全文获取类型
收费全文 | 704篇 |
免费 | 6篇 |
国内免费 | 18篇 |
专业分类
化学 | 200篇 |
力学 | 57篇 |
数学 | 248篇 |
物理学 | 223篇 |
出版年
2020年 | 6篇 |
2016年 | 12篇 |
2015年 | 7篇 |
2013年 | 47篇 |
2012年 | 16篇 |
2011年 | 29篇 |
2010年 | 6篇 |
2009年 | 15篇 |
2008年 | 23篇 |
2007年 | 26篇 |
2006年 | 20篇 |
2005年 | 20篇 |
2004年 | 17篇 |
2003年 | 6篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 22篇 |
1992年 | 10篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 18篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 12篇 |
1973年 | 19篇 |
1972年 | 9篇 |
1971年 | 5篇 |
1969年 | 6篇 |
1967年 | 6篇 |
1966年 | 6篇 |
1965年 | 5篇 |
排序方式: 共有728条查询结果,搜索用时 15 毫秒
11.
Friedman LA You F Sabat M Harman WD 《Journal of the American Chemical Society》2003,125(49):14980-14981
The rhenium furan complexes TpRe(CO)(MeIm)(eta2-2-methylfuran) (1) and TpRe(CO)(MeIm)(eta2-2,5-dmethylfuran) (2) undergo Lewis acid-promoted cyclopentannulation reactions with enones and enals to generate 3-acetylcyclopentene complexes. During the reaction, a rearrangement occurs such that the alpha and beta carbons of the enone are incorporated into the new carbocycle. Treatment of these complexes with an oxidant (H2O2 or silver triflate) liberates the acetylcyclopentene. When a resolved form of the rhenium complex is used, the acetylcyclopentenes can be obtained enantioselectively. 相似文献
12.
Friedman M 《Journal of chromatography. A》2004,1054(1-2):143-155
Potatoes and tomatoes, members of the Solanaceae plant family, serve as major, inexpensive low-fat food sources providing for energy, high-quality protein, fiber, vitamins, pigments, as well as other nutrients. These crops also produce biologically active secondary metabolites, which may have both adverse and beneficial effects in the diet. This limited overview, based largely on our studies with the aid of HPLC, TLC, ELISA, GC-MS, and UV spectroscopy, covers analytical aspects of two major potato trisaccharide glycoalkaloids, alpha-chaconine and alpha-solanine, and their hydrolysis products (metabolites) with two, one, and zero carbohydrate groups; the potato water-soluble nortropane alkaloids calystegine A3 and B2; the principal potato polyphenolic compound chlorogenic acid; potato inhibitors of digestive enzymes; the tomato tetrasaccharide glycoalkaloids dehydrotomatine and alpha-tomatine and hydrolysis products; the tomato pigments beta-carotene, lycopene, and chlorophyll; and the anticholinergic alkaloids atropine and scopolamine present in Datura stramonium (jimson weed) seeds that contaminate grain and animal feed. Related studies by other investigators are also mentioned. Accurate analytical methods for these food ingredients help assure the consumer of eating a good-quality and safe diet. 相似文献
13.
Brooks BC Meiere SH Friedman LA Carrig EH Gunnoe TB Harman WD 《Journal of the American Chemical Society》2001,123(15):3541-3550
The mechanisms for the interconversion of facial diastereomers of a variety of TpRe(CO)(L)(eta(2)-L(Ar)) complexes [L = (t)BuNC, pyridine (py), PMe(3), or 1-methylimidazole (MeIm); L(Ar) = benzene, anisole, naphthalene, 1-methylpyrrole, furan, or thiophene; Tp = hydridotris(pyrazolyl)borate] have been investigated by (1)H NMR spin saturation experiments. In addition, the rates and free energies of activation for these processes were calculated from spin saturation experiments and T(1) measurements. The operative mechanisms for interconversion of the pi diastereomers were found to be nondissociative, undergoing either an interfacial or intrafacial linkage isomerization. A comparison of the kinetic parameters for isomerization of related eta(2)-olefin complexes of the [TpRe(CO)(PMe(3))] and [CpRe(NO)(PPh(3))](+) fragments is also presented. 相似文献
14.
An interesting energy cascade is observed in the phosphorescence spectra of 1% biphenyl-h10 in biphenyl-d10 (2–15 K); strongly perturbed host sites, with energy levels below that of the protonated guest, quench the guest sites at higher temperatures (11–15 K). The identification of the perturbed sites is based on vibrational characteristics (both intensity and frequency), obtained with the help of phosphorescence spectra of biphenyl-h10 and biphenyl-d10 in an argon matrix, indicating an isotope dependent vibronic structure. A partial vibrational analysis is presented, resulting in confirmation of the first triplet state of biphenyl as orbitally ungerade. The dynamics of the triplet excitation are discussed, including several possible mechanisms explaining the non-Boltzmann nature of the low-temperature steady state. 相似文献
15.
Joel Friedman 《Combinatorica》1995,15(1):31-42
For any prime,p, we construct a Cayley graph on the group,G, of affine linear transformations ofℤ/pℤ of degree 2(p−1) and second eigenvalue
with the following special property: the adjacency matrix of the graph is supported on the “blocks” associated to the trivial
representation and the irreducible representation of sizep−1. SinceG is of orderp(p−1), the correspondingt-uniform Cayley hypergraph has essentially optimal second eigenvalue for this degree and size of the graph (see [2] for definitions).
En route we give, for any integerk>1, a simple Cayley graph onp
k nodes of degreep of second eigenvalue
.
The author wishes to acknowledge the National Science Foundation for supporting this research in part under Grant CCR-8858788,
and the Office of Naval Research under Grant N00014-87-K-0467. 相似文献
16.
17.
18.
Given a measurable function f on (0, ) with Mellin transformF(s), let |f|p denote the Lp-norm of f with respect to the measuredx/x. We prove that under certain assumptions, for instanceif f is real and non-negative and F() converges for in an openinterval and F() 0, then
wherecp (2e)1. We derive similar inequalities for complex-valuedf, for the Lp-norm of the derivative of f, and for the supremumof real-valued f and of its derivative. The lower bounds areeminently applicable when f is a convolution product. 相似文献
19.
20.