全文获取类型
收费全文 | 29552篇 |
免费 | 928篇 |
国内免费 | 356篇 |
专业分类
化学 | 20070篇 |
晶体学 | 135篇 |
力学 | 728篇 |
综合类 | 23篇 |
数学 | 5550篇 |
物理学 | 4330篇 |
出版年
2022年 | 174篇 |
2021年 | 292篇 |
2020年 | 378篇 |
2019年 | 344篇 |
2018年 | 302篇 |
2017年 | 319篇 |
2016年 | 650篇 |
2015年 | 673篇 |
2014年 | 717篇 |
2013年 | 1477篇 |
2012年 | 1485篇 |
2011年 | 1795篇 |
2010年 | 1025篇 |
2009年 | 867篇 |
2008年 | 1567篇 |
2007年 | 1549篇 |
2006年 | 1625篇 |
2005年 | 1450篇 |
2004年 | 1398篇 |
2003年 | 1128篇 |
2002年 | 1089篇 |
2001年 | 495篇 |
2000年 | 449篇 |
1999年 | 431篇 |
1998年 | 405篇 |
1997年 | 438篇 |
1996年 | 416篇 |
1995年 | 377篇 |
1994年 | 399篇 |
1993年 | 370篇 |
1992年 | 338篇 |
1991年 | 336篇 |
1990年 | 294篇 |
1989年 | 296篇 |
1988年 | 266篇 |
1987年 | 229篇 |
1986年 | 250篇 |
1985年 | 369篇 |
1984年 | 377篇 |
1983年 | 322篇 |
1982年 | 356篇 |
1981年 | 319篇 |
1980年 | 345篇 |
1979年 | 275篇 |
1978年 | 305篇 |
1977年 | 280篇 |
1976年 | 227篇 |
1975年 | 195篇 |
1974年 | 197篇 |
1973年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
991.
Graham Smith Urs D. Wermuth Peter C. Healy Jonathan M. White 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):o203-o207
The structures of two pseudopolymorphic hydrates of brucine, C23H26N2O4·4H2O, (I), and C23H26N2O4·5.25H2O, (II), have been determined at 130 K. In both (I) and (II) (which has two independent brucine molecules together with 10.5 water molecules of solvation in the asymmetric unit), the brucine molecules form head‐to‐tail sheet substructures, which associate with the water molecules in the interstitial cavities through hydrogen‐bonding associations and, together with water–water associations, give three‐dimensional framework structures. 相似文献
992.
Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids 总被引:3,自引:0,他引:3
Stieger M Richtering W Pedersen JS Lindner P 《The Journal of chemical physics》2004,120(13):6197-6206
The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids. 相似文献
993.
Stefan Furegati Walter Ganci Fabrizio Gorla Urs Ringeisen Peter Rüedi 《Helvetica chimica acta》2004,87(10):2629-2661
Phosphorylation of suitable piperidine precursors yielded a series of novel decalin‐type O,N,P‐heterocycles. The title compounds, P(3)‐axially and P(3)‐equatorially X‐substituted, cis‐ and trans‐configurated 2,4‐dioxa‐7‐aza‐, 2,4‐dioxa‐8‐aza‐, and 2,4‐dioxa‐9‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides (X=Cl, F, 4‐nitrophenoxy, and 2,4‐dinitrophenoxy), are configuratively fixed and conformationally constrained P‐analogues of acetylcholine and as such represent acetylcholine (7‐aza and 9‐aza isomers) or γ‐homo‐acetylcholine mimetics (8‐aza isomers). Being irreversible inhibitors of acetylcholinesterase (AChE), the compounds are considered to be suitable probes for the investigation of the stereochemical course of the inhibition reaction by 31P‐NMR spectroscopy. Moreover, the design of these mimetics will enable studies of molecular interactions with AChE, in particular, the recognition conformation of acetylcholine. 相似文献
994.
Fractionation of a steam distilled residue of Commiphora confusa resin has yielded four novel dammarane triterpenes characterised as (20S)-3beta-acetoxy-12beta,16beta-trihydroxydammar-24-ene, (20S)-12beta,16beta-trihydroxydammar-24-ene-3beta-O-beta-glucopyranoside, (20S)-3beta-acetoxy-12beta,16beta,25-tetrahydroxydammar-23-ene, and (20S)-3beta,12beta,16beta,25-pentahydroxydammar-23-ene. The known compounds beta-amyrin, 3beta-amyrinacetate, 2-methoxyfuranodienone, 2-acetoxyfuranodienone, (20R)-3beta-acetoxy-16beta-dihydroxydammar-24-ene, (20R)-3beta,16beta-trihydroxydammar-24-ene, 3beta-acetoxy-16beta-hydroxydammar-24-ene, 3beta-hydroxydammar-24-ene, 3beta-acetoxydammar-24-ene, and beta-sistosterol were also isolated from the same extract. The structures of the compounds were determined using spectroscopic, physical, and chemical methods. 相似文献
995.
A Variety of potential–time waveforms are useful in pulsed electrochemical detection (PED) when applied for the amperometric detection of numerous polar organic compounds following their separation by liquid chromatography (LC). Here, we compare the waveforms for pulsed amperometric detection (PAD) and integrated voltammetric detection (IVD) applied for detection of organosulfur compounds at Au electrodes in acidic media. In PAD waveforms, electrodes response is measured at a constant detection potentials. In IVD waveforms, electrodes current is integrated throughout a fast cyclic scan of the detection potential. As a consequence of this difference in detection strategy, the background signal for IVD is significantly smaller for PAD in the detection of organosulfur compounds whose response mechanisms require the concomitant formation of surface oxides on Au electrodes. Furthermore, in comparison to Pad, IVD has a larger sensitivity and a diminished system peak from 02 dissolved in the sample. Use of a preadsorption step increases detection sensitivity in both PAD and IVD. The limit of detection (S/N=3)for cysteine in LC-IVD is ca. 6 nM for a 50-μl injection (i.e., 300 fmol) using a detection waveform that includes a 1000-ms preadsorption period. 相似文献
996.
Kristian Syberg Maria Bille Nielsen Lauge Peter Westergaard Clausen Geert van Calster Annemarie van Wezel Chelsea Rochman Albert A. Koelmans Richard Cronin Sabine Pahl Steffen Foss Hansen 《Current Opinion in Green and Sustainable Chemistry》2021
- Download : Download high-res image (101KB)
- Download : Download full-size image
997.
Proton transfer reaction ion trap mass spectrometer 总被引:1,自引:0,他引:1
Prazeller P Palmer PT Boscaini E Jobson T Alexander M 《Rapid communications in mass spectrometry : RCM》2003,17(14):1593-1599
Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H(3)O(+) as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is "soft" and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results. 相似文献
998.
Comprehensive two-dimensional liquid chromatography-size-exclusion chromatography (LC x SEC) was investigated as a tool for the characterization of functional poly(methyl methacrylate) (PMMA) polymers. Ultraviolet-absorbance and evaporative light-scattering detection (ELSD) were used. A simple method to quantify ELSD data is presented. Each data point from the ELSD chromatogram can be converted into a mass concentration using experimental calibration curves. The qualitative and quantitative information obtained on two representative samples is used to demonstrate the applicability of LC x SEC for determining the mutually dependent molar-mass distributions (MMD) and functionality-type distributions (FTD) of functional polymers. The influence of the molar mass on the retention behavior in LC was investigated using LC x SEC for hydroxyl-functional PMMA polymers. The critical conditions, at which retention is--by definition--independent of molar mass, were not exactly the same for PMMA series with different end-groups. Our observations are in close agreement with theoretical curves reported in the literature. However, for practical applications of LC x SEC it is not strictly necessary to work at the exact critical solvent composition. Near-critical conditions are often sufficient to determine the mutually dependent distributions (MMD and FTD) of functional polymers. 相似文献
999.
Wolff SA Alemán EA Banerjee D Rinaldi PL Modarelli DA 《The Journal of organic chemistry》2004,69(14):4571-4576
Three disubstituted N-confused porphyrins (2-4) were prepared in ca. 4% yield using a one-pot synthesis. These porphyrins bear 3,5-di-tert-butylphenyl groups substituted at the C(5) and C(20) meso positions and para-substituted (Br, NO(2), ethynyl) phenyl groups at the C(10) and C(15) meso positions. The specific orientation of the aryl rings around the macrocycle in porphyrin 2 was definitively determined using a combination of 1D ((1)H and (13)C) and 2D (gHMQC and gHMBC) NMR spectroscopy. The absorption spectra of 2-4 in CH(2)Cl(2) and dimethylacetamide are similar to those of N-confused tetraphenylporphyrin in the same solvents but have Soret and Q-bands that are shifted to lower energies. Steady-state fluorescence measurements revealed Q(x)(0,0) and Q(x)(0,1) bands similar in energy to the unsubstituted NCPs 1i and 1e. The fluorescence quantum yield results for two of these NCPs (2, 4) are atypical of porphyrin behavior and are being further investigated by time-resolved spectroscopy. 相似文献
1000.
E. Clementi W. Koos G. C. Lie G. Ranghino 《International journal of quantum chemistry》1980,17(3):377-398
The energy of (H2O)3 has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long-range induction interaction. 相似文献