全文获取类型
收费全文 | 207068篇 |
免费 | 35035篇 |
国内免费 | 19935篇 |
专业分类
化学 | 152360篇 |
晶体学 | 2166篇 |
力学 | 11458篇 |
综合类 | 1160篇 |
数学 | 23843篇 |
物理学 | 71051篇 |
出版年
2024年 | 580篇 |
2023年 | 3419篇 |
2022年 | 5983篇 |
2021年 | 6654篇 |
2020年 | 7751篇 |
2019年 | 8344篇 |
2018年 | 6794篇 |
2017年 | 6050篇 |
2016年 | 11013篇 |
2015年 | 10941篇 |
2014年 | 12803篇 |
2013年 | 16383篇 |
2012年 | 18097篇 |
2011年 | 17800篇 |
2010年 | 13397篇 |
2009年 | 12989篇 |
2008年 | 13298篇 |
2007年 | 11666篇 |
2006年 | 10689篇 |
2005年 | 9179篇 |
2004年 | 7346篇 |
2003年 | 6020篇 |
2002年 | 6297篇 |
2001年 | 4992篇 |
2000年 | 4531篇 |
1999年 | 3986篇 |
1998年 | 3160篇 |
1997年 | 2961篇 |
1996年 | 2975篇 |
1995年 | 2531篇 |
1994年 | 2276篇 |
1993年 | 1924篇 |
1992年 | 1710篇 |
1991年 | 1513篇 |
1990年 | 1241篇 |
1989年 | 958篇 |
1988年 | 714篇 |
1987年 | 626篇 |
1986年 | 622篇 |
1985年 | 498篇 |
1984年 | 328篇 |
1983年 | 264篇 |
1982年 | 220篇 |
1981年 | 133篇 |
1980年 | 92篇 |
1979年 | 42篇 |
1978年 | 25篇 |
1976年 | 24篇 |
1975年 | 24篇 |
1957年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The cyclopentadienes 3a/b/c, 8b/c, 12a/b/d , and 16a/b/d were prepared as mixtures of regioisomers from the D -mannitol-derivatives 1,6,10 , and 14 and transformed into the ferrocenes 17, 18 , and 19 (73%; 38:17:45), 23, 24 , and 25 (70%; 6:42 :52), 26 (31%), and 27 (27%), respectively. Deprotection of 17–19 with HCl/MeOH gave the H2O-soluble ferrocenes 20–22 ; chloromercuration and iodination of 17 via 29 led to the C2-symmetric diiodo-ferrocene 30 . The mono(chloromercurio) derivative 28 , obtained as a by-product, was also transformed into 29 . The structure of the ferrocenes 18 and 19 , and of the bis(chloromercurio)ferrocene 29 has been established by X-ray analysis. The starting cyclopentadienes 3 were obtained in 50% yield from 1 by dialkylation of CpNa, followed by thermolysis of the spiro-annulated 2 . Similarly, dimesylate 6 (from 4 ) gave the spiro-annulated diene 7 and the annulated dienes 8 in about equal amounts; thermolysis transformed 7 into 8 (62%). The dienes 12 were prepared in 15% yield from the ditriflate 10 via 11 , the dimesylate 9 proving insufficiently reactive, and the dienes 16 (49%) from 14 via 15 . 相似文献
992.
Nicola Armaroli Gianluca Accorsi JohnN. Clifford Jean‐Franois Eckert Jean‐Franois Nierengarten 《化学:亚洲杂志》2006,1(4):564-574
Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C60 by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C60G1 – C60G4 , reaching a 99:1 ratio for C60G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C60G1 – C60G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C60 singlet energy transfer. In CH2Cl2, the OPV→C60 electron transfer from the lowest fullerene singlet level (1C60*) is slightly exergonic (ΔGCS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C60G2 – C60G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C60 electron transfer observed for C60G3 and C60G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer. 相似文献
993.
Ni(OH)2 hollow microspheres with beta-Ni(OH)2 nanosheets as the in situ formed building units were fabricated via a novel template-free approach in a strong alkaline solution of glycine, and can be converted into NiO hollow microspheres by a thermal decomposition process. 相似文献
994.
J. Losada I. Cuadrado B. González J. Zhang 《Journal of organometallic chemistry》2004,689(17):2799-2807
Cyclosiloxane and silsesquioxane-based ferrocenyl and permethylferrocenyl polymers have been used as mediators in amperometric enzyme electrodes for the detection of glucose. Biosensors have been prepared by electrostatically immobilizing the enzyme glucose oxidase (GOx) on electrodes modified with the polymers. The steady-state amperometric response of the sensors is investigated as a function of the applied potential and substrate concentration. The dependence of the sensors response on the structure of the siloxane-framework and on the presence or not of methyl groups on the ferrocenyl units is discussed. 相似文献
995.
Self-assembly and characterization of paclitaxel-loaded N-octyl-O-sulfate chitosan micellar system 总被引:3,自引:0,他引:3
N-octyl-O-sulfate chitosan micellar system loaded paclitaxel was prepared by using dialysis method. The critical micelle concentration (CMC) of the modified chitosan was found to be 0.45 mg/ml. Compared with the amount of N-octyl-O-sulfate chitosan, the paclitaxel loading amount in the system was up to 25% (w/w), depending on both of the solvents used in dialysis and the feed weight ratio of paclitaxel to the derivative. The polymeric micelles forming and loading occurred simultaneously in the dialysis process when ethanol and water were utilized as the solvents for paclitaxel and the polymer, respectively. Paclitaxel-loaded micellar system of N-octyl-O-sulfate chitosan was characterized by DSC, WXRD and TEM. TEM photograph revealed that paclitaxel existed as the colloid particulates in ethanol before loading and in the cores of the spherical polymeric micelles after loading. The results of DSC and WXRD indicated that paclitaxel was transferred from the crystalline state to amorphous state after loading. The lyophilized powder of micellar system (25% (w/w) loading) could be reconstituted easily in aqueous media even after 2 months storage at 4 degrees C without the change of paclitaxel entrapment and micelle size. The reconstituted solution (2.1 mg paclitaxel/ml) also showed good stability. The dilution with saline may decrease the loading and physical stability based on the dilution times which was related with CMC of the polymer. In vitro tests showed that paclitaxel was slowly released from micellar solution and the release lasted up to 220 h by means of the dialysis method. 相似文献
996.
Yuanyou Yang Ning Liu Shunzhong Luo Jiali Liao Jiannan Jin Taiming Zhang Pengji Zhao 《Journal of Radioanalytical and Nuclear Chemistry》2004,260(3):659-663
Biosorption of 241Am by a fungus A. niger, including the spore and hyphae, was investigated. The preliminary results showed that the adsorption of 241Am by the microorganism was efficient. More than 96% of the total 241Am could be removed from 241Am solutions of 5.6-111 MBq/l (C
o) by spore and hyphaeof A. niger, with adsorbed 241Am metal (Q) of 7.2-142.4 MBq/g biomass, and 5.2-106.5 MBq/g, respectively. The biosorption equilibrium was achieved within 1 hour and
the optimum pH range was pH 1-3. No obvious effects on 241Am adsorption by the fungus were observed at 10-45 °C, or in solutions containing Au3+ or Ag+, even 2000 times above the 241Am concentration. The 241Am biosorption by the fungus obeys the Freundlich adsorption equation. There was no significant difference between the adsorption
behavior of A. nigerspore and hyphae.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
997.
Rapid and reversible G-quadruplex hairpin dimer formation is observed for bis(oligonucleotide) conjugates possessing stilbenediether (Sd) linkers connecting two short poly(G) sequences. 相似文献
998.
Chunli Zhang Xueshen Liu Peizhu Ding Yueying Qi 《Journal of mathematical chemistry》2006,39(3-4):451-463
Asymptotic boundary condition (ABC) of laser-atom interaction presented recently is applied to transform the initial value problem of the time-dependent Schrödinger equation (TDSE) in infinite space into the initial and boundary value problem in the finite space, and then the TDSE is discretized into linear canonical equations by substituting the symmetry difference quotient for the 2-order partial derivative. The canonical equation is solved by symplectic algorithm. The ground state and the equal weight coherent superposition of the ground state and the first excited state have been taken as the initial conditions, respectively, while we calculate the population of bound states, the evolution of average distance and the high-order harmonic generation (HHG). The conversion efficiency of HHG can be enhanced by initial coherent superposition state and moderate laser intensities 相似文献
999.
A novel one-dimensional complex [Zn(NIT4py)2(DTB)2(H2O)2] (1), with mixed ligands [where NIT4py is 2-(4′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and DTB is 3,5-dinitrobenzoate] has been synthesized and characterized by elemental analyses, i.r., u.v.–vis spectra, thermogravimetric analysis, X-ray single crystal diffraction and magnetic measurements. The complex crystallizes in the triclinic crystal system and space group Pî. The Zn II ion is in a distorted octahedral environment: two nitrogen atoms from two NIT4py entities, two oxygen atoms from two DTB units in the basal plane; and two oxygen atoms from the two water molecules in the axial position. The [Zn(NIT4py)2(DTB)2(H2O)2] units are connected as a one dimension chain by the intermolecular hydrogen bonds. The complex exhibits intramolecular antiferromagnetic interactions between the two radicals. 相似文献
1000.
Lei He Hui‐Min Liu Xiao‐Lan Luo Wen‐Qin Zhang 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(2):o104-o106
The title compound, 2‐{N‐[2‐(2‐hydroxybenzamido)ethylammonioethyl]aminocarbonyl}phenolate, C18H21N3O4, crystallizes in a zwitterionic form as a result of intermolecular proton transfer and possesses a negatively charged phenolate group and a protonated amino group. The 2‐hydroxybenzamide and 2‐(aminocarbonyl)phenolate moieties attached to the two ends of the C—C—N—C—C backbone adopt a cis conformation in relation to this backbone. All N‐ and O‐bound H atoms are involved in hydrogen‐bond formation; the zwitterions are first linked into head‐to‐tail dimers, which are further organized into a two‐dimensional network parallel to the crystallographic bc plane. 相似文献