全文获取类型
收费全文 | 14116篇 |
免费 | 330篇 |
国内免费 | 59篇 |
专业分类
化学 | 8041篇 |
晶体学 | 134篇 |
力学 | 523篇 |
数学 | 2593篇 |
物理学 | 3214篇 |
出版年
2022年 | 158篇 |
2021年 | 226篇 |
2020年 | 228篇 |
2019年 | 245篇 |
2018年 | 240篇 |
2017年 | 216篇 |
2016年 | 334篇 |
2015年 | 286篇 |
2014年 | 344篇 |
2013年 | 870篇 |
2012年 | 633篇 |
2011年 | 739篇 |
2010年 | 477篇 |
2009年 | 442篇 |
2008年 | 615篇 |
2007年 | 550篇 |
2006年 | 501篇 |
2005年 | 437篇 |
2004年 | 455篇 |
2003年 | 354篇 |
2002年 | 358篇 |
2001年 | 265篇 |
2000年 | 235篇 |
1999年 | 178篇 |
1998年 | 140篇 |
1997年 | 167篇 |
1996年 | 205篇 |
1995年 | 157篇 |
1994年 | 150篇 |
1993年 | 192篇 |
1992年 | 191篇 |
1991年 | 192篇 |
1990年 | 142篇 |
1989年 | 139篇 |
1988年 | 155篇 |
1987年 | 214篇 |
1986年 | 182篇 |
1985年 | 231篇 |
1984年 | 210篇 |
1983年 | 167篇 |
1982年 | 197篇 |
1981年 | 201篇 |
1980年 | 193篇 |
1979年 | 202篇 |
1978年 | 193篇 |
1977年 | 181篇 |
1976年 | 159篇 |
1975年 | 178篇 |
1974年 | 139篇 |
1973年 | 135篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
181.
The results of a CNDO/2 conformation analysis on dimethylmethylphosphonate are reported. Six stable conformers were found; their relative stabilities can be understood in terms of steric hindrance and gauche effect. Calculated barriers to internal rotation around P—O and O—C bonds are tabulated. 相似文献
182.
13CT1 relaxation times for the different carbons of the sodium dodecyl sulphate chain in micellar systems have been measured, using Gd3+ as a paramagnetic relaxation reagent. The fluorescence decay of ω-(-naphthyl) dodecanoic acid, solubflized in the sodium dodecyl sulphate micelles was obtained in the presence of various amounts of counterion quencher Both series of experiments point to the occurrence of chain folding and to the fact that the terminal group can approach the Stern region of the micelle. 相似文献
183.
W. Pyckhout P. Van Nuffel C. Van Alsenoy L. Van Den Enden H. J. Geise 《Journal of Molecular Structure》1983,102(3-4):333-345
The gas phase molecular structure of methyl vinyl ether at room temperature has been studied by joint analysis of electron diffraction and microwave data. Constraints on geometrical and thermal parameters were derived from the geometry and force field of the s-cis form, obtained by ab-initio calculations (4–21 G basis set) after complete geometry relaxation. A range of models was investigated that fits all available data (infrared, microwave and electron diffraction). The following rg/r-parameters were obtained: C=C: 1.337 Å, C(sp2)---O: 1.359 Å, C(sp3)---O: 1.427 Å, : 1.102 Å C=C---O : 127.3° and COC: 116.8°. Experimental rg---re (ab initio) corrections are given for C=C, C(sp2)---O and Csp3)---O.
This investigation demonstrates that molecular orbital constrained electron diffraction is sufficiently reliable and in such a manner that it can be applied to more complicated problems. 相似文献
184.
Gas-phase derivatization of [C3H5]+ ions using tandem mass spectrometry methods A13C labelling study
Nguyen Kieu My Marita Schilling Helmut Schwarz 《Journal of mass spectrometry : JMS》1987,22(5):254-258
The study of specifically 13C-labelled precursors sheds further light on the gas-phase chemistry of allyl and 2-propenyl cations. It is demonstrated that both species are formed from allyl and 2-propenyl bromide upon 70 eV electron impact ionization without skeletal reorganization. Gas-phase derivatization of the [C3 H5]+ ions with benzene facilitates, as suggested and observed earlier, the distinction of the two isomers using collision-induced dissociation of the Wheland complexes (or isomers thereof). The 13C labelling data clearly demonstrate that 64% of allyl cations survive the derivatization while 36% isomerize to 2-phenylpropyl cations; the latter are also formed via the reaction of 2-propenyl cation with benzene, protonation of α-methylstyrene and water loss from protonated 2-phenyl-2-propanol, respectively. Unimolecular loss of C2H4 from protonated allylbenzene proceeds via two competing reaction channels: one involves heterolysis of 1-phenylpropyl cations (~30%); the major pathway (~70%), however, involves decomposition via propylene benzenium ions. 相似文献
185.
Mass spectrometric analysis of ceramide perturbations in brain and fibroblasts of mice and human patients with peroxisomal disorders 总被引:2,自引:0,他引:2
Pettus BJ Baes M Busman M Hannun YA Van Veldhoven PP 《Rapid communications in mass spectrometry : RCM》2004,18(14):1569-1574
In this study, the levels and composition of ceramides in brains of newborn mice lacking peroxisomes (Pex5-/-, Zellweger mice) were analyzed using normal-phase high-performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (HPLC/APCI-MS). Total ceramide compositions were found to be comparable to that of control animals. However, a minor ceramide species, containing hexacosanoic/hexacosenoic acid as the amide fatty acid, was 9-fold increased. Also, in the sphingomyelin-derived ceramides this species was elevated. Subsequent analysis of extracts from fibroblasts of Pex5-/- mice and mice with a defective peroxisomal beta-oxidation (lacking D-specific multifunctional protein 2 (MFP2)), revealed, again, a similar rise in this particular ceramide. Further, this ceramide was elevated in human X-ALD fibroblasts as well. Whether C26:1/0-ceramide is linked to some of the pathology seen in Zellweger syndrome remains to be investigated. However, an increase in this sphingolipid can be considered as a diagnostic criterion for diseases caused by defects in peroxisome biogenesis or peroxisomal beta-oxidation. 相似文献
186.
Pham-Tran NN Bouchoux G Delaere D Nguyen MT 《The journal of physical chemistry. A》2005,109(12):2957-2963
We have investigated the basicity of phosphinine (C5H5P, phosphabenzene) in reevaluating its proton affinity (PA) and gas-phase basicity (GB) and the pK(a) value of its protonated form. As a necessary step, we have first determined its gas-phase proton affinity. Using both mass spectrometric and quantum chemical methods, we have obtained the values PA(C5H5P) = 195.8 +/- 1.0 kcal mol(-1) and GB(298)(C5H5P) = 188.1 +/- 1.0 kcal mol(-1), in good agreement with previous results. We then derived a value of pK(a)(C5H6P+) = -16.1 +/- 1.0 in aqueous solution using three different approaches: the latter markedly differs from the currently available value of -10. The reason for such a discrepancy in the pK(a) of protonated phosphinine in solution is discussed. In the theoretical determination of PAs, evaluation of the basis set superposition error (BSSE) showed that this effect is quite small, being 0.1-0.2 kcal mol(-1) for phosphinine, when a density functional theory (DFT) method in conjunction with a large basis set were used. 相似文献
187.
José M. Bartoloméa Jesús AlcázarJ.Ignacio Andrés Marcel De BruynJavier Fernández Encarna MatesanzKristof Van Emelen 《Tetrahedron letters》2003,44(47):8545-8548
The synthesis of a series of novel analogues of the 3-substituted-2,3-dihydro-1,4-dioxino[2,3-b]pyridine core, modified in the non-aromatic ring, is described. Due to the presence of a versatile hydroxymethyl group in their structure, these novel scaffolds are attractive intermediates for the preparation of potential new therapeutic agents. 相似文献
188.
Broussy S Coppel Y Nguyen M Bernadou J Meunier B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2034-2038
Isoniazid (INH) is easily oxidized with manganese(III) pyrophosphate, a chemical model of the KatG protein involved in activation of INH inside the bacteria Mycobacterium tuberculosis. Performed in the presence of NAD(+), this oxidation generates a family of isomeric INH-NAD(H) adducts, which have been shown to be effective inhibitors of InhA, an enzyme essential in mycolic acid biosynthesis. In this work, we fully characterized by (1)H and (13)C NMR spectroscopy four main species of INH-NAD(H) adducts that coexist in solution. Two of them are open diastereoisomers consisting of the covalent attachment of the isonicotinoyl radical at position four of the nicotinamide coenzyme. The other two result from a cyclization involving the amide group from the nicotinamide and the carbonyl group from the isonicotinoyl radical to give diastereoisomeric hemiamidals. Although an INH-NAD(H) adduct with a 4S configuration has been characterized within the active site of InhA from Xray crystallography and this bound adduct interpreted as an open form (Rozwarski et al., Science 1998, 279, 98-102), it is legitimate to raise the question about the effective active form(s), open or cyclic, of INH-NAD(H) adduct(s). Is there a single active form or are several forms able to inhibit the InhA activity with different levels of inhibitory potency? 相似文献
189.
A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst. 相似文献
190.
Perfluoromethyl Element Ligands. XXIX. Preparation and Spectroscopic Investigation of M(CO)4L2 Complexes (M ? Cr, Mo, W; L ? Me2PSMe, Me2PSeMe, (CF3)2PSMe, (CF3)2PSMe) The complexes M(CO)4L2 (see Inhaltsübersicht) have been prepared by the reaction of tetracarbonyl norbornadiene metal compounds M(CO)4NBD with L at room temperature or 35°C, respectively. The cis-complexes formed in the first step undergo rearrangement to trans-isomers at higher temperatures. New compounds have been characterized by analytical and spectroscopic (IR, NMR, MS) methods. 相似文献