全文获取类型
收费全文 | 1929篇 |
免费 | 55篇 |
国内免费 | 9篇 |
专业分类
化学 | 1204篇 |
晶体学 | 21篇 |
力学 | 38篇 |
数学 | 147篇 |
物理学 | 367篇 |
无线电 | 216篇 |
出版年
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 24篇 |
2020年 | 23篇 |
2019年 | 42篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 45篇 |
2015年 | 29篇 |
2014年 | 36篇 |
2013年 | 93篇 |
2012年 | 108篇 |
2011年 | 125篇 |
2010年 | 61篇 |
2009年 | 51篇 |
2008年 | 116篇 |
2007年 | 108篇 |
2006年 | 137篇 |
2005年 | 99篇 |
2004年 | 68篇 |
2003年 | 50篇 |
2002年 | 60篇 |
2001年 | 45篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 25篇 |
1994年 | 20篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 24篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 20篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 24篇 |
1984年 | 20篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 19篇 |
1976年 | 15篇 |
1975年 | 16篇 |
1974年 | 20篇 |
1973年 | 21篇 |
排序方式: 共有1993条查询结果,搜索用时 93 毫秒
91.
Carolyn J. Koester James F. Blankenship Patrick M. Grant 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(3):2015-2019
Using cyanoacrylate or “superglue” fuming to develop latent dermatoglyphic prints significantly altered the volatile and semivolatile compounds within the material of polyethylene zip-lock bags. Comparisons of SPME-GC/MS analyses of poly bags obtained before and after application of a glue fuming fingermark-developing technique resulted in markedly different material profiles of the bags. Not only were species added to the chemical composition of a bag, but other compounds that had been initially present were removed. These effects are particularly important for nuclear forensic investigations in the realm of route (pathway) analyses, and may also be of general interest to criminalistics laboratories that examine illicit drugs and their packaging. 相似文献
92.
Ewan Cruickshank Mirosław Salamończyk Damian Pociecha Grant J. Strachan John M. D. Storey Cheng Wang 《Liquid crystals》2013,40(10):1595-1609
ABSTRACTThe synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 34-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[11,21:24,31-terphenyl]-14-carbonitrile (CT6SCB), is also reported and shows enantiotropic NTB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-NTB transition corresponds to 4 longitudinal molecular distances. 相似文献
93.
Dr. J. Grant Hill Xiaojun Hu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(11):3620-3628
Benchmark quality geometries and interaction energies for the prereactive halogen‐bonded complexes of dihalogens and ammonia, including hypothetical astatine containing dihalogens, have been produced via explicitly correlated coupled cluster methods. The application of local electron correlation partitioning reveals dispersion, electrostatics and ionic substitutions all contribute significantly to the interaction energy, with a linear relationship between the ionic substitutions and the degree of charge transfer. Potential energy curves for H3N???ClF show that as the relative orientations of the two subunits are manipulated appreciable interactions can be found at considerably angular displaced geometries, signifying lower directionality in halogen bonding than previously supposed. 相似文献
94.
Manool is converted into ring-B-nor analogues of commercially important perfumery compounds via a novel ring-B-nor intermediate methylene ketone 9. Compound 9 is synthesised via the formation of an exocyclic bromonium ion and the concomitant ring contraction of the B-ring of a diterpene skeleton derived from manool. Oxidation and base treatment of the ring-contracted product result in dehydrohalogenation and decarboxylation to afford methylene ketone compound 9, which is then converted to ring-B-nor analogues of Ambrox® and amberketal. 相似文献
95.
Sultan J. Alsufyani Lauren R. Liegey Valeriia N. Starovoitova 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(1):623-629
We performed computational and experimental studies of the feasibility of the gold bearing ore assay utilizing the 197Au(γ,n)196Au photonuclear reaction. Gold bearing silicate samples were irradiated using bremsstrahlung produced by an electron accelerator with endpoint energies ranging from 25 to 40 MeV. 196Au yield simulations were benchmarked and experimental results were in good agreement with the predictions. Optimum electron beam energy for photon activation analysis was found to be around 32 MeV which corresponded to a detection limit of 80 ppb. Two-hour gamma-spectroscopy measurements were repeated every 24 h and the optimum sample cooling time was found to be about 100–160 h. 相似文献
96.
Harper JK Strohmeier M Grant DM 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,189(1):20-31
The (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment is demonstrated to provide shift assignments in certain powders that have two or more structurally independent molecules in the unit cell (i.e. multiple molecules per asymmetric unit). Although this class of solids is often difficult to characterize using other methods, HETCOR provides both the conventional assignment of shifts to molecular positions and associates many resonances with specific molecules in the asymmetric unit. Such assignments facilitate conformational characterization of the individual molecules of the asymmetric unit and the first such characterization solely from solid-state NMR data is described. HETCOR offers advantages in sensitivity over prior methods that assign resonances in the asymmetric unit by (13)C-(13)C correlations and therefore allows shorter average analysis times in natural abundance materials. The (1)H-(13)C analysis is demonstrated first on materials with known shift assignments from INADEQUATE data (santonin and Ca(OAc)(2) phase I) to verify the technique and subsequently is extended to a pair of unknown solids: (+)-catechin and Ca(OAc)(2) phase II. Sufficient sensitivity and resolution is achieved in the spectra to provide assignments to one of the specific molecules of the asymmetric unit at over 54% of the sites. 相似文献
97.
Grant Korensky Xinye Chen Mengdi Bao Abbi Miller Blanca Lapizco-Encinas Myeongkee Park Ke Du 《Electrophoresis》2021,42(1-2):95-102
A planar, transparent, and adaptable nanosieve device is developed for efficient microalgae/bacteria separation. In the proposed method, a sacrificial layer is applied with dual photolithography patterning to achieve a 1D channel with a very low aspect ratio (1:10 000). A microalgae/bacteria mixture is then introduced into the deformable PDMS nanochannel. The hydrodynamic deformation of the nanochannel is regulated to allow the bacteria cells to pass through while leaving the microalgae cells trapped in the device. At a flow rate of 4 μL/min, the supernatant collected from the device is indistinguishable from a control solution, indicating that nearly all the microalgae cells are trapped in the device. Additionally, this device is capable of single cell auto-fluorescence tracking. These microalgae cells demonstrate minimal photobleaching over 250 s laser exposure and could be used to monitor hazardous compounds in the sample with a continuous flow. This method will be valuable to purify microalgae samples containing contaminations and study single-cell heterogeneity. 相似文献
98.
99.
In this paper we characterize the Schatten p class membership of Toeplitz operators with positive measure symbols acting on generalized Fock spaces for the full range 0<p<∞. 相似文献
100.
A Supramolecular Sorting Hat: Stereocontrol in Metal–Ligand Self‐Assembly by Complementary Hydrogen Bonding
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Michael C. Young Lauren R. Holloway Amber M. Johnson Prof. Richard J. Hooley 《Angewandte Chemie (International ed. in English)》2014,53(37):9832-9836
A combination of self‐complementary hydrogen bonding and metal–ligand interactions allows stereocontrol in the self‐assembly of prochiral ligand scaffolds. A unique, non‐tetrahedral M4L6 structure is observed upon multicomponent self‐assembly of 2,7‐diaminofluorenol with 2‐formylpyridine and Fe(ClO4)2. The stereochemical outcome of the assembly is controlled by self‐complementary hydrogen bonding between both individual ligands and a suitably sized counterion as template. This hydrogen‐bonding‐mediated stereoselective metal–ligand assembly allows the controlled formation of nonsymmetric discrete cage structures from previously unexploited ligand scaffolds. 相似文献