全文获取类型
收费全文 | 3086篇 |
免费 | 59篇 |
国内免费 | 8篇 |
专业分类
化学 | 1799篇 |
晶体学 | 39篇 |
力学 | 57篇 |
数学 | 252篇 |
物理学 | 1006篇 |
出版年
2021年 | 19篇 |
2020年 | 37篇 |
2019年 | 38篇 |
2018年 | 18篇 |
2016年 | 54篇 |
2015年 | 41篇 |
2014年 | 50篇 |
2013年 | 93篇 |
2012年 | 93篇 |
2011年 | 128篇 |
2010年 | 66篇 |
2009年 | 60篇 |
2008年 | 116篇 |
2007年 | 102篇 |
2006年 | 105篇 |
2005年 | 109篇 |
2004年 | 74篇 |
2003年 | 79篇 |
2002年 | 73篇 |
2001年 | 85篇 |
2000年 | 78篇 |
1999年 | 49篇 |
1998年 | 62篇 |
1997年 | 28篇 |
1996年 | 56篇 |
1995年 | 55篇 |
1994年 | 47篇 |
1993年 | 50篇 |
1992年 | 64篇 |
1991年 | 49篇 |
1990年 | 45篇 |
1989年 | 42篇 |
1988年 | 48篇 |
1987年 | 44篇 |
1986年 | 48篇 |
1985年 | 54篇 |
1984年 | 50篇 |
1983年 | 34篇 |
1982年 | 42篇 |
1981年 | 50篇 |
1980年 | 45篇 |
1979年 | 57篇 |
1978年 | 52篇 |
1977年 | 47篇 |
1976年 | 36篇 |
1975年 | 38篇 |
1974年 | 37篇 |
1973年 | 33篇 |
1970年 | 25篇 |
1969年 | 20篇 |
排序方式: 共有3153条查询结果,搜索用时 203 毫秒
41.
Chemical Transport of Iron Sulphide. II. Experimental Results on the Transport of FeSx with Iodine It is possible to deposite monocrystals of iron sulphide FeSx by means of CTR method using iodine as transport agent. The presence of sufficient, high sulfur partial pressures is the most important condition for the FeSx being transported, in other words: x must be equal or greater than approximately 1.04. The transport velocity and the shape of the deposited crystals depend on temperature, amount of iodine, and composition of the original material. The expected differences between the composition of the deposited solid and that of the residue can be verified experimentally. 相似文献
42.
Lead tetraacetate (LTA) oxidation of the allylic alcohols 1, 10, 14 and 19 leads to the formation of the epoxides 2, 11, 15 and 20 , products of a novel internal addition reaction of the electron deficient alcohol oxygen to the allylic double bond. In some cases ( 10, 14 ) the formation of a new type of acetoxylated enolethers ( 12, 16 ) is observed. The LTA oxidation of the allylic dienols 21 and 29 gives rise to the formation of the epoxyacetates 25 and 33 , products of a similar internal addition reaction. Furthermore, a variety of cyclization products ( 22, 23, 24, 26, 30, 31, 32 and 34 ) has been isolated whose formation requires an isomerisation of the allylic trans double bond to a cis one. 相似文献
43.
Jakub Wojciechowski Henryk Krawczyk Marcin
liwiski Karolina Kafarska Wojciech M. Wolf 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(5):o280-o282
The title compound, C14H20O2, adopts a conformation in which the δ‐valerolactone and cyclohexane rings are almost coplanar with one another. The γ‐methyl substituent occupies an axial position with respect to the cyclohexane ring. The δ‐valerolactone moiety adopts an envelope arrangement, while the cyclohexane ring exists in a chair conformation. 相似文献
44.
Steffen Wolf Benedikt Sohmen Bjrn Hellenkamp Johann Thurn Gerhard Stock Thorsten Hugel 《Chemical science》2021,12(9):3350
We report on a study that combines advanced fluorescence methods with molecular dynamics (MD) simulations to cover timescales from nanoseconds to milliseconds for a large protein. This allows us to delineate how ATP hydrolysis in a protein causes allosteric changes at a distant protein binding site, using the chaperone Hsp90 as test system. The allosteric process occurs via hierarchical dynamics involving timescales from nano- to milliseconds and length scales from Ångstroms to several nanometers. We find that hydrolysis of one ATP is coupled to a conformational change of Arg380, which in turn passes structural information via the large M-domain α-helix to the whole protein. The resulting structural asymmetry in Hsp90 leads to the collapse of a central folding substrate binding site, causing the formation of a novel collapsed state (closed state B) that we characterise structurally. We presume that similar hierarchical mechanisms are fundamental for information transfer induced by ATP hydrolysis through many other proteins.We report on a study that combines advanced fluorescence methods with molecular dynamics simulations to cover timescales from nanoseconds to milliseconds for a large protein, the chaperone Hsp90. 相似文献
45.
A novel approach to the preparation of perfluorotail-functionalized triarylphosphines using a p-silyl substituent as the branching point has been developed. This approach enabled the attachment of between three and nine perfluorotails per phosphorus atom, resulting in the production of highly fluorous tris[p-(1H,1H,2H, 2H-perfluoroalkylsilyl)aryl]phosphines, P[C(6)H(4)-p-SiMe(3)(-)(n)()(CH(2)CH(2)C(x)()F(2)(x)()(+1))(n)()](3) (n = 1, 2, 3; x = 6, 8), containing between 50 and 67 wt % fluorine. (31)P NMR studies indicate that the phosphorus atoms, and consequently the sigma-donor and pi-acceptor properties of these phosphines, are not influenced by the electron-withdrawing perfluoroalkyltails. The fluorous triarylphosphines are readily soluble in fluorous solvents and display fluorous phase preference in several fluorous biphasic systems. The phase partitioning of these fluorous ligands, as well as their donor properties, is discussed in relation to their potential for fluorous biphasic catalyst separation. 相似文献
46.
The widespread occurrence of the neutral loss of one to six amino acid residues as neutral fragments from doubly protonated tryptic peptides is documented for 23 peptides with individual sequences. Neutral loss of amino acids from the N-terminus of doubly charged tryptic peptides results in doubly charged y-ions, forming a ladder-like series with the ions [M + 2H](2+) = y(max) (2+), y(max - 1) (2+), y(max - 2) (2+), etc. An internal residue such as histidine, proline, lysine or arginine appears to favor this type of fragmentation, although it was sometimes also observed for peptides without this structure. For doubly protonated non-tryptic peptides with one of these residues at or near the N-terminus, we observed neutral loss from the C-terminus, resulting in a doubly charged b-type ion ladder. The analyses were performed by Q-TOF tandem mass spectrometry, facilitating the recognition of neutral loss ladders by their 2+ charge state and the conversion of the observed mass differences into reliable sequence information. It is shown that the neutral loss of amino acid residues requires low collision offset values, a simple mechanistic explanation based on established fragmentation rules is proposed and the utility of this neutral loss fragmentation pathway as an additional source for dependable peptide sequence information is documented. 相似文献
47.
Recently the subcommittee on Food Definition of the AOAC INTERNATIONAL Task Force on Methods for Nutrition Labeling proposed a Food Matrix Organizational System [4] to systematically judge the applicability of collaboratively studied methods over a range of food matrices. This system describes a food matrix by its location in one of nine sectors in a triangle, with each point of the triangle defined as representing 100% and the opposite side representing 0% of the normalized contents of each of three major components of FAT, PROTEIN and CARBOHYDRATE. Foods falling within the same sector would be chemically similar and thus should behave in a similar analytical manner. This same scheme can be used to select one or two food matrices representing each sector, for development of a series of reference materials representing all foods. The list of 5250 foods contained in the USDA Nutrient Data Base for Standard Reference has been sorted and matched to this schematic to determine the scope of the selection process. In addition the list of foods in the USDA Data Base for Food Consumption Surveys has been examined. Results and progress of this selection process are reported. 相似文献
48.
49.
50.