全文获取类型
收费全文 | 3489篇 |
免费 | 117篇 |
国内免费 | 11篇 |
专业分类
化学 | 2065篇 |
晶体学 | 130篇 |
力学 | 108篇 |
数学 | 378篇 |
物理学 | 936篇 |
出版年
2023年 | 24篇 |
2022年 | 44篇 |
2021年 | 51篇 |
2020年 | 52篇 |
2019年 | 64篇 |
2018年 | 72篇 |
2017年 | 64篇 |
2016年 | 101篇 |
2015年 | 100篇 |
2014年 | 110篇 |
2013年 | 279篇 |
2012年 | 177篇 |
2011年 | 178篇 |
2010年 | 145篇 |
2009年 | 138篇 |
2008年 | 175篇 |
2007年 | 145篇 |
2006年 | 130篇 |
2005年 | 123篇 |
2004年 | 106篇 |
2003年 | 84篇 |
2002年 | 89篇 |
2001年 | 45篇 |
2000年 | 40篇 |
1999年 | 42篇 |
1998年 | 34篇 |
1997年 | 26篇 |
1996年 | 40篇 |
1995年 | 40篇 |
1994年 | 30篇 |
1993年 | 41篇 |
1992年 | 49篇 |
1991年 | 40篇 |
1990年 | 33篇 |
1989年 | 43篇 |
1988年 | 34篇 |
1987年 | 31篇 |
1986年 | 35篇 |
1985年 | 38篇 |
1984年 | 50篇 |
1983年 | 35篇 |
1982年 | 35篇 |
1981年 | 42篇 |
1980年 | 32篇 |
1979年 | 33篇 |
1978年 | 30篇 |
1977年 | 37篇 |
1976年 | 21篇 |
1975年 | 24篇 |
1974年 | 23篇 |
排序方式: 共有3617条查询结果,搜索用时 250 毫秒
91.
Geometrical parameters associated with N-H ... N types of hydrogen bonds have been analysed using crystal structure data on
nucleic acids, amino acids and related compounds. Histograms depicting the frequency distribution of N-H ... N length (l) and H-N ... N angle (θ) have been drawn and conclusions on the favoured geometry of such bonds have been arrived at. The
distribution ofl shows a pronounced maximum in the range between 2.9? and 3.0? with an overall average of 2.98 ?. The θ distribution shows
a pronounced maximum for the hydrogen bond angle in the range 0°-10°, with a rapid fall-off in frequency for nonlinear hydrogen
bonds. The frequency shows a cos6θ dependence as compared to cos2θ dependence term used earlier to predict the angular dependence of hydrogen bond potential energy in proteins and polypeptides. 相似文献
92.
Y. Sesha Sayi K. L. Ramakumar Rajendra Prasad C. S. Yadav P. S. Shankaran G. C. Chhapru H. C. Jain 《Journal of Radioanalytical and Nuclear Chemistry》1998,230(1-2):5-9
A hot vacuum extraction technique for the determination of hydrogen in metal and alloy samples has been standardised. After measuring the total pressure of the evolved gases, individual hydrogen and deuterium intensities are measured using an on-line quadrupole mass spectrometer. Synthetic mixtures of H2 and D2, in known concentrations, have been analysed by QMS and an analytical expression correlating the measured [D2]/[HD] intensity ratio with the mole fraction of deuterium in the synthetic mixture has been arrived at. The precision and accuracy in the measurement of hydrogen is about 10% at 50 ppmw level. 相似文献
93.
Syntheses of substituted pyrazolo[3,4-b]quinolines, 3,4-dihydro-4-oxopyriraido[4′,5′:4,5]theino[2,3-b]quinoline and 12-phenylpyrido[1′,2′:1,2[pyrimido[4,5-b]quinoline are described. 相似文献
94.
Davis FA Nolt MB Wu Y Prasad KR Li D Yang B Bowen K Lee SH Eardley JH 《The Journal of organic chemistry》2005,70(6):2184-2190
[reaction: see text] Diverse organometallic reagents readily add to enantiopure N-sulfinyl beta-amino Weinreb amides providing the corresponding, stable, N-sulfinyl beta-amino carbonyl compounds in good to excellent yields. This new methodology represents a general solution to the problem of beta-amino carbonyl syntheses, which are important chiral building blocks and constituents of natural products. N-Sulfinyl beta-amino Weinreb amides are prepared by reaction of the potassium enolate of N-methoxy N-methylacetamide with sulfinimines (N-sulfinyl imines) or lithium N,O-dimethylhydroxylamine with N-sulfinyl beta-amino esters. 相似文献
95.
Prasenjit Maity Susmit Basu Sumit Bhaduri Goutam Kumar Lahiri 《Journal of molecular catalysis. A, Chemical》2007,270(1-2):117-122
High nuclearity platinum carbonyl cluster anions (Chini's clusters) have been used as precursors to prepare a platinum nanocatalyst. The ionic polyelectrolyte poly(diallyldimethylammonium chloride) has been used as the support material for anchoring [Pt30(CO)60]2− via ion-pairing and subsequent stabilization of the nanoparticles. The polymer-supported material has been studied by spectroscopy (NIR, 13C NMR, and IR) and TEM before and after its use as a water soluble hydrogenation catalyst. The nanocatalyst is found to be effective for the chemoselective hydrogenation of olefinic, aldehydic and ketonic double bonds. For most of the substrates isolation of the product and reuse of the catalyst are extremely easy due to the automatic phase separation of the products from the catalyst. The spectral features of the fresh catalyst show retention of the carbonyl ligands and molecular identity of the parent cluster, but after use the carbonyl ligands appear to be lost. TEM of the supported material before and after use as a catalyst shows the presence of platinum nanoparticles with majority (≥70%) of the particles in the range of 2–6 nm. Smaller particles are dominant in the used catalyst and this observation is rationalized on the basis of the known reactivity of Chini's clusters with dihydrogen. 相似文献
96.
Solvent-adaptable silver nanoparticles 总被引:4,自引:0,他引:4
Prasad BL Arumugam SK Bala T Sastry M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(3):822-826
A simple and efficient way of obtaining silver nanoparticles that are dispersible both in organic and in aqueous solvents using a single capping agent is described. The silver nanoparticles are initially prepared in water in the presence of aerosol OT [sodium bis(2-ethylhexyl)-sulfosuccinate, AOT]. Thereafter, transfer of the AOT-capped silver nanoparticles to an organic phase is induced by the addition of a small amount of orthophosphoric acid during shaking of the biphasic mixture. The AOT-stabilized silver nanoparticles could be separated out from the organic phase in the form of a powder. The hydrophobic nanoparticles thus prepared are stable and are readily resuspended in a variety of other polar (including water) and nonpolar solvents without further surface treatment. The amphiphatic nature of the silver surface is brought about by a small orientational change in the AOT monolayer on the silver surface in response to the polarity of the solvent. 相似文献
97.
[reaction: see text] N-Sulfinyl beta-amino Weinreb amides are prepared by condensation of sulfinimines with the potassium enolate of N-methoxy-N-methylacetamide. These new chiral building blocks are useful for the asymmetric synthesis of beta-amino carbonyl compounds, as illustrated here by the concise enantioselective syntheses of sedum alkaloids (+)-sedridine and (-)-allosedridine. 相似文献
98.
L. C. Tribedi K. G. Prasad P. N. Tandon 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,27(2):143-148
Single and double K-vacancy fractions at sulphur ions in Fe and Ni are measured for ion velocities between 7v 0 and 14v 0 (v 0=c/137) using the probe layer technique. The derived values for the degree of polarization of these ions from the available transient magnetic field data at a velocity of 8v 0 in Fe and Ni agrees with the general trends seen for light ions. 相似文献
99.
A general partial summation method for including arbitrary classes of diagrams to all orders in the coupled cluster based size consistent energy functional for closed shell states is developed. Since the various reduced density matrices which appear in the energy functional are essentially the time-independent analogues of the corresponding many body Green functions, it is possible to derive Dyson-like equations for these quantities. By expanding the associated proper self energy parts in terms of the T-amplitudes, one can carry out partial summations in the reduced density matrices and thus in energy. At a higher level, higher order terms in a proper self energy can also be generated by renormalizing the internal propagators in it, and considering only the irreducible self-energy terms. 相似文献
100.
Euchrenone a2 (7) isolated from the roots ofEuchresta japonica has been synthesised from 3-prenylphloroacetophenone (1) by other workers. We carried out its cyclodehydrogenation with dichloro dicyano quinone (DDQ) to obtain 6-acetyl-5,7-dihydroxy-2,2-dimethylchromene
(2) which was ethoxymethylated in the 7-position to give 6-acetyl-7-ethoxymethoxy-5-hydroxychromene (3). Chalcone condensation of3 and 4-ethoxymethoxy-3-C-prenylbenzaldehyde (4) gave 4,6′-bisethoxymethoxy-2′-hydroxy-6″, 6″-dimethyl-3-C-prenylpyrano (2″, 3″–4,3) chalcone (5) which cyclised with methanolic sodium acetate to give protected 5,4′-bisethoxymethoxy-6″, 6″-dimethyl-3′-C-prenylpyrano (2″, 3″–7,8) flavanone (6). Deprotection of6 with 4% methanolic HCl yielded (7) with melting point and spectral data identical to that of the natural compound. 相似文献