for 2, orthorhombic space group Cmc2(1), a = 17.164(2) A, b = 8.6270(6) A, c = 8.8140(14) A, Z = 4; for 3, orthorhombic space group Pna2(1), a = 6.1478(15) A, b = 10.396(3) A, c = 17.787(5) A, Z = 4. Compound 3 is a rare example of an oxide possessing aluminum exclusively in trigonal bipyramidal coordination. 相似文献