全文获取类型
收费全文 | 2927篇 |
免费 | 138篇 |
国内免费 | 9篇 |
专业分类
化学 | 2126篇 |
晶体学 | 3篇 |
力学 | 46篇 |
数学 | 428篇 |
物理学 | 471篇 |
出版年
2023年 | 26篇 |
2022年 | 23篇 |
2021年 | 51篇 |
2020年 | 59篇 |
2019年 | 68篇 |
2018年 | 37篇 |
2017年 | 32篇 |
2016年 | 90篇 |
2015年 | 112篇 |
2014年 | 107篇 |
2013年 | 168篇 |
2012年 | 231篇 |
2011年 | 223篇 |
2010年 | 129篇 |
2009年 | 118篇 |
2008年 | 150篇 |
2007年 | 160篇 |
2006年 | 174篇 |
2005年 | 150篇 |
2004年 | 170篇 |
2003年 | 121篇 |
2002年 | 96篇 |
2001年 | 56篇 |
2000年 | 32篇 |
1999年 | 37篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 25篇 |
1995年 | 38篇 |
1994年 | 40篇 |
1993年 | 20篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 24篇 |
1978年 | 13篇 |
1977年 | 12篇 |
1976年 | 8篇 |
1974年 | 7篇 |
1973年 | 11篇 |
1971年 | 4篇 |
1970年 | 4篇 |
排序方式: 共有3074条查询结果,搜索用时 15 毫秒
71.
Hamada T Chieffi A Ahman J Buchwald SL 《Journal of the American Chemical Society》2002,124(7):1261-1268
A new catalyst system for the enantioselective alpha-arylation of ketones is reported. This catalyst, prepared from Pd(2)(dba)(3) and a bulky dialkylphosphino-binaphthyl ligand, is able to effect the asymmetric arylation of ketone enolates with aryl bromides utilizing NaO(t)()Bu as base. These new catalysts enjoy much higher reactivity than previous systems; arylation reactions could be effected at room temperature with only 2 mol % of the Pd catalyst. The coupling of alpha-alkyl-alpha'-protected cyclopentanones proceeded in high yield, and the resulting quaternary stereogenic center was installed in up to 94% ee. 相似文献
72.
Petersen M Bondensgaard K Wengel J Jacobsen JP 《Journal of the American Chemical Society》2002,124(21):5974-5982
Locked nucleic acids (LNAs) containing one or more 2'-O,4'-C-methylene-linked bicyclic ribonucleoside monomers possess a number of the prerequisites of an effective antisense oligonucleotide, e.g. unprecedented helical thermostability when hybridized with cognate RNA and DNA. To acquire a detailed understanding of the structural features of LNA giving rise to its remarkable properties, we have conducted structural studies by use of NMR spectroscopy and now report high-resolution structures of two LNA:RNA hybrids, the LNA strands being d(5'-CTGAT(L)ATGC-3') and d(5'-CT(L)GAT(L)AT(L)GC-3'), respectively, T(L) denoting a modified LNA monomer with a thymine base, along with the unmodified DNA:RNA hybrid. In the structures, the LNA nucleotides are positioned as to partake in base stacking and Watson-Crick base pairing, and with the inclusion of LNA nucleotides, we observe a progressive change in duplex geometry toward an A-like duplex structure. As such, with the inclusion of three LNA nucleotides, the hybrid adopts an almost canonical A-type duplex geometry, and thus it appears that the number of modifications has reached a saturation level with respect to structural changes, and that further incorporations would furnish only minute changes in the duplex structure. We attempt to rationalize the conformational steering induced by the LNA nucleotides by suggesting that the change in electronic density at the brim of the minor groove, introduced by the LNA modification, is causing an alteration of the pseudorotational profile of the 3'-flanking nucleotide, thus shifting this sugar equilibrium toward N-type conformation. 相似文献
73.
Warnau Judith Wichmann Karin Reinisch Jens 《Journal of computer-aided molecular design》2021,35(7):813-818
Journal of Computer-Aided Molecular Design - We applied the COSMO-RS method to predict the partition coefficient logP between water and 1-octanol for 22 small drug like molecules within the... 相似文献
74.
Tengzhou Ma Ban Xuan Dong Jonathan W. Onorato Jens Niklas Oleg Poluektov Christine K. Luscombe Shrayesh N. Patel 《Journal of polymer science. Part A, Polymer chemistry》2021,59(22):2797-2808
Molecular doping of conjugated polymers (CPs) plays a vital role in optimizing organic electronic and energy applications. For the case of organic thermoelectrics, it is commonly believed that doping CPs with a strong dopant could result in higher conductivity (σ) and thus better power factor (PF). Herein, by investigating thermoelectric performance of a polar side-chain bearing CP, poly(3-(methoxyethoxyethoxy)thiophene) (P3MEET), vapor doped with fluorinated-derivative of tetracyanoquinodimethane FnTCNQ (n = 1, 2, 4), we show that using strong dopants can in fact have detrimental effects on the thermoelectric performance of CPs. Despite possessing higher electron affinity, doping P3MEET with F4TCNQ only results in a σ (27.0 S/cm) comparable to samples doped with other two weaker dopants F2TCNQ and F1TCNQ (26.4 and 20.1 S/cm). Interestingly, F4TCNQ-doped samples display a marked reduction in the Seebeck coefficient (α) compared to F1TCNQ- and F2TCNQ-doped samples from 42 to 13 μV/K, leading to an undesirable suppression of the PF. Structural characterizations coupled with Kang-Snyder modeling of the α–σ relation show that the reduction of α in F4TCNQ-doped P3MEET samples originates from the generation of low mobility carrier within P3MEET's amorphous domain. Our results demonstrate that factors such as dopant distribution and doping efficiency within the crystalline and amorphous domains of CPs should play a crucial role in advancing rational design for organic thermoelectrics. 相似文献
75.
Markus Follmann Nils Griebenow Michael G. Hahn Ingo Hartung Franz‐Josef Mais Joachim Mittendorf Martina Schfer Hartmut Schirok Johannes‐Peter Stasch Friederike Stoll Alexander Straub Peter Jeschke Ralf Nauen Michael Edmund Beck Hans‐Wilhelm Engels Hans‐Georg Pirkl Reinhard Albers Rolf W. Albach Jens Krause Andreas Hoffmann Holger Casselmann Jeff Dormish 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2013,125(36):9503-9503
76.
77.
Frontispiece: Probing Transient Conformational States of Proteins by Solid‐State R1ρ Relaxation‐Dispersion NMR Spectroscopy 下载免费PDF全文
78.
Previous studies of kinetic transport in the Lorentz gas have been limited to cases where the scatterers are distributed at random (e.g., at the points of a spatial Poisson process) or at the vertices of a Euclidean lattice. In the present paper we investigate quasicrystalline scatterer configurations, which are non-periodic, yet strongly correlated. A famous example is the vertex set of a Penrose tiling. Our main result proves the existence of a limit distribution for the free path length, which answers a question of Wennberg. The limit distribution is characterised by a certain random variable on the space of higher dimensional lattices, and is distinctly different from the exponential distribution observed for random scatterer configurations. The key ingredients in the proofs are equidistribution theorems on homogeneous spaces, which follow from Ratner’s measure classification. 相似文献
79.
80.
Jens A. Pedersen 《Molecular physics》2013,111(4):1031-1035
A set of equations for assigning hyperfine coupling constants of substituted 1,4-benzosemiquinones are presented. The equations, based on additivity rules, yield assignments for a range of substituents with a correlation between predicted and experimental constants to better than five per cent. The investigation shows that t-butyl groups in semiquinones rotate freely against prior assertions. 相似文献