全文获取类型
收费全文 | 23532篇 |
免费 | 1462篇 |
国内免费 | 3452篇 |
专业分类
化学 | 19216篇 |
晶体学 | 1057篇 |
力学 | 218篇 |
综合类 | 192篇 |
数学 | 3686篇 |
物理学 | 4077篇 |
出版年
2024年 | 11篇 |
2023年 | 169篇 |
2022年 | 495篇 |
2021年 | 738篇 |
2020年 | 542篇 |
2019年 | 538篇 |
2018年 | 423篇 |
2017年 | 558篇 |
2016年 | 590篇 |
2015年 | 550篇 |
2014年 | 816篇 |
2013年 | 1777篇 |
2012年 | 1291篇 |
2011年 | 1487篇 |
2010年 | 1483篇 |
2009年 | 1678篇 |
2008年 | 1625篇 |
2007年 | 1643篇 |
2006年 | 1555篇 |
2005年 | 1341篇 |
2004年 | 1308篇 |
2003年 | 1108篇 |
2002年 | 946篇 |
2001年 | 709篇 |
2000年 | 695篇 |
1999年 | 530篇 |
1998年 | 519篇 |
1997年 | 425篇 |
1996年 | 361篇 |
1995年 | 367篇 |
1994年 | 323篇 |
1993年 | 303篇 |
1992年 | 286篇 |
1991年 | 213篇 |
1990年 | 122篇 |
1989年 | 153篇 |
1988年 | 136篇 |
1987年 | 89篇 |
1986年 | 61篇 |
1985年 | 71篇 |
1984年 | 90篇 |
1983年 | 23篇 |
1982年 | 44篇 |
1981年 | 60篇 |
1980年 | 51篇 |
1979年 | 38篇 |
1978年 | 27篇 |
1977年 | 22篇 |
1976年 | 11篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
Stereochemistry of 7-aryl-1,7,8,8a-tetrahydro-3(2H)-indolizinones was studied by1H and13C NMR. Complete assignment of1H NMR signals and analysis of1H-1H coupling constants were performed using the iterative PANIC program. Values of3
J
6,7,3
J
7,8endo, and4
J
5,7 allow one to unambiguously identify the correspondingexo- andendo-stereoisomers. For stereoisomers with exo-orientation of H(7), complete assignment of13C NMR signals was performed on the basis of analysis of the13C-1H coupling constants using two dimensional heteronuclear shift-correlating spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 591–593, March, 1996. 相似文献
932.
Gerhard Buchbauer Angela Fischlmayr Ernst Haslinger Wolfgang Robien Horst Völlenkle Carla Wassmann 《Monatshefte für Chemie / Chemical Monthly》1984,115(6-7):739-748
2-Hydroxy-13-oxo-tricyclo[7.3.1.02,7]tridecane derivatives can be obtained by reaction of cyclohexanone with alcohols under alkaline conditions. The unambiguous assignment of all signals of the1H-NMR- and13C-NMR-spectrum is possible by 2D-1H-13C-shift correlation and 2D-INADEQUATE. Compound1 crystallizes in space group P21/n witha=8.518 (1),b=14.789 (2),c=19.321 (2) Å, =94.91 (1)°,Z=8,D
c
=1.22 Mg cm–3. The structure refined toR=0.100 andR
w
=0.097 for 1719 observed reflections. Two independent molecules form centrosymmetric hydrogen-bonded dimers.Part V:Haslinger E.,Kalchhau-ser H.,Robien W.,Steindl H., Monatsh. Chem.115, 597 (1984). 相似文献
933.
Herta Effenberger 《Monatshefte für Chemie / Chemical Monthly》1986,117(10):1099-1106
Crystals of PbCu3(OH)(NO3)(SeO3)3·1/2H2O [a=7.761(3)Å,b=9.478(4)Å,c=9.514(4)Å, =66.94(2)°, =69.83(2)°, =81.83(2)°, space group P
,Z=2] and Pb2Cu3O2(NO3)2(SeO3)2 [a=5.884(2)Å,b=12.186(3)Å,c=19.371(4)Å, space group Cmc21,Z=4] were synthesized under hydrothermal conditions. Their crystal structures were refined with three-dimensional X-ray data toR
w=0.033 resp. 0.055. In PbCu3(OH)(NO3)(SeO3)3·1/2H2O the Cu atoms are [4+1] and [4+2] coordinated and via SeO3 groups a three-dimensional atomic arrangement is built up. In Pb2Cu3O2(NO3)2(SeO3)2 there are
sheets, which are connected only via Pb-O bonds ranging from 2.98 Å to 3.16 Å. 相似文献
934.
Various kinds of aluminum species in dealuminated mordenite were investigated in detail, and the quadrupole coupling constants
(QCCs) for aluminum atoms associated with these species were obtained by means of the newly introduced1H/27 AI TRAPWR method as well as27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). QCC values of 11.3, 15.3, 13.3 and (14.0± 0.6) MHz were determined
from the TRAPDOR profiles for Lewis acid sites, Bronsted acid sites (SiOHAl) and two kinds of non-framework aluminum species
Al(OH)
n
, respectively. The source of the “invisible Al” is discussed on the basis of the NMR experimental results. 相似文献
935.
Synthesis and spectral characterization of some complexes of platinum(II) containing η-methyleugenol
Several complexes of the formula trans-[Pt(Meug)(Am)Cl2], Meug: methyleugenol (4-allyl-1,2-dimethoxybenzene), a η2-coordinated olefin, and Am: ammine, methylamine, diethylamine, o-toluidine, m-toluidine, p-toluidine, o-anisidine, m-anisidine and p-anisidine have been prepared. UV, IR, Raman, 1H NMR, 13C NMR and 2D NMR spectra of the complexes were recorded and analyzed. 相似文献
936.
V. V. Bakharev A. A. Ghidaspov D. B. Krivolapov E. V. Mironova I. A. Litvinov 《Chemistry of Heterocyclic Compounds》2006,42(8):1051-1058
2-Amino-4-azido-1,3,5-triazin-6(1H)-ones were synthesized by successive substitution of the trinitromethyl groups in 2-amino-4,6-bis(trinitromethyl)-1,3,5-triazines
under the influence of azide and nitrite ions. Interaction of 2-amino-4-azido-1,3,5-triazin-6(1H)-ones with bases led to the
azido-tetrazole tautomeric conversion give salts of 5-aminotetrazolo[1,5-a]-1,3,5-triazin-7-one.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1211–1219, August, 2006. 相似文献
937.
Mordvintsev DY Polyak YL Levtsova OV Tourleigh YV Kasheverov IE Shaitan KV Utkin YN Tsetlin VI 《Computational Biology and Chemistry》2005,29(6):398-411
Short-chain alpha-neurotoxins from snakes are highly selective antagonists of the muscle-type nicotinic acetylcholine receptors (nAChR). Although their spatial structures are known and abundant information on topology of binding to nAChR is obtained by labeling and mutagenesis studies, the accurate structure of the complex is not yet known. Here, we present a model for a short alpha-neurotoxin, neurotoxin II from Naja oxiana (NTII), bound to Torpedo californica nAChR. It was built by comparative modeling, docking and molecular dynamics using 1H NMR structure of NTII, cross-linking and mutagenesis data, cryoelectron microscopy structure of Torpedo marmorata nAChR [Unwin, N., 2005. Refined structure of the nicotinic acetylcholine receptor at 4A resolution. J. Mol. Biol. 346, 967-989] and X-ray structures of acetylcholine-binding protein (AChBP) with agonists [Celie, P.H., van Rossum-Fikkert, S.E., van Dijk, W.J., Brejc, K., Smit, A.B., Sixma, T.K., 2004. Nicotine and carbamylcholine binding to nicotinic acetylcholine receptors as studied in AChBP crystal structures. Neuron 41 (6), 907-914] and antagonists: alpha-cobratoxin, a long-chain alpha-neurotoxin [Bourne, Y., Talley, T.T., Hansen, S.B., Taylor, P., Marchot, P., 2005. Crystal structure of Cbtx-AChBP complex reveals essential interactions between snake alpha-neurotoxins and nicotinic receptors. EMBO J. 24 (8), 1512-1522] and alpha-conotoxin [Celie, P.H., Kasheverov, I.E., Mordvintsev, D.Y., Hogg, R.C., van Nierop, P., van Elk, R., van Rossum-Fikkert, S.E., Zhmak, M.N., Bertrand, D., Tsetlin, V., Sixma, T.K., Smit, A.B., 2005. Crystal structure of nicotinic acetylcholine receptor homolog AChBP in complex with an alpha-conotoxin PnIA variant. Nat. Struct. Mol. Biol. 12 (7), 582-588]. In complex with the receptor, NTII was located at about 30 A from the membrane surface, the tip of its loop II plunges into the ligand-binding pocket between the alpha/gamma or alpha/delta nAChR subunits, while the loops I and III contact nAChR by their tips only in a 'surface-touch' manner. The toxin structure undergoes some changes during the final complex formation (for 1.45 rmsd in 15-25 ps according to AMBER'99 molecular dynamics simulation), which correlates with NMR data. The data on the mobility and accessibility of spin- and fluorescence labels in free and bound NTII were used in MD simulations. The binding process is dependent on spontaneous outward movement of the C-loop earlier found in the AChBP complexes with alpha-cobratoxin and alpha-conotoxin. Among common features in binding of short- and long alpha-neurotoxins is the rearrangement of aromatic residues in the binding pocket not observed for alpha-conotoxin binding. Being in general very similar, the binding modes of short- and long alpha-neurotoxins differ in the ways of loop II entry into nAChR. 相似文献
938.
V. L Gein E. N. Bezmaternykh L. F. Gein I. V. Krylova 《Chemistry of Heterocyclic Compounds》2004,40(10):1332-1334
It has been shown that 5-aryl-3-arylamino-4-benzoyl-1H-3-pyrrolin-2-ones cyclize in the presence of conc. H2SO4 to give pyrrolo[3,4-b]quinolines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1543–1545, October, 2004. 相似文献
939.
Cellulose-based hydrogel materials were prepared and modified with tannic acid and l-methionine using ionic liquid as the solvent. The gels were prepared to develop a sustained release medium for selenourea (SeU). The drug delivery characteristics of selenourea-loaded cellulose (CSeU), selenourea-loaded tannic acid-modified cellulose (CTSeU), and selenourea-loaded L-methionine-modified cellulose (CMSeU) were investigated in aqueous media and simulated gastric fluid (SGF) media. This modified gel beads have been characterized using field emission scanning electron microscope, X-ray energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, thermogravimetry–differential thermal analysis and swelling properties and compared with those of the unmodified ones. We also investigated the inhibitory effects of SeU released from these gels on the activity of mushroom tyrosinase. Out of all the gel materials, CTSeU showed maximum SeU release both in water and SGF media. However, tyrosinase inhibitory action in PBS medium was comparable for all the three gel materials. 相似文献
940.
Vashchenko V. V. Kutulya L. A. Pivnenko M. N. Shkolnikova N. I. 《Russian Chemical Bulletin》2003,52(11):2406-2418
(1R,4R)-2-(4-Hydroxybenzylidene)- and (1R,4R)-2-(4"-hydroxybiphenyl-4-yl)methylene-p-menthan-3-ones were synthesized by condensation of (–)-menthone with O-tetrahydropyran-2-yl derivatives of 4-hydroxybenzaldehyde and 4"-hydroxy-4-formylbiphenyl, respectively, in a DMSO—base medium followed by the removal of the protective group. The reactions of these hydroxy derivatives with 4-alkylbenzoic, 4-alkyloxybenzoic, trans-4-alkylcyclohexane-4-carboxylic, and 4"-alkylbiphenyl-4-carboxylic acids afforded three series of new chiral esters. Compounds containing the arylidene moiety with three benzene rings were found to exhibit liquid-crystalline properties. The characteristic features of these compounds are discussed based on the results of studies by polarizing microscopy, differential scanning calorimetry, and small-angle X-ray scattering. It was found that the mesomorphic compounds under study can form a smectic A mesophase, twist grain boundary mesophases (TGBA), and blue phases in a wide temperature range. Upon dissolution of certain of chiral compounds in 4"-cyano-4-pentylbiphenyl, a rather high twisting power and the thermal stabilizing effect on mesophases were observed. 相似文献