全文获取类型
收费全文 | 16345篇 |
免费 | 435篇 |
国内免费 | 133篇 |
专业分类
化学 | 11283篇 |
晶体学 | 134篇 |
力学 | 510篇 |
数学 | 2509篇 |
物理学 | 2477篇 |
出版年
2022年 | 107篇 |
2021年 | 152篇 |
2020年 | 218篇 |
2019年 | 199篇 |
2018年 | 184篇 |
2017年 | 147篇 |
2016年 | 313篇 |
2015年 | 269篇 |
2014年 | 311篇 |
2013年 | 780篇 |
2012年 | 760篇 |
2011年 | 1037篇 |
2010年 | 485篇 |
2009年 | 437篇 |
2008年 | 844篇 |
2007年 | 879篇 |
2006年 | 929篇 |
2005年 | 854篇 |
2004年 | 779篇 |
2003年 | 710篇 |
2002年 | 682篇 |
2001年 | 227篇 |
2000年 | 197篇 |
1999年 | 159篇 |
1998年 | 190篇 |
1997年 | 199篇 |
1996年 | 232篇 |
1995年 | 179篇 |
1994年 | 184篇 |
1993年 | 147篇 |
1992年 | 151篇 |
1991年 | 157篇 |
1990年 | 131篇 |
1989年 | 113篇 |
1988年 | 147篇 |
1987年 | 142篇 |
1986年 | 107篇 |
1985年 | 212篇 |
1984年 | 234篇 |
1983年 | 169篇 |
1982年 | 241篇 |
1981年 | 220篇 |
1980年 | 224篇 |
1979年 | 206篇 |
1978年 | 198篇 |
1977年 | 181篇 |
1976年 | 182篇 |
1975年 | 152篇 |
1974年 | 162篇 |
1973年 | 155篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
181.
Kim W McMillan RA Snyder JP Conticello VP 《Journal of the American Chemical Society》2005,127(51):18121-18132
Stereoelectronic effects have been identified as contributing factors to the conformational stability of collagen-mimetic peptide sequences. To assess the relevance of these factors within other protein structural contexts, three polypeptide sequences were prepared in which the sequences were derived from the canonical repeat unit (Val-Pro-Gly-Val-Gly) of the protein material elastin. These elastin-mimetic polypeptides, elastin-1, elastin-2, and elastin-3, incorporate (2S)-proline, (2S,4S)-4-fluoroproline, and (2S,4R)-4-fluoroproline, respectively, at the second position of the elastin repeat. Calorimetric and spectroscopic investigations of these three polypeptides indicate that the incorporation of the substituted proline residues had a dramatic effect upon the self-assembly of the corresponding elastin peptide. The presence of (2S,4R)-4-fluoroproline in elastin-3 lowered the temperature of the phase transition and increased the type II beta-turn population with respect to the parent polypeptide, while the presence of (2S,4S)-4-fluoroproline in elastin-2 had the opposite effect. These results suggest that stereoelectronic effects could either enhance or hinder the self-assembly of elastin-mimetic polypeptides, depending on the influence of the proline analogue on the energetics of the beta-turn conformation that develops within the pentapeptide structural repeats above the phase transition. Density functional theory (DFT) was employed to model three possible turn types (betaI-, betaII-, and inverse gamma-turns) derived from model peptide segments (MeCO-Xaa-Gly-NHMe) (Xaa = Pro, 4S-F-Pro, or 4R-F-Pro) corresponding to the turn-forming residues of the elastin repeat unit (Val-Pro-Gly-Val-Gly). The results of the these calculations suggested a similar outcome to the experimental data for the elastin-mimetic polypeptides, in that type II beta-turn structures were stabilized for peptide segments containing (2S,4R)-fluoroproline and destabilized for segments containing (2S,4S)-fluoroproline relative to the canonical proline residue. 相似文献
182.
James J. Gallagher Edward K. Chess Saleh M. Arghestani Michael L. Gross 《Journal of mass spectrometry : JMS》1983,18(3):118-126
Mechanisms for decomposition of 1- and 2-phenyltetralins were investigated using low resolution mass spectrometry and metastable ion techniques. Four primary decompositions were observed for 1-phenyltetralin radical cations: (1) the loss of C6H6 via a 1,4-elimination; (2) the elimination of ethene via competing losses from carbons 3 + 4 and carbons 2 + 3; (3) the loss of C8H8, probably through a stepwise Diels-Alder cycloreversion to expel styrene; and (4) the loss of methyl radical involving carbon 2 and possibly carbon 4. Three major decompositions were observed for 2-phenyltetralin radical cations: (1) the loss of C8H8, possibly through a Diels-Alder cycloreversion to expel styrene; (2) the loss of C6H6 via a 1,3 elimination; and (3) the loss of methyl radical from carbon 1. Various exchange reactions occur prior to these losses, but they proved to be incomplete even for metastable ions. 相似文献
183.
High-field n.m.r. analysis of four 3-methoxy-14β-estra-1,3,5(10), 15-tetraen-17-ones provides evidence for conformational deformation of ring C to a twist-boat form in solution. These observations are supported by molecular mechanics (MM2) calculations, which predict that the ring C chair and ring C twist-boat conformers have similar steric energies, slightly favouring the latter. An X-ray crystal structure determination on 3-methoxy-14-methyl-14β-estra-1,3,5(10), 15-tetraen-17-one revealed that ring C does indeed adopt a twist-boat conformation in the solid state. 相似文献
184.
Mono-, di- and tri-alkyl-substituted allenes were potentiostatically oxidized in methanol at graphite and Pt anodes. At the former electrode, α-methoxylated ketones (due to 4F/mole electricity consumption) and esters (6F/mole) were the major products. At a Pt anode, intermediate products such as vinyl-ether derivatives (2F/mole) were characterised too. Unlike the anodic oxidation of alkenes and alkynes previously reproted in the literature, dimerisation is not a typical process in the allenes' oxidation, since of all the products obtained only a sole dimer has been observed. The mechanism for the formation of most products is discussed. 相似文献
185.
Christopher J. Chandler Leslie W. Deady James A. Reiss Vasilios Tzimos 《Journal of heterocyclic chemistry》1982,19(5):1017-1019
The preparation of a series of macrocycles, formed by reaction of HO-(-CH2-CH2-O-)nH with 1,10-phenanthroline-2,9-dicarbonyl chloride (n = 2,3,4) and 1,8-naphthyridine-2,7-dicarbonyl chloride (n = 3,4), is described. An improved synthetic route to 2,7-dimethyl-1,8-naphthyridine 9 is also reported. 相似文献
186.
The rate constants of para-/orthohydrogen (p-/o-H2) nuclear spin isomerization have been measured by means of 1H NMR in deuterated solvents at 298.2 K. The indicated reaction is catalyzed by paramagnetic complex ions giving rate constants that are proportional to the concentrations of the catalysts. The second-order rate constants are directly proportional to the squares of the magnetic moments for the solvated metal complexes for two classifications: M(solv)m2+, M = 3d transition metals; Ln(solv)n3+, where in 1:9 D2O-CD3CN the aqua complexes are the predominant species, Ln = lanthanides. The other 3d transition metal complexes with different ligands show rate constants that also depend on the sizes of ligands. Whereas the correlation between the second-order rate constants and magnetic moments is consistent with Wigner's theory, the size of catalyst shows a more modest effect on the rate constants than expected. The effective collision radii of the complexes, calculated from the rate constants, proved to be approximately constant for each series of solvated metal complexes. 相似文献
187.
[reaction: see text] Combinatorial screening of five catalyst precursors and nine ligands with three substituted aniline trapping reagents uncovered a catalyst system that promotes efficient palladium-catalyzed cyclization-trapping with a series of substituted anilines of varying steric and electronic character. The results of the parallel optimization study illustrate the interdependency of the key reaction variables. 相似文献
188.
Zhang Z Blom DA Gai Z Thompson JR Shen J Dai S 《Journal of the American Chemical Society》2003,125(25):7528-7529
One-dimensional (1D) magnetic nanomaterials have attracted much attention recently because of their applications in magnetic recording and spintronics. Nevertheless, it remains a challenge to prepare free-standing magnetic nanowires in high yield. This Communication reports the successful high-yield synthesis of an interesting 1D ferromagnetic CoPt alloy by direct decomposition of platinum acetylacetonate and cobalt carbonyl compound in ethylenediamine solvent through a solvothermal reaction. The CoPt alloy nanowires obtained have a tunable diameter of 10-50 nm and a length along the longitudinal axis of up to several microns, depending on crystallization temperature and reaction time. A unique formation mechanism involving coarsening and ripening under solvothermal conditions was discovered. This research opens new opportunities in synthesizing nanomaterials through low-temperature solvothermal processes. 相似文献
189.
Initially in this review the sodalite framework [T12O24]6– (T=Al, Si] is modeled with regular tetrahedra and disordered T atoms. Equations are given for calculating atomic coordinates from the unit cell parameter a and the T—O distancet; the expansion or contraction of the sodalite-cage is related quantitatively to changes ina through the cooperative twists of TO4 tetrahedra about 4 axes and changes in < TOT bridge angles. The fully expanded cage has=0° and the maximum value ofa. The equations are general for any framework formed by isomorphous substitution of T by atoms other than Al, Si and for any SiAl ratio. The model and equations are extended to the zeolite A framework, which can be built from fully expanded sodalite cages. With the cooperative tilt of the TO4 tetrahedra of zeolite A, described by Depmeier, the major variable features of the zeolite A framework are explained quantitatively. The faujasite framework has twisted-cages (>0), as in sodalite examples, and is quantitatively modeled most conveniently from sodalite examples with similar-cage contents. The review is extended to structures with T-ordering and distorted tetrahedra. Methods are given for estimating a for sodalites from a knowledge of the cavity contents, especially the sizes of cations and anions, and so on, present. Ways of predicting cavity sites in zeolite A as a function of cation size are presented, and the principal cavity sites in the faujasite-cage region are discussed. Finally the review considers isomorphous replacement of T atoms (Si or Al) by B, Be, Fe, Ga, Ge, and P; many of these substituted frameworks are stabilized by templates, or guest molecules, which reside in the cavities. Templates also stabilize Si, Al frameworks with high SiAl ratios. The modeling approach reviewed here is tested on a range of isomorphously substituted frameworks isotypic with sodalite; observed and calculated values of twist and 12O24]12– with distorted tetrahedra; these are structures with Al-O-Al bridges, which violate Loewenstein's Rule. 相似文献
190.
Abstract Porphyrins, which may be introduced into the eye as a result of abnormal porphyrin metabolism (uroporphyrin–Uro) or when used in the diagnosis or photodynamic therapy of certain tumors, including intraocular tumors (hematoporphyrin–Hp and'hematoporphyrin derivative'–Hpd and mesotetra( P -sulfonatophenyl)porphyrin–TPPS) are efficient photosensitizers in biological systems. We have been studying the potential phototoxic side effects of these drugs in the lens of the eye. Encapsulated in the human lens is a mixture of soluble protein crystallins. With little turnover of protein in the lens, any photosensitized modifications will accumulate and may result in an opacification of the lens. To evaluate the potential of different porphyrins to induce such damage, a series of porphyrins were photolyzed (transmission above 295 nm) in the presence of calf lens protein (2 mg m−1 ). Marked photopolymerization and histidine destruction were observed for the lens protein photolyzed in the presence of all of the drugs. We have found that the relative effectiveness of the following porphyrins to induce that damage is: Uro = TPPS Hpd = Hp. Both the singlet oxygen quencher, azide, and the free radical scavenger, penicillamine, decrease this photosensitized oxidative damage to lens protein. TPPS binds significantly to lens protein and this binding leads to conformational changes in that protein. 相似文献