全文获取类型
收费全文 | 5854篇 |
免费 | 127篇 |
国内免费 | 34篇 |
专业分类
化学 | 4057篇 |
晶体学 | 88篇 |
力学 | 136篇 |
数学 | 992篇 |
物理学 | 742篇 |
出版年
2022年 | 36篇 |
2021年 | 43篇 |
2020年 | 41篇 |
2019年 | 62篇 |
2018年 | 58篇 |
2017年 | 47篇 |
2016年 | 114篇 |
2015年 | 114篇 |
2014年 | 114篇 |
2013年 | 315篇 |
2012年 | 264篇 |
2011年 | 325篇 |
2010年 | 149篇 |
2009年 | 147篇 |
2008年 | 287篇 |
2007年 | 313篇 |
2006年 | 264篇 |
2005年 | 301篇 |
2004年 | 250篇 |
2003年 | 281篇 |
2002年 | 234篇 |
2001年 | 102篇 |
2000年 | 87篇 |
1999年 | 84篇 |
1998年 | 63篇 |
1997年 | 86篇 |
1996年 | 94篇 |
1995年 | 92篇 |
1994年 | 84篇 |
1993年 | 64篇 |
1992年 | 54篇 |
1991年 | 60篇 |
1990年 | 49篇 |
1989年 | 63篇 |
1988年 | 72篇 |
1987年 | 55篇 |
1986年 | 52篇 |
1985年 | 78篇 |
1984年 | 99篇 |
1983年 | 67篇 |
1982年 | 90篇 |
1981年 | 95篇 |
1980年 | 91篇 |
1979年 | 74篇 |
1978年 | 77篇 |
1977年 | 72篇 |
1976年 | 67篇 |
1975年 | 58篇 |
1974年 | 48篇 |
1973年 | 36篇 |
排序方式: 共有6015条查询结果,搜索用时 15 毫秒
991.
Ram P. Kashyap Mahendra N. Deshpande Dayanada Rajapaksa Alan P. Marchand William H. Watson 《Journal of chemical crystallography》1995,25(9):573-578
Hexacyclo[6.5.0.02,7.04,12.05,10.09.13]tridecane (HCTD) contains two four-membered, two five-membered and two six-membered rings fused into a cage structure which contains about 77.0 kcal/mol of strain energy. Attempts to prepare the thioketal from the diketone of HCTD led to a skeletal rearrangement to produce a cage with one four-membered, four five-membered, and two six-membered rings fused into a cage (RHCTD). The corresponding RHCTD hydrocarbon has a strain energy 13.7 kcal/mol less than that of the starting tridecane (HCTD) which provides the driving force for the rearrangement. The X-ray structures of two HCTD derivatives and one RHCTD derivative are reported. The bond lengths in the three reported structures are normal for cages of this type. The structure of tetracyclo[6.3.0.03,7.04,11]undecane-5,10-dione mono(ketene 1,3-propanedithioacetal) is discussed also. 相似文献
992.
The structure of a bis(benzyloxy)trishomocubanone with molecular C2 symmetry reveals that substitution of benzyloxy groups has little effect on the trishomocubane cage. 相似文献
993.
A. K. Das M. Datta S. K. Mazumdar N. Das A. Van Aerschot J. N. Low R. Alan Howie 《Journal of chemical crystallography》1992,22(4):439-442
The title compound (C9H12N2O5,M
r
=228.2) crystallizes in the trigonal space groupP31,21 witha=b=9.438(3) andc=19.775(3) Å,V=1525.5 Å3,D
x
=1.490 gm cm–3,Z=6,=0.12 mm–1,F(000)=720.0,T=293 K. The structure was solved by direct methods and refined by full-matrix least-squares calculations to anR value of 0.044 on 843 unique observed reflections. The dioxane ring adopts a chair conformation. The crystal structure is stabilized by N-HO hydrogen bonds and C-HO close contacts. 相似文献
994.
Alan Calvo Romolo Faggiani Debra Ann Harvey Helen Elaine Howard-Lock Walter Fairbairn Kean Colin James Lyne Lock 《Journal of chemical crystallography》1984,14(1):59-70
The X-ray crystal structure ofd-,-dimethylcysteic acid, C5H11NO5S, has been determined. The compound is monoclinic, space groupP214 (No. 4), with cell dimensionsa=8.133(1),b=8.094(1),c=6.3138(7) Å,=96.36(1)° andZ=2. The structure was solved by Patterson and electron density difference methods and refined toR=0.025,R
w=0.022 for 1081 reflections. Bond lengths and angles do not differ from those in similar structures. The crystal is held together by intermolecular hydrogen bonds. The vibrational spectra have been recorded and assigned and the1 H and13C NMR spectra were measured. 相似文献
995.
Simon G. Bott Alan P. Marchand Duddu Raja Gopal 《Journal of chemical crystallography》1993,23(11):849-855
The syntheses and crystal structures of pentacyclo[9.4.1.05,14.07,13.012,15]hexadecane-3,9-dione (2), a novel C16 diketone with potential inclusion ability, and the products of the reaction of2 withn-butyllithium andmeso-erythritol (i.e.,meso-1,2,3,4-butanetetrol) are described. Crystal data:2·H2O, orthorhombic,Pmmn,a=10.7385(8),b=9.0188(6),c=6.8411(4)Å,V=662.55(7)Å3, andR=0.039 (379 reflections);3 (product from reaction withn-butyllithium), triclinic,P¯1,a=8.7192(7),b=10.9351(8),c=12.1842(8)Å,=68.933(5),=75.850(6), =80.309(6)°,V=1047.0(1)Å3, andR=0.081 (1811 reflections);4 (product from reaction with erythritol), triclinic,P¯1,a=7.7224(8),b=10.1976(8),c=10.7021(9)Å,=85.405(7),=85.766(8), =76.056(7)°,V=814.0(1)Å3, andR=0.052 (1471 reflections). 相似文献
996.
Canoy WL Cooley BE Corona JA Lovelace TC Millar A Weber AM Xie S Zhang Y 《Organic letters》2008,10(6):1103-1106
A one-step diastereoselective (up to 98:2) synthesis of the bis-furan alcohol of Darunavir and other HIV drug candidates has been achieved utilizing the novel cyclization of glycolaldehyde and 2,3-dihydrofuran. The cycloaddition was catalyzed by a variety of catalysts including those formed from tin(II) triflate and common chiral ligands such as BINAP and Evans's box ligands. An efficient and unique enzymatic process enhanced the enantiomeric purity to provide the target in optically pure form. 相似文献
997.
Lutz I Kloas W Springer TA Holden LR Wolf JC Krueger HO Hosmer AJ 《Analytical and bioanalytical chemistry》2008,390(8):2031-2048
Xenopus laevis has been introduced as a model to study effects of endocrine-active compounds (EAC) on development and sexual differentiation.
However, variable and inconsistent data have raised questions about the reliability of the test methods applied. The current
study was conducted in two laboratories to develop, refine, and standardize procedures and protocols. Larvae were exposed
in flow-through systems to 17β-estradiol (E2), at concentrations from 0.2 to 6.0 μg E2 L−1 in Experiment 1A, and 0.015 to 2.0 μg E2 L−1 in Experiment 1B. In both studies survival (92%, 99%) and percentage of animals that completed metamorphosis (97%, 99%) indicated
reproducible biological performance. Furthermore, minor variations in husbandry led to significant differences in snout-to-vent
length, weight, and gonad size. In Experiment 1A, almost complete feminization occurred in all E2 treatment groups whereas
a concentration response was observed in Experiment 1B resulting in an EC50 of 0.12 μg E2 L−1. The final verified protocol is suitable for determining effects of EAC on development and sexual differentiation in X. laevis. 相似文献
998.
Harinandini Paramahamsan Anthony J. Pearson Nathalie M. Pinkerton A. Alan Pinkerton 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(4):m147-m148
The tricarbonylchromium unit bound to the arene ring of the chiral title complex, [Cr(C19H26O3)(CO)3], is rotated by ca 25° in agreement with the proposed mechanism for 1,5‐asymmetric induction of nucleophilic attack. 相似文献
999.
A (K4-e)-design on v+w points embeds a Steiner triple system (STS) if there is a subset of v points on which the graphs of the design induce the blocks of a STS. It is established that wv/3, and that when equality is met that such a minimum embedding of an STS(v) exists, except when v=15. 相似文献
1000.
The first example of a beta-diketiminato-supported hydroxyphosphenium cation has been prepared, structurally characterized, and modeled by DFT calculations. 相似文献