全文获取类型
收费全文 | 85535篇 |
免费 | 742篇 |
国内免费 | 411篇 |
专业分类
化学 | 29621篇 |
晶体学 | 805篇 |
力学 | 6898篇 |
数学 | 33213篇 |
物理学 | 16151篇 |
出版年
2023年 | 79篇 |
2022年 | 112篇 |
2021年 | 138篇 |
2020年 | 190篇 |
2019年 | 191篇 |
2018年 | 10500篇 |
2017年 | 10335篇 |
2016年 | 6380篇 |
2015年 | 1142篇 |
2014年 | 575篇 |
2013年 | 767篇 |
2012年 | 4295篇 |
2011年 | 11072篇 |
2010年 | 5975篇 |
2009年 | 6332篇 |
2008年 | 7050篇 |
2007年 | 9166篇 |
2006年 | 643篇 |
2005年 | 1705篇 |
2004年 | 1859篇 |
2003年 | 2268篇 |
2002年 | 1294篇 |
2001年 | 414篇 |
2000年 | 410篇 |
1999年 | 273篇 |
1998年 | 288篇 |
1997年 | 248篇 |
1996年 | 301篇 |
1995年 | 202篇 |
1994年 | 135篇 |
1993年 | 155篇 |
1992年 | 108篇 |
1991年 | 97篇 |
1990年 | 85篇 |
1989年 | 87篇 |
1988年 | 94篇 |
1987年 | 84篇 |
1986年 | 89篇 |
1985年 | 71篇 |
1984年 | 65篇 |
1983年 | 50篇 |
1982年 | 63篇 |
1981年 | 59篇 |
1980年 | 65篇 |
1979年 | 55篇 |
1978年 | 56篇 |
1914年 | 45篇 |
1912年 | 41篇 |
1909年 | 42篇 |
1908年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In cetyltrimethylammonium/n-pentanol/H2O W/O (W/O = water in oil microemulsion) mixtures and bi-continuous microemulsions, phenothiazine (PTZ) molecules exist in
the membrane phase of the dispersion either with the N atom or with the S atom pointed toward the polar head of cetyltrimethylammonium
(CTAB). Cyclic voltammetry has been used to investigate the effects of the compositions and structures of the microemulsions,
pH, and the salt on the location distribution of PTZ in the membrane phase of the dispersion in CTAB/n-C5H11OH/H2O W/O and bi-continuous microemulsions. The results show that the location distribution of PTZ in the membrane phase of the
dispersion in microemulsions is mainly dependent on the hydrogen bond between PTZ and n-C5H11OH (or the counterion), and on the electrostatic attractive interaction between the N atom in PTZ and the polar head of CTAB. 相似文献
992.
The thermodynamic properties of the mixed aqueous electrolyte of ammonium and alkaline earth metal nitrates have been studied using the hygrometric method at 25?°C. The water activities of these {yNH4NO3+(1?y)Y(NO3)2}(aq) systems with Y ≡ Ba2+, Mg2+ and Ca2+ were measured at total molalities ranging from 0.10 mol?kg?1 to saturation for different NH4NO3 ionic-strength fractions of y=0.20, 0.50 and 0.80. These data allow the calculation of osmotic coefficients. From these measurements, the ionic mixing parameters are determined and used to calculate the solute activity coefficients in the mixtures at different ionic-strength fractions. The results of these ternary solution measurements are compared with those for binary solutions of the alkaline earth nitrates of magnesium, calcium and barium with ammonium nitrates. The behavior of the aqueous electrolyte solutions containing mixtures of barium or calcium or magnesium with ammonium nitrates are correlated and show that ionic interactions are more important for the system containing Mg2+ than for Ca2+ or Ba2+. The trends are mainly due to the effects of the ionic size, polarizability and the hydration of the ions in these solutions. 相似文献
993.
Prithwiraj Byabartta 《Transition Metal Chemistry》2007,32(6):716-726
[Au(C6F5)(tht)], which on reaction with P, O, S-coordinating phosphines in CH2Cl2 medium leads to [Au(C6F5)(X)] [X = PPh3 H, (1a), oMe, (1b), pMe, (1c), mMe, (1d), AsPh3
(2), OPPh3
(3), SPPh3
(4), dppm, dppe, dppa = diphenylphosphino-methane,-ethane,-ammine(5, 6, 7), TPA = 135-tetraaza-7-phosphino adamentane(8), Py4H (9a), 4Bu (9b), 4Ac (9c), tht = tetrahydrothiophen, C6F5 is the pentafluorophenyl ring]. The maximum molecular peak of the corresponding molecule is observed in the ESI mass spectrum.
I.r. spectra of the complexes show –C = C– and C6F5 stretching near at 1610 and 1510, 955, 800 cm−1. The 1H-n.m.r. spectra as well as 31P- (1H)n.m.r. suggest solution stereochemistry, proton movement, phosphorus proton interaction. 13C-n.m.r. spectrum reflect the carbon skeleton in the molecule. In the 1H–1H COSY spectrum of the present complexes and contour peaks in the 1H–13C-HMQC spectrum, assign the solution structure and stereoretentive conformation in each step. 相似文献
994.
A zwitterionic trichloroplatinum(II) complex PtCl3(4,4′-bpy-N-nBu) (1) was prepared by an aqueous reaction of [4,4′-bpy-N-nBu]Cl with K2PtCl4, and was characterized by 1H-n.m.r. and IR spectroscopy as well as elemental analysis. Dissolving (1) into dmso-d
6 at 25 °C yields a mixture of the complexes, cis-PtCl2(4,4′-bpy-N-nBu)(dmso-d
6) (2-cis), trans-PtCl2(4,4′-bpy-N-nBu)(dmso-d
6) (2-trans), [4,4′-bpy-N-nBu]Cl and PtCl2(dmso-d
6)2. Ratio of the products in the dmso-d
6 solution changed depending on the temperature and the total concentration of the complexes. These compounds are in equilibrium
via isomerization reaction between (2-cis) and (2-trans) and via displacement reaction of the alkylbipyridinium ligand of
(2-cis) and (2-trans) with dmso-d
6 to form [4,4′-bpy-N-nBu]Cl and PtCl2(dmso-d
6)2. 相似文献
995.
Satish M. Annigeri M. P. Sathisha Vidyanand K. Revankar 《Transition Metal Chemistry》2007,32(1):81-87
Binuclear cobalt(II), nickel(II), copper(II) and zinc(II) complexes of general composition [M2L1-2(μ-Cl)Cl2] · nH2O with the Schiff-base ligands (where L1H and L2H are the potential pentadentate ligands derived by condensing 2,6-diformyl-4-methylphenol with 4-amino-3-antipyrine and 2-hydroxy-3-hydrazinoquinoxiline,
respectively) have been synthesized and characterized. Analytical and spectral studies support the above formulation. 1H-NMR and IR spectra of the complexes suggest they have an endogenous phenoxide bridge, with chloride as the exogenous bridge
atom. The electronic spectra of all the complexes are well characterized by broad d–d and a high intensity charge-transfer
transitions. The complexes are chloro-bridged as evidenced by two intense far-IR bands centered around 270–280 cm−1. Magnetic susceptibility measurements show that complexes are antiferromagnetic in nature. The compounds show significant
growth inhibitory activity against fungi Aspergillus niger and Candida albicans and moderate activity against bacteria Bacillus cirroflagellosus and Pseudomonas auresenosa. 相似文献
996.
(2S)- and (2R)-2-Amino-4-bromobutanoic acid were prepared starting from N-Boc-glutamic acid α tert-butyl ester. The double tert-butyl protection was necessary to prevent a partial racemisation during Barton’s radical decarboxylation used to transform the γ-carboxylic group into a bromide. This bromide reacted with different nitrogen, oxygen and sulphur nucleophiles to give nonnatural amino acids characterised by basic or heterocyclic side chains. The title compound was also used to prepare a conformationally constrained peptidomimetic. 相似文献
997.
Pekka Juusola Pentti Minkkinen Lasse Leino Jarmo K. Laihia 《Monatshefte für Chemie / Chemical Monthly》2007,138(10):951-965
Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation
constants (pK
a1 and pK
a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation
(pK
a1 = 3.43 ± 0.12 and pK
a2 = 5.80 ± 0.04) and (pK
a1 = 2.7 ± 0.3 and pK
a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride
solutions up to the isotonic salt concentration (0.154 mol dm−3), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at
temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK
a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric
dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived.
The obtained pK
a1 and pK
a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.
An erratum to this article is available at . 相似文献
998.
The molar conductivities (Λ) of solutions of n-tetrabutylammonium tetraphenylborate (NBu4BPh4) in 3-pentanone have been measured in the temperature range from 283.15 to 329.15 K. The conductance data have been analyzed
using the Lee-Wheaton conductivity equation with the distance parameter (a) set at Bjerrum’s pairing distance, and the limiting molar conductivities (Λo) and the association equilibrium constants (K
A) have been derived. The limiting ion conductivities (λ_±o) have been evaluated according to the method of Krumgalz. The λ+
o values have been compared with λ+
o values calculated from the empirical equation of Gill. The thermodynamic functions, Gibbs energy (Δ G
A
o), enthalpy (Δ H
A
o) and entropy (Δ S
A
o) for the process of ion-pair formation as well as the activation energy of the ionic movement (ΔH
∗) have been evaluated. The obtained results are discussed in terms of ion-ion and ion-solvent interactions. 相似文献
999.
Stefan Marinov 《Foundations of Physics》1976,6(5):571-581
Proceeding from our absolute space-time conceptions, we obtain the formula for the gravitational frequency shift in an extremely simple way. Using our burst model for photons, we show that the different rates of clocks placed in spatial regions with different gravitational potentials appear as a direct result of the gravitational frequency shift and the axiomatic assumption that at any space point the time unit is to be defined by light clocks with equal arms, i.e., that at any space point the light velocity (in moving frames the there-and-back velocity) has the same numerical valuec. Considering the principle of equivalence, we come to the logical conclusion that the kinematic (Einstein-Lorentz) time dilation is an absolute phenomenon. 相似文献
1000.
Mariyana Atanasova Ivan Dimitrov Stefan Ivanov Borislav Georgiev Strahil Berkov Dimitrina Zheleva-Dimitrova Irini Doytchinova 《Molecules (Basel, Switzerland)》2022,27(10)
Acetylcholinesterase (AChE) is one of the classical targets in the treatment of Alzheimer’s disease (AD). Inhibition of AChE slows down the hydrolysis of acetycholine and increases choline levels, improving the cognitive function. The achieved success of plant-based natural drugs acting as AChE inhibitors, such as galantamine (GAL) from Galanthus genus and huperzine A from Huperzia serrate (approved drug in China), in the treatment of AD, and the fact that natural compounds (NCs) are considered as safer and less toxic compared to synthetic drugs, led us to screen the available NCs (almost 150,000) in the ZINC12 database for AChE inhibitory activity. The compounds were screened virtually by molecular docking, filtered for suitable ADME properties, and 32 ligands from 23 structural groups were selected. The stability of the complexes was estimated via 1 μs molecular dynamics simulation. Ten compounds formed stable complexes with the enzyme and had a vendor and a reasonable price per mg. They were tested for AChE inhibitory and antioxidant activity. Five compounds showed weak AChE inhibition and three of them exhibited high antioxidant activity. 相似文献