全文获取类型
收费全文 | 11771篇 |
免费 | 518篇 |
国内免费 | 101篇 |
专业分类
化学 | 8272篇 |
晶体学 | 58篇 |
力学 | 284篇 |
数学 | 2116篇 |
物理学 | 1660篇 |
出版年
2023年 | 82篇 |
2022年 | 178篇 |
2021年 | 348篇 |
2020年 | 234篇 |
2019年 | 257篇 |
2018年 | 230篇 |
2017年 | 187篇 |
2016年 | 435篇 |
2015年 | 398篇 |
2014年 | 436篇 |
2013年 | 749篇 |
2012年 | 820篇 |
2011年 | 943篇 |
2010年 | 589篇 |
2009年 | 534篇 |
2008年 | 779篇 |
2007年 | 777篇 |
2006年 | 740篇 |
2005年 | 656篇 |
2004年 | 531篇 |
2003年 | 396篇 |
2002年 | 377篇 |
2001年 | 136篇 |
2000年 | 132篇 |
1999年 | 98篇 |
1998年 | 107篇 |
1997年 | 102篇 |
1996年 | 126篇 |
1995年 | 75篇 |
1994年 | 71篇 |
1993年 | 70篇 |
1992年 | 84篇 |
1991年 | 67篇 |
1990年 | 39篇 |
1989年 | 46篇 |
1988年 | 33篇 |
1987年 | 34篇 |
1986年 | 41篇 |
1985年 | 57篇 |
1984年 | 56篇 |
1983年 | 37篇 |
1982年 | 47篇 |
1981年 | 39篇 |
1980年 | 38篇 |
1979年 | 28篇 |
1978年 | 28篇 |
1977年 | 22篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
A study of the viability of three derivatizing reagents for obtaining amino acid profiles in honey through high performance liquid chromatography (HPLC) is presented. A method using diode array detection based on a reaction with diethyl ethoxymethylene malonate (DEMM) and two other methods using fluorescence detection based on derivatization with fluorenylmethyl chloroformate (FMOC-Cl) and 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) have been developed. The three methods yield detection limits close to the ppb level, but vary in relation to other analytical characteristics. The use of methyl chloroformate derivatives allows the profile to be obtained with the greatest sensitivity within a short time frame. On applying such methods to honey samples of diverse botanical origin, we observe that the proline values obtained are always lower than those found using the official spectrophotometric method, thereby underlining the advisability of using HPLC methods to reduce uncertainty in these results. 相似文献
102.
Alberto Avenoza Jesús H Busto Carlos Cativiela Francisco Corzana Jesús M Peregrina María M Zurbano 《The Journal of organic chemistry》2002,67(2):598-601
A short and efficient asymmetric synthesis of the (6R,7S)-7-tert-butoxycarbonylamino-2-ketocarbacepham is described. The key step involves the hetero Diels-Alder reaction of the benzylimine derived from the enantiomer of Garner's aldehyde with Danishefsky's diene. 相似文献
103.
Ricardo Baggio Juan Carlos Muoz Mireille Perec 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(10):m498-m500
The title compound, [Dy2(C2H3O2)6(H2O)4]·4H2O, crystallizes in the form of dimeric units related by an inversion centre. Each cation is nine‐coordinate, binding to two water molecules and three acetate groups, two of which are bidentate and the third tridentate. This last acetate group acts as a bridge between neighbouring metal atoms, leading to an intradimer Dy?Dy separation of 4.170 (1) Å. 相似文献
104.
105.
The rate of the Wolff-Kishner reaction of benzophenone hydrazone in butyl carbitol increases as the cation of the alkoxide base is varied in the order K>Na>Li>Mg. The replacement of butyl carbitol by 1-decanol also accelerates the reaction, and an additional increase is caused by the presence of crown ether. On the basis of changes in the activation parameters, it is concluded that the reactivity of the hydrazone anion increases as the contact ion pair becomes more fully solvated and dissociated. 相似文献
106.
Reinoso S Vitoria P Gutiérrez-Zorrilla JM Lezama L San Felices L Beitia JI 《Inorganic chemistry》2005,44(26):9731-9742
Reaction of in situ generated copper(II)-monosubstituted Keggin polyoxometalates and copper(II)-bipyridine-oxalate complexes in the corresponding alkaline acetate buffer led to the formation of hybrid metal organic-inorganic compounds K(2)[{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(H(2)O)(2)(mu-ox)}(2)].14H(2)O (1), K(14)[{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(mu-ox)}](2)[SiW(11)O(39)Cu(H(2)O)].55H(2)O (2), (NH(4))(4)[{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(mu-ox)}].10H(2)O (3), and Rb(4)[{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(mu-ox)}].10H(2)O (4). Their structures have been established by single-crystal X-ray diffraction. The main structural feature of these compounds is the presence of copper(II)-monosubstituted alpha-Keggin polyoxoanions as inorganic building blocks, on which the mu-oxalatodicopper metalorganic blocks are supported. Compound 1contains the discrete hybrid polyanion [{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(H(2)O)(2)(mu-ox)}(2)](2)(-), whereas the polymeric hybrid polyanion [{SiW(11)O(39)Cu(H(2)O)}{Cu(2)(bpy)(2)(mu-ox)}(2)](n)(4)(n)(-) gives a monodimensional character to compounds 2-4. Magnetic and EPR results are discussed with respect to the crystal structure of the compounds. DFT calculations on both the [Cu(2)(bpy)(2)(H(2)O)(4)(mu-ox)](2+) cationic complex and the metalorganic blocks have been performed in order to determine the optimized geometry and the magnetic coupling constants, respectively. 相似文献
107.
A novel asymmetric, edge-sharing bioctahedral complex with formamidinato ligands (
2-DAniF)Mo(-DAniF)2(-O,Cl)MoCl2, 1, (DAniF=N,N-di-p-anisylformamidinate) has been isolated as a byproduct from the preparation of the dimolybdenum(II,III) compound Mo2(DAniF)3Cl2. An X-ray crystallographic study shows that the structure consists of edge-sharing bioctahedra, with the shared edge defined by the vector joining the: -O and -Cl ligands. Compound 1 is best formulated as a mixed-valent MoIIIMoV compound, the Mo(V) ion being that with the two terminal Cl– ligands. The cyclic voltammogram of 1 shows a reversible reduction at –0.472 V and a reversible oxidation at 1.028 V vs. the Ag/AgCl reference electrode, while the absorption spectrum of 1 reveals an intense low energy absorption at 523 nm (
max=12 500) which is attributed to an intervalence charge transfer transition. Crystallographic data for 1 are as follows: a=13.387(1) Å, b=15.500(2) Å, c=21.855(2) Å, =98.786(2)°, V=4481.8(8) Å3, P21/c, R1 (wR2)=0.082 (0.177). 相似文献
108.
Luis JM Torrent-Sucarrat M Solà M Bishop DM Kirtman B 《The Journal of chemical physics》2005,122(18):184104
Our new simple method for calculating accurate Franck-Condon factors including nondiagonal (i.e., mode-mode) anharmonic coupling is used to simulate the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum. An improved vibrational basis set truncation algorithm, which permits very efficient computations, is employed. Because the torsional mode is highly anharmonic it is separated from the other modes and treated exactly. All other modes are treated through the second-order perturbation theory. The perturbation-theory corrections are significant and lead to a good agreement with experiment, although the separability assumption for torsion causes the C2D4 results to be not as good as those for C2H4. A variational formulation to overcome this circumstance, and deal with large anharmonicities in general, is suggested. 相似文献
109.
Synthetic erythropoietic proteins: tuning biological performance by site-specific polymer attachment
Chen SY Cressman S Mao F Shao H Low DW Beilan HS Cagle EN Carnevali M Gueriguian V Keogh PJ Porter H Stratton SM Wiedeke MC Savatski L Adamson JW Bozzini CE Kung A Kent SB Bradburne JA Kochendoerfer GG 《Chemistry & biology》2005,12(3):371-383
Chemical synthesis in combination with precision polymer modification allows the systematic exploration of the effect of protein properties, such as charge and hydrodynamic radius, on potency using defined, homogeneous conjugates. A series of polymer-modified synthetic erythropoiesis proteins were constructed that had a polypeptide chain similar to the amino acid sequence of human erythropoietin but differed significantly in the number and type of attached polymers. The analogs differed in charge from +5 to -26 at neutral pH and varied in molecular weight from 30 to 54 kDa. All were active in an in vitro cell proliferation assay. However, in vivo potency was found to be strongly dependent on overall charge and size. The trends observed in this study may serve as starting points for the construction of more potent synthetic EPO analogs in the future. 相似文献
110.
Barriga S Fuertes P Marcos CF Rakitin OA Torroba T 《The Journal of organic chemistry》2002,67(18):6439-6448
We report the synthesis of some new polysulfur-nitrogen heterocycles by cycloaddition reactions to readily available tricyclic condensed 1,2-dithiole-3-thiones. Thus, treatment of bis[1,2]dithiolopyrrole ketothione 1 with diacyl acetylenes gave the bis-aducts 2a-d. On the other hand, cycloaddition of bis[1,2]dithiolo[1,4]thiazine ketothione 3 with 1 equiv of acyl or diacyl acetylenes gave [1,3]dithiolylidenyl[1,2]dithiolo[1,4]thiazines 4a-f in fair to high yields. Catalysis by scandium triflate was used in the reactions that implied the less reactive dipolarophiles. Treatment of 3 with 2 equiv of DBA gave the bis-aduct 5a, and reaction of 4c with DMAD gave the mixed bis-adduct 5b. Cyclic voltammetry of selected examples showed irreversible processes that were influenced by the electrochemical activity of peripheral groups bonded to the heterocyclic system. 相似文献