全文获取类型
收费全文 | 2429篇 |
免费 | 97篇 |
国内免费 | 10篇 |
专业分类
化学 | 1801篇 |
晶体学 | 20篇 |
力学 | 73篇 |
数学 | 245篇 |
物理学 | 397篇 |
出版年
2023年 | 16篇 |
2022年 | 11篇 |
2021年 | 19篇 |
2020年 | 46篇 |
2019年 | 35篇 |
2018年 | 19篇 |
2017年 | 13篇 |
2016年 | 42篇 |
2015年 | 50篇 |
2014年 | 55篇 |
2013年 | 126篇 |
2012年 | 137篇 |
2011年 | 191篇 |
2010年 | 66篇 |
2009年 | 64篇 |
2008年 | 172篇 |
2007年 | 184篇 |
2006年 | 145篇 |
2005年 | 163篇 |
2004年 | 134篇 |
2003年 | 112篇 |
2002年 | 82篇 |
2001年 | 27篇 |
2000年 | 19篇 |
1999年 | 23篇 |
1998年 | 24篇 |
1997年 | 23篇 |
1996年 | 23篇 |
1995年 | 29篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 13篇 |
1991年 | 22篇 |
1990年 | 15篇 |
1989年 | 19篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 21篇 |
1985年 | 25篇 |
1984年 | 21篇 |
1983年 | 24篇 |
1982年 | 27篇 |
1981年 | 29篇 |
1980年 | 21篇 |
1979年 | 15篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 13篇 |
1974年 | 14篇 |
1973年 | 13篇 |
排序方式: 共有2536条查询结果,搜索用时 17 毫秒
71.
Andrew P. Mendham John Spencer Babur Z. Chowdhry Trevor J. Dines Muhammad Mujahid Rex A. Palmer Graham J. Tizzard Simon J. Coles 《Journal of chemical crystallography》2011,41(9):1323-1327
Abstract
The cyclic di-amino acid peptide N,N′-diacetyl-cyclo(Gly-Gly), C8H10N2O4, crystallizes in the triclinic space group P[`1] P\bar{1} with unit cell parameters a = 9.4855(4) ?, b = 10.0250(3) ?, c = 10.0763(4) ?, α = 73.682(2)°, β = 82.816(2)°, γ = 81.733(2)°, V = 906.40(6) ?3, Z = 4 (2 molecules, A and B, per asymmetric unit), Dc = 1.452 g cm−3 and linear absorption coefficient 0.118 mm−1. The crystal structure determination was carried out with MoKα X-ray data measured at 120(2) K. In the final refinement cycle the data/restraints/parameter ratios were 4124/0/258 and goodness-of-fit on F2 = 1.0008. Final R indices for [I > 2σ(I)] were R1 = 0.0501, wR2 = 0.1007 and R indices (all data) R1 = 0.0864, wR2 = 0.11180. The largest electron density difference peak and hole were 0.241 and −0.232 e ?−3, respectively. The DKP rings in both molecules A and B have boat conformations with pseudo mm2 (C2v) symmetry if the N atoms and CH2 groups are considered identical. In each case, the prow and stern of the boat are the α-carbons C(3) and C(6). The overall molecular symmetry of molecules A and B is approximately C2 with the twofold symmetry axis of the DKP boat being maintained through the centre of the DKP ring. Details of the molecular geometry are compared with that of the parent compound cyclo(Gly-Gly) in which the DKP ring is planar with exact symmetry [`1] \bar{1} (Ci). 相似文献72.
73.
Georges Assaf Gemma Cansell Doug Critcher Stuart Field Stewart Hayes Suju Mathew Alan Pettman 《Tetrahedron letters》2010,51(38):5048-5051
We report our results on the construction of a morpholine ring system from the corresponding epoxide and amino alcohol. From this study, we were able to convert a previous four-step synthesis into a more efficient two-step process. 相似文献
74.
Globular Polymer Grafts Require a Critical Size for Efficient Molecular Sieving of Enzyme Substrates
Stuart A. McNelles Victoria M. Marando Alex Adronov 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(25):8536-8541
A series of 2,2‐bis(hydroxymethyl)propionic acid dendrons of generation 2 through 8 having a strained cyclooctyne at the core and hydroxy groups at the periphery were prepared by a divergent method and used to functionalize azide‐decorated α‐chymotrypsin. The ability of the appended dendrons to selectively block enzyme activity (through a molecular sieving effect) was investigated using a small molecule substrate (benzoyl‐l ‐tyrosine p‐nitroanilide), as well as two proteins of different size (casein and bovine serum albumin). Additionally, the ability of dendrons to block complexation with a chymotrypsin antagonist, α‐antichymotrypsin, was investigated, and it was found that the dendron coating effectively prevented inhibition by this antagonist. We found that a critical generation is required to achieve efficient sieving with bis‐MPA dendrons, which illustrates the importance of macromolecular architecture and size in the shielding of proteins. 相似文献
75.
Ryan J. Schwamm Mathew D. Anker Matthias Lein Martyn P. Coles 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(5):1503-1507
The potassium aluminyl complex K[Al(NONAr)] (NON=NONAr=[O(SiMe2NAr)2]2?, Ar=2,6‐iPr2C6H3) reacts with 1,3,5,7‐cyclooctatetraene (COT) to give K[Al(NONAr)(COT)]. The COT‐ligand is present in the asymmetric unit as a planar μ2‐η2:η8‐bridge between Al and K, with additional K???π‐aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT]2?. Addition of 18‐crown‐6 causes a rearrangement of the C8‐carbocycle to form the isomeric 9‐aluminabicyclo[4.2.1]nona‐2,4,7‐triene anion. 相似文献
76.
An improved visualization-based force-measurement technique for short-duration hypersonic facilities 总被引:2,自引:0,他引:2
This article is concerned with describing and exploring the limitations of an improved version of a recently proposed visualization-based
technique for the measurement of forces and moments in short-duration hypersonic wind tunnels. The technique is based on tracking
the motion of a free-flying body over a sequence of high-speed visualizations; while this idea is not new in itself, the use
of high-speed digital cinematography combined with a highly accurate least-squares tracking algorithm allows improved results
over what have been previously possible with such techniques. The technique precision is estimated through the analysis of
artificially constructed and experimental test images, and the resulting error in acceleration measurements is characterized.
For wind-tunnel scale models, position measurements to within a few microns are shown to be readily attainable. Image data
from two previous experimental studies in the T5 hypervelocity shock tunnel are then reanalyzed with the improved technique:
the uncertainty in the mean drag acceleration is shown to be reduced to the order of the flow unsteadiness, 2–3%, and time-resolved
acceleration measurements are also shown to be possible. The response time of the technique for the configurations studied
is estimated to be ∼0.5 ms. Comparisons with computations using the DLR TAU code also yield agreement to within the overall
experimental uncertainty. Measurement of the pitching moment for blunt geometries still appears challenging, however. 相似文献
77.
78.
Jakob Knelles Dr. Stuart Beardsworth Korinna Bader Dr. Johanna R. Bruckner Andrea Bühlmeyer Robert Forschner Kevin Schweizer Dr. Wolfgang Frey Prof. Dr. Frank Giesselmann Prof. Dr. Yann Molard Prof. Dr. Sabine Laschat 《Chemphyschem》2019,20(17):2210-2216
A series of tetraguanidinium tetraphenylethene (TPE) arylsulfonates with different chain lengths was prepared via ionic self-assembly of tetraguanidinium TPE chloride and the respective methyl arylsulfonates. Liquid crystalline properties were studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. Tetraguanidinium TPE arylsulfonates with chain lengths of C8–C12 displayed hexagonal columnar mesophases over a broad temperature range, while derivatives with longer chains showed oblique columnar phases. In solution all compounds displayed aggregation-induced emission behaviour. Temperature-dependent luminescence spectra of the bulk phase of the tetraguanidinium TPE arylsulfonate with C14 side chains revealed a strong luminescence both in the solid state and the oblique columnar mesophase. The emission behaviour was rationalized by a unique combination of restriction of intramolecular rotation of the TPE core, Coulomb interaction between the guanidinium cations and π–π interactions of the anionic arylsulfonate moieties. 相似文献
79.
We perform a differential temperature Carnot analysis of the changes in energy and entropy (degrees of freedom) associated with an ideal classical computing machine. Assuming that Carnot’s maximum efficiency law is as equally applicable to a computing machine as to a mechanical machine, we find that useful computation is necessarily dissipative and thermodynamically irreversible. In addition, we find that copying or cloning of information is as dissipative as the original process employed to create the information (through a computation) in the first place. We prove minimum heat dissipation kT ln 2 per output calculation bit, where T is the thermodynamic temperature of unavoidable by-product bits (i.e. not the output calculation bits) rather than a generally assumed ‘surrounding environment’ temperature. Overall, this places computers into the same category as conventional machines, obeying the second law of thermodynamics and always operating below 100% efficiency, such that a perpetual calculating machine cannot exist. 相似文献
80.
Stuart D. Jackson 《Optics Communications》2008,281(14):3837-3840
High power and highly efficient operation of a Tm3+, Ho3+-doped silica fibre laser that is pumped with diode lasers operating at 1150 nm is demonstrated. Internal slope efficiencies approaching the Stokes limit were produced and the maximum output power was 2.9 W. High power diode lasers operating at 1150 nm are valuable pump sources for a range of fibre lasers offering output in the shortwave infrared spectrum. 相似文献