全文获取类型
收费全文 | 4408篇 |
免费 | 123篇 |
国内免费 | 27篇 |
专业分类
化学 | 3111篇 |
晶体学 | 37篇 |
力学 | 117篇 |
数学 | 613篇 |
物理学 | 680篇 |
出版年
2021年 | 42篇 |
2020年 | 45篇 |
2019年 | 53篇 |
2018年 | 35篇 |
2017年 | 32篇 |
2016年 | 84篇 |
2015年 | 66篇 |
2014年 | 81篇 |
2013年 | 204篇 |
2012年 | 211篇 |
2011年 | 261篇 |
2010年 | 126篇 |
2009年 | 98篇 |
2008年 | 253篇 |
2007年 | 253篇 |
2006年 | 237篇 |
2005年 | 227篇 |
2004年 | 223篇 |
2003年 | 172篇 |
2002年 | 164篇 |
2001年 | 55篇 |
2000年 | 77篇 |
1999年 | 58篇 |
1998年 | 44篇 |
1997年 | 52篇 |
1996年 | 61篇 |
1995年 | 55篇 |
1994年 | 57篇 |
1993年 | 45篇 |
1992年 | 43篇 |
1991年 | 40篇 |
1990年 | 46篇 |
1989年 | 40篇 |
1988年 | 50篇 |
1987年 | 51篇 |
1986年 | 47篇 |
1985年 | 81篇 |
1984年 | 62篇 |
1983年 | 67篇 |
1982年 | 81篇 |
1981年 | 64篇 |
1980年 | 71篇 |
1979年 | 54篇 |
1978年 | 43篇 |
1977年 | 48篇 |
1976年 | 31篇 |
1975年 | 36篇 |
1974年 | 43篇 |
1973年 | 42篇 |
1969年 | 16篇 |
排序方式: 共有4558条查询结果,搜索用时 31 毫秒
1.
Haixing Guan Qianwen Zhang Prof. Dr. Patrick J. Walsh Prof. Dr. Jianyou Mao 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(13):5210-5215
A unique nickel/organic photoredox co-catalyzed asymmetric reductive cross-coupling between α-chloro esters and aryl iodides is developed. This cross-electrophile coupling reaction employs an organic reductant (Hantzsch ester), whereas most reductive cross-coupling reactions use stoichiometric metals. A diverse array of valuable α-aryl esters is formed under these conditions with high enantioselectivities (up to 94 %) and good yields (up to 88 %). α-Aryl esters represent an important family of nonsteroidal anti-inflammatory drugs. This novel synergistic strategy expands the scope of Ni-catalyzed reductive asymmetric cross-coupling reactions. 相似文献
2.
3.
Gary W. Breton Lauren A. Hahn Kenneth L. Martin 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(9):1208-1212
Tetrahydrotetrazoles are five‐membered‐ring heterocycles containing four contiguous saturated nitrogen atoms. Very few examples of such compounds have been reported in the literature. Our previous attempt at the synthesis of a member of this class of compound suggested that the N—N bonds may be more labile than expected. This finding raised the question as to whether the structures of any of the previously reported tetrahydrotetrazoles had been properly assigned. We have reproduced the synthesis of a reported tetrahydrotetrazole, namely 1,2‐di‐tert‐butyl 3‐phenyl‐1H,2H,3H,10bH‐[1,2,3,4]tetrazolo[5,1‐a]isoquinoline‐1,2‐dicarboxylate, C25H30N4O4, and have now confidently confirmed its structure via X‐ray crystallography. However, while sufficiently stable in the crystal phase, we discovered that it remains very labile in solution (having a half‐life of only 15 min at 20 °C in CDCl3). A tentative reaction pathway for its dissociation based on 1H NMR spectral evidence is provided. 相似文献
4.
Synthesis of optically active sesquiterpenes with a eudesmane C-skeleton from the chiral starting material thujone involves transformation of a tricyclic intermediate (1R,2R,4S)-1,7-dimethyl-4-(1-methylethyl)tricyclo[4.4.0.02,4]dec-6-en-8-one ( 2 ) into the bicyclic compound β -cyperone ( 5 ). Hydroxylation of 2 at C(5) or C(11) permits subsequent opening of the cyclopropane ring and rearrangement to β -cyperone. In this publication, studies involving hydroxylation of 2 by fungal cultures are presented. The resultant products are useful intermediates in efficient synthesis of eudesmane sesquiterpenes. Of five fungi tested, Rhizopus oryzae ATCC 11145 proved most versatile. It hydroxylates at the exocyclic C(11) position in high yield (70%) and, to a lesser extent, at C(5) (5%). Enzymatic activity appears at the end of growth phase and at least 2.2 g of 2 per liter can be metabolized without significant loss of product yield. A second fungus, Cunninghamella echinulata ATCC 9244, proved most useful for hydroxylation of derivatives of 2 for the preparation of derivatives of β -cyperone, although product yields were low (2–20%), some derivatives were nonreactive, and hydroxylation at C(9) occurred. The relationship between precursor structure and enzyme affinity is discussed. 相似文献
5.
Charles J. Colbourn Sosina S. Martirosyan Gary L. Mullen Dennis Shasha George B. Sherwood Joseph L. Yucas 《组合设计杂志》2006,14(2):124-138
A covering array CA(N;t,k, v is an N × k array such that every N × t subarray contains all t‐tuples from v symbols at least once, where t is the strength of the array. Covering arrays are used to generate software test suites to cover all t‐sets of component interactions. The particular case when t = 2 (pairwise coverage) has been extensively studied, both to develop combinatorial constructions and to provide effective algorithmic search techniques. In this paper, a simple “cut‐and‐paste” construction is extended to covering arrays in which different columns (factors) admit different numbers of symbols (values); in the process an improved recursive construction for covering arrays with t = 2 is derived. © 2005 Wiley Periodicals, Inc. J Combin Designs 14: 124–138, 2006 相似文献
6.
The singular-perturbation expansion of the plasma cold-fluid equations for crossed fields in a planar geometry is presented. The general expansion is carried out to third order. Various instabilities that occur in the first, second, and third orders are discussed. 相似文献
7.
The method described in this paper allows an investigator to determine the intrinsic stress of a polymer layer in a way that
does not result in damage to devices or test structures. The method requires that a small area of the polymer be released
from the substrate to form a diaphragm. The diaphragm is stimulated with acoustic white noise and the diaphragm movement is
monitored with a laser vibrometer. The first few resonance frequencies of the diaphragm are obtained using a laser vibrometer
and then those frequencies are used to calculate the membrane intrinsic bi-axial tension. 相似文献
8.
9.
10.
[3,3]-Sigmatropic rearrangements of allyl tetronates and allyl tetramates to give 3-allyltetronic or -tetramic acids, respectively, proceed within 20-60 min under microwave irradiation (300 W, 130-190 °C). Consecutive (homo)sigmatropic [1,5] H-shifts such as oxa-ene reactions are promoted less effectively, which allows the isolation of Claisen intermediates of sigmatropic domino sequences, in contrast to conventional heating. 相似文献