全文获取类型
收费全文 | 14671篇 |
免费 | 665篇 |
国内免费 | 98篇 |
专业分类
化学 | 10407篇 |
晶体学 | 81篇 |
力学 | 237篇 |
数学 | 2157篇 |
物理学 | 2552篇 |
出版年
2023年 | 106篇 |
2022年 | 101篇 |
2021年 | 208篇 |
2020年 | 349篇 |
2019年 | 307篇 |
2018年 | 192篇 |
2017年 | 157篇 |
2016年 | 490篇 |
2015年 | 377篇 |
2014年 | 447篇 |
2013年 | 748篇 |
2012年 | 909篇 |
2011年 | 1081篇 |
2010年 | 592篇 |
2009年 | 465篇 |
2008年 | 858篇 |
2007年 | 857篇 |
2006年 | 837篇 |
2005年 | 783篇 |
2004年 | 611篇 |
2003年 | 443篇 |
2002年 | 490篇 |
2001年 | 224篇 |
2000年 | 217篇 |
1999年 | 189篇 |
1998年 | 190篇 |
1997年 | 169篇 |
1996年 | 197篇 |
1995年 | 167篇 |
1994年 | 185篇 |
1993年 | 164篇 |
1992年 | 142篇 |
1991年 | 121篇 |
1990年 | 101篇 |
1989年 | 87篇 |
1988年 | 66篇 |
1987年 | 74篇 |
1985年 | 122篇 |
1984年 | 103篇 |
1983年 | 75篇 |
1982年 | 118篇 |
1981年 | 96篇 |
1980年 | 83篇 |
1979年 | 109篇 |
1978年 | 95篇 |
1977年 | 88篇 |
1976年 | 97篇 |
1975年 | 67篇 |
1974年 | 63篇 |
1973年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
In the structure Ba12F19Cl5 [hexagonal space group P6 2m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba12F19ClδBr5–δ with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF2 and 25 mol% BaX2 with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl?/Br? population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1). 相似文献
142.
Wolfgang Oppolzer Mark Kurth Daniel Reichlin Frank Moffatt 《Tetrahedron letters》1981,22(27):2545-2548
The chiral induction in the Diels-Alder addition → , assessed reliably by 19F-NMR-spectroscopy of the endo-esters , varied between 47 - 93% in favor of the 2-(R)-adducts depending on the auxiliary chiral group and the Lewis-acid catalyst. 相似文献
143.
57Fe electric and magnetic hyperfine parameters were calculated for a series of 10 iron model complexes, covering a wide range of oxidation and spin states. Employing the B3LYP hybrid method, results from nonrelativistic density functional theory (DFT) and quasi-relativistic DFT within the zero-order regular approximation (ZORA) were compared. Electron densities at the iron nuclei were calculated and correlated with experimental isomer shifts. It was shown that the fit parameters do not depend on a specific training set of iron complexes and are, therefore, more universal than might be expected. The nonrelativistic and quasi-relativistic electron densities gave fit parameters of similar quality; the ZORA densities are only shifted by a factor of 1.32, upward in the direction of the four-component Dirac-Fock value. From a correlation of calculated electric field gradients and experimental quadrupole splittings, the value of the 57Fe nuclear quadrupole moment was redetermined to a value of 0.16 barn, in good agreement with other studies. The ZORA approach gave no additional improvement of the calculated quadrupole splittings in comparison to the nonrelativistic approach. The comparison of the calculated and measured 57Fe isotropic hyperfine coupling constants (hfcc's) revealed that both the ZORA approach and the inclusion of spin-orbit contributions lead to better agreement between theory and experiment in comparison to the nonrelativistic results. For all iron complexes with small spin-orbit contributions (high-spin ferric and ferryl systems), a distinct underestimation of the isotropic hfcc's was found. Scaling factors of 1.81 (nonrelativistic DFT) and 1.69 (ZORA) are suggested. The calculated 57Fe isotropic hfcc's of the remaining model systems (low-spin ferric and high-spin ferrous systems) contain 10-50% second-order contributions and were found to be in reasonable agreement with the experimental results. This is assumed to be the consequence of error cancellation because g-tensor calculations for these systems are of poor quality with the existing DFT approaches. Excellent agreement between theory and experiment was found for the 57Fe anisotropic hfcc's. Finally, all of the obtained fit parameters were used for an application study of the [Fe(H2O)6]3+ ion. The calculated spectroscopic data are in good agreement with the Mossbauer and electron paramagnetic resonance results discussed in detail in a forthcoming paper. 相似文献
144.
Kratzsch C Tenberken O Peters FT Weber AA Kraemer T Maurer HH 《Journal of mass spectrometry : JMS》2004,39(8):856-872
A liquid chromatographic/mass spectrometric assay with atmospheric pressure chemical ionization (LC/APCI-MS) is presented for fast and reliable screening and identification and also for precise and sensitive quantification in plasma of the 23 benzodiazepines alprazolam, bromazepam, brotizolam, camazepam, chlordiazepoxide, clobazam, clonazepam, diazepam, flunitrazepam, flurazepam, desalkylflurazepam, lorazepam, lormetazepam, medazepam, metaclazepam, midazolam, nitrazepam, nordazepam, oxazepam, prazepam, temazepam and tetrazepam, triazolam, their antagonist flumazenil and the benzodiazepine BZ1 (omega 1) receptor agonists zaleplone, zolpidem and zopiclone. It allows confirmation of the diagnosis of an overdose situation and monitoring of psychiatric patients' compliance. The analytes were isolated from plasma using liquid-liquid extraction and were separated on a Merck LiChroCART column with Superspher 60 RP Select B as the stationary phase. Gradient elution was performed using aqueous ammonium formate and acetonitrile. After screening and identification in the scan mode using the authors' LC/MS library, the analytes were quantified in the selected-ion monitoring mode. The quantification assay was fully validated. It was found to be selective proved to be linear from sub-therapeutic to over therapeutic concentrations for all analytes, except bromazepam. The corresponding reference levels the assay's accuracy and precision data for all studied substances are listed. The accuracy and precision data were within the required limits with the exception of those for bromazepam. The analytes were stable in frozen plasma for at least 1 month. The validated assay was successfully applied to several authentic plasma samples from patients treated or intoxicated with various benzodiazepines or with zaleplone, zolpidem or zopiclone. It has proven to be appropriate for the isolation, separation, screening, identification and quantification of the drugs mentioned above in plasma for clinical toxicology, e.g. in cases of poisoning, and forensic toxicology, e.g. in cases of driving under the influence of drugs. 相似文献
145.
Pozzoli L Bolzacchini E Van Dingenen R Hjiorth J Dentener F Perrone G Rindone B Librando V 《Annali di chimica》2003,93(4):447-456
The model BAGS (Boxmodel for Aerosol and Gasphase Simulations) has been developed. It is composed of two major modules: the first one describes the system of the chemical reactions in the gaseous phase, the second one calculates the aerosol chemical composition and the dimensional distribution of the particles. The boxmodel has been developed with the introduction of new chemical and physical processes, not previously included, in particular the formation of Secondary Organic Aerosol. The other implemented processes are a module for the dynamic of the particle population, nucleation, coagulation and dry deposition. The last phase of the work has been a check of the BAGS capabilities by a series of tests, that have permitted to compare it with other models (MAPS and MADM). The tests in particular have concerned the aerosol water content prediction, the photochemistry, the condensation of the inorganic compounds and the formation of Secondary Organic Aerosol. 相似文献
146.
[reaction: see text] Treatment of 1,4-dilithio-1,3-butadiyne (1) with dichalcogenides RSSR or RSeSeR affords dithio- and diseleno-1,3-butadiynes (2, 3), perthio- and perseleno-[3]-cumulenes (4, 5), perthio- and perseleno-1,3-butadienes (6, 7), and/or perthio- and perseleno-but-1-ene-3-ynes (8, 9). The products can be controlled by stoichiometry and temperature, by the presence or absence of oxygen, and by choice of the "R" group. By X-ray crystallography, hexa(methylthio)-1,3-butadiene is highly twisted, with a torsion angle [Phi(CCCC)] of 84.7 degrees and an elongated C(2)-C(3) distance of 1.484(3) A. 相似文献
147.
Frank R. Fronczek Richard D. Gandour Laura M. B. Gehrig Lyman R. Caswell Karen A. McDowell I. Alam 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(3):379-383
The first crystal structure of a potassium cation complex with dicyclohexano-18-crown-6 is reported. The potassium 2-nitrophenoxide complex ofsyn-cis-syn dicyclohexano-18-crown-6 crystallizes in the triclinic space group P
with cell constantsa=8.604(2),b=10.772(4),C=16.123(5)Å, =73.86(3)°,=77.61(3)°, =82.68(3)° andZ=2 forD
c
=1.31 g cm–3. Least-squares refinement based on 2742 observed reflections led to a final conventionalR value of 0.040. Dicyclohexano-18-crown-6 has the shape of a saddle with the potassium cation sitting at the saddlepoint. The structure of the 2-nitrophenoxide anion is dominanted by the quinoid resonance contributor. Because the complex is devoid of significant intercomplex interactions, it is a prototypical 1:1:1 complex.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82043 (26 pages).Now Mrs. K. M. Balo. 相似文献
148.
In a series of methyl substituted 1-(2′,6′-dichlorobenzyl)-1,4-dihydronicotinamides those compounds with the methyl α to the pyridine nitrogen are at least one order of magnitude more reactive than the analogues without methyl or methyl on the 5-position. 相似文献
149.
Diorganogallium Fluorides. The Crystal Structure of the Mixed Crystal [B(CH2Ph)3]0.92[Ga(CH2Ph)3]0.08 · NCMe The reaction of GaR3 with BF3 · OEt2 in diethylether leads to the diorganogallium fluorides R2GaF [R = i-Pr ( 1 ), CH2Ph ( 2 ), Mes ( 3 )]. Compound 1 is also available by the reaction of i-Pr2GaBr ( 6 ) with KF at ?20°C in acetonitrile. The by-product B(CH2Ph)3, formed together with 2 during the first reaction, crystallizes with ca. 8% Ga(CH2Ph)3 in acetonitrile as [B(CH2Ph)3]0.92[Ga(CH2Ph)3]0.08 · NCMe ( 4 ) in the space group P21/n with a = 1050.32(7) pm, b = 1159.5(2) pm, c = 1591.6(1) pm and β = 96.931(6)°. 相似文献
150.
Classifying wine according to geographical origin via quadrupole-based ICP–mass spectrometry measurements of boron isotope ratios 总被引:1,自引:0,他引:1
The potential of quadrupole-based ICP–MS as a tool for B-isotopic analysis of wines and its usefulness in provenance determinations
were assessed. A precision of 0.1–0.25% RSD (corresponding to a relative standard deviation of the mean of three replicate
measurements of 0.06–0.12%) was sufficient to establish small differences in the B isotope ratios in wines from different
geographical origins. Each sample measurement was bracketed by measurements of a standard and mass bias drift correction made
by interpolation. Sample preparation was kept to a minimum to avoid possible fractionation. Dilution of the wine samples by
a factor of 100 with 0.65% HNO3 was found to reduce matrix-induced mass discrimination substantially. Wines from three wine-producing regions, Stellenbosch,
Robertson, and Swartland, in the Western Cape Province of South Africa, and wines from specific regions in France (Bergerac)
and Italy (Valpolicella) were analyzed by ICP–QMS for their B-isotopic compositions. It was concluded that the 11B/10B ratios can be used to characterize wines from different geographical origins. Average 11B/10B ratios in red wines from South Africa (Stellenbosch), France (Bergerac), and Italy (Valpolicella) were found to differ by
between 0.5 and 1.5%. 相似文献