全文获取类型
收费全文 | 1024篇 |
免费 | 43篇 |
国内免费 | 8篇 |
专业分类
化学 | 811篇 |
晶体学 | 16篇 |
力学 | 13篇 |
数学 | 94篇 |
物理学 | 141篇 |
出版年
2022年 | 9篇 |
2021年 | 10篇 |
2020年 | 22篇 |
2019年 | 24篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 20篇 |
2015年 | 20篇 |
2014年 | 25篇 |
2013年 | 60篇 |
2012年 | 70篇 |
2011年 | 73篇 |
2010年 | 47篇 |
2009年 | 33篇 |
2008年 | 63篇 |
2007年 | 58篇 |
2006年 | 52篇 |
2005年 | 64篇 |
2004年 | 44篇 |
2003年 | 39篇 |
2002年 | 40篇 |
2001年 | 15篇 |
2000年 | 17篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 10篇 |
1996年 | 15篇 |
1995年 | 5篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 3篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1985年 | 3篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 14篇 |
1975年 | 11篇 |
1974年 | 8篇 |
1973年 | 21篇 |
1972年 | 7篇 |
1971年 | 3篇 |
1957年 | 3篇 |
排序方式: 共有1075条查询结果,搜索用时 15 毫秒
11.
M R Winchester W R Kelly J L Mann W F Guthrie B S MacDonald G C Turk 《Fresenius' Journal of Analytical Chemistry》2001,370(2-3):234-240
The S mass fractions of coal SRMs 2682b, 2684b, and 2685b are certified by direct comparison with coal SRMs 2682a, 2684a, and 2685a, respectively, using high-temperature combustion analysis with infrared (IR) absorption detection. The S mass fractions of the "a" materials used for calibration were previously determined by means of isotope-dilution thermal-ionization mass spectrometry (ID-TIMS). Therefore, the comparisons performed with the combustion-IR absorption method establish direct traceability links to accurate and precise ID-TIMS measurements. The expanded uncertainties associated with the certified S mass fractions are of approximately the same magnitude as would be expected for the ID-TIMS methodology. An important aspect of these certifications is that each "b" material is essentially identical with the corresponding "a" material, because both were produced from the same bulk, homogenized coal. As a test of the efficacy of the new certification approach when calibrant and unknown are not identical, the S mass fraction of coal SRM 2683b has been determined by direct comparison to coal SRM 2683a. These two coals, which have both previously been analyzed with ID-TIMS, are different in terms of S content and other properties. Whereas the S mass fraction for SRM 2683b determined with the new methodology agrees statistically with the ID-TIMS value, there is reason for caution in such cases. In addition to the usefulness of the alternative approach for certification activities within NIST, this approach might also be an excellent way of establishing NIST traceability during the value assignment process for reference materials not issued by NIST. Further research is needed, however, to understand better the scope of applicability. 相似文献
12.
Fry BG Wüster W Ryan Ramjan SF Jackson T Martelli P Kini RM 《Rapid communications in mass spectrometry : RCM》2003,17(18):2047-2062
The evolution of the venomous function of snakes and the diversification of the toxins has been of tremendous research interest and considerable debate. It has become recently evident that the evolution of the toxins in the advanced snakes (Colubroidea) predated the evolution of the advanced, front-fanged delivery mechanisms. Historically, the venoms of snakes lacking front-fanged venom-delivery systems (conventionally grouped into the paraphyletic family Colubridae) have been largely neglected. In this study we used liquid chromatography with mass spectrometry (LC/MS) to analyze a large number of venoms from a wide array of species representing the major advanced snake clades Atractaspididae, Colubrinae, Elapidae, Homalopsinae, Natricinae, Psammophiinae, Pseudoxyrhophiinae, Xenodontinae, and Viperidae. We also present the first sequences of toxins from Azemiops feae as well as additional toxin sequences from the Colubrinae. The large body of data on molecular masses and retention times thus assembled demonstrates a hitherto unsuspected diversity of toxins in all lineages, having implications ranging from clinical management of envenomings to venom evolution to the use of isolated toxins as leads for drug design and development. Although definitive assignment of a toxin to a protein family can only be done through demonstrated structural studies such as N-terminal sequencing, the molecular mass data complemented by LC retention information, presented here, do permit formulation of reasonable hypotheses concerning snake venom evolution and potential clinical effects to a degree not possible till now, and some hypotheses of this kind are proposed here. The data will also be useful in biodiscovery. 相似文献
13.
Novel aliphatic polyesters with pendent acetylene groups were prepared by controlled ring-opening polymerization and subsequently used for grafting poly(ethylene glycol) and oligopeptide moieties by the Cu(I)-catalyzed addition of azides and alkynes, a type of "click" chemistry. These aliphatic polyesters possess an acetylene graft density that can be tailored by ring-opening copolymerization of alpha-propargyl-delta-valerolactone (1) with epsilon-caprolactone. Since the mild conditions associated with the click reaction are shown to be compatible with the polyester backbone, this method is a generally useful means for grafting numerous types of functionality onto aliphatic polyesters. The amphiphilic graft polyesters prepared in this study are shown to be biocompatible by in vitro cytotoxicity evaluation, suggesting their suitability for a range of biomaterial applications. 相似文献
14.
The problem of defining N(E), the density of states, for molecular systems is discussed. It is shown that a numeric evaluation of N(E) is often only approximate, even for continuous data, and is not well defined for quantized systems. The application of the concept of density of states is discussed, particularly with regard to the RRKM theory of unimolecular reactions. The sum of states, W(E), and density of states curves are evaluated for several harmonic and anharmonic model systems and the results discussed in order to illustrate the foregoing considerations. 相似文献
15.
The total synthesis of a 3-thia- and a 3-aza-1-dethiacepham ( and ) are described. Neither compound possessed antibacterial activity . . An unsaturated analog, a 3-aza-1-dethiaceph-1-em exhibited weak antibacterial activity. 相似文献
16.
Matsumoto K Davis BG Jones JB 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(18):4129-4137
A significant enhancement of the applicability of the serine protease subtilisin Bacillus lentus (SBL) in peptide synthesis was achieved by using the strategy of combined site-directed mutagenesis and chemical modification to create chemically modified mutant (CMM) enzymes. The introduction of polar and/or homochiral auxiliary substituents, such as X=oxazolidinones, alkylammonium groups, and carbohydrates at position 166 at the base of the primary specificity S(1) pocket created SBL CMMs S166C-S-X with strikingly broad structural substrate specificities. These CMMs are capable of catalyzing the coupling reactions of not only L-amino acid esters but also D-amino acid esters as acyl donors with glycinamide to give the corresponding dipeptides in good yields. These powerful enzymes are also applicable to the coupling of L-amino acid acyl donors with alpha-branched acyl acceptor, L-alaninamide. Typical increases in isolated yields of dipeptides of 60-80 % over SBL-WT (e.g. 0 % yield of Z-D-Glu-GlyNH(2) using SBL-WT-->74 % using S166C-S-(CH(2))(2) NMe(3) (+)) demonstrate the remarkable synthetic utility of this "polar patch" strategy. Such wide-ranging systems displaying broadened and therefore similarly high, balanced yields of products (e.g. 91 % Z-L-Ala-GlyNH(2) and 86 % yield of Z-D-Ala-GlyNH(2) using S166C-S-(3R,4S)-indenooxazolidinone) may now allow the use of biocatalysts in parallel library synthesis. 相似文献
17.
When complexed by tetrabenzo-24-crown-8, the cesium ion can accommodate unprecedented ligation. The structures of the following complexes are presented. [Cs(tetrabenzo-24-crown-8)(eta 1-NCMe)2(eta 2-NCMe)][NO3] (1): triclinic P1, a = 12.0119(14) A, b = 13.3680(15) A, c = 13.7859(12) A, alpha = 89.124(8) degrees, beta = 66.928(9) degrees, gamma = 71.536(10) degrees, V = 1916.7(4) A3, Z = 2. [Cs(tetrabenzo-24-crown-8)(eta 1-NCMe)2(eta 2-CH2Cl2)][NO3] (2): triclinic, P1, a = 12.295(3) A, b = 13.295(3) A, c = 13.782(3) A, alpha = 89.105(17) degrees, beta = 66.096(18) degrees, gamma = 71.980(19) degrees, V = 1929.5(8) A3, Z = 2. These structures are the first reported examples of linear eta 2-acetonitrile coordination to any metal ion and the first structures illustrating eta 2-acetonitrile and dichloromethane ligation to an alkali metal ion. Possible steric and electronic origins of these unusual metal-ligand interactions are discussed. 相似文献
18.
Eric G. Bryan Brian F.G. Johnson Jack Lewis 《Journal of organometallic chemistry》1976,122(2):249-252
Treatment of H2Os3(CO)10 with cyclonona-l,2-diene produced HOs3(CO)9C9H13 and Os2(CO)6(C9H4)2. Single crystal X ray analysis has shown that the latter is not isostructural with Fe2(CO)6(C9H14)2. 相似文献
19.
20.
Mei Y Wu T Xu C Langenbach KJ Elliott JT Vogt BD Beers KL Amis EJ Washburn NR 《Langmuir : the ACS journal of surfaces and colloids》2005,21(26):12309-12314
A simple yet versatile method was developed to prepare a low-density polymerization initiator gradient, which was combined with surface-initiated atom transfer radical polymerization (ATRP) to produce a well-defined poly(2-hydroxyethyl methacrylate) (HEMA) gradient substrate. A smooth variation in film thickness was measured across the gradient, ranging from 20 A to over 80 A, but we observed a nonmonotonic variation in water contact angle. Fits of X-ray reflectivity profiles suggested that at the low graft density end, the polymer chain structure was in a "mushroom" regime, while the polymer chains at high graft density were in a "brush" regime. It was found that the "mushroom" region of the gradient could be made adhesive to cells by adsorbing adhesion proteins, and cell adhesion could be tuned by controlling the density of the polymer grafts. Fibroblasts were seeded on gradients precoated with fibronectin to test cellular responses to this novel substrate, but it was found that cell adhesion did not follow the expected trend; instead, saturated cell adhesion and spreading was found at the low grafting density region. 相似文献