全文获取类型
收费全文 | 6274篇 |
免费 | 155篇 |
国内免费 | 47篇 |
专业分类
化学 | 3938篇 |
晶体学 | 39篇 |
力学 | 210篇 |
数学 | 1286篇 |
物理学 | 1003篇 |
出版年
2022年 | 63篇 |
2021年 | 66篇 |
2020年 | 71篇 |
2019年 | 77篇 |
2018年 | 63篇 |
2017年 | 57篇 |
2016年 | 116篇 |
2015年 | 129篇 |
2014年 | 142篇 |
2013年 | 351篇 |
2012年 | 309篇 |
2011年 | 341篇 |
2010年 | 236篇 |
2009年 | 217篇 |
2008年 | 317篇 |
2007年 | 334篇 |
2006年 | 314篇 |
2005年 | 279篇 |
2004年 | 273篇 |
2003年 | 225篇 |
2002年 | 218篇 |
2001年 | 100篇 |
2000年 | 96篇 |
1999年 | 100篇 |
1998年 | 75篇 |
1997年 | 70篇 |
1996年 | 95篇 |
1995年 | 68篇 |
1994年 | 74篇 |
1993年 | 83篇 |
1992年 | 66篇 |
1991年 | 43篇 |
1990年 | 77篇 |
1989年 | 63篇 |
1988年 | 49篇 |
1987年 | 57篇 |
1986年 | 50篇 |
1985年 | 72篇 |
1984年 | 86篇 |
1983年 | 53篇 |
1982年 | 83篇 |
1981年 | 83篇 |
1980年 | 97篇 |
1979年 | 78篇 |
1978年 | 81篇 |
1977年 | 76篇 |
1976年 | 72篇 |
1975年 | 53篇 |
1974年 | 39篇 |
1973年 | 60篇 |
排序方式: 共有6476条查询结果,搜索用时 0 毫秒
41.
Jean Michel Brunel 《Tetrahedron》2007,63(18):3899-3906
A new and efficient mild Pd/P(t-Bu)3 catalyst for selective reduction of various alkenes under transfer hydrogen conditions has been developed leading to the corresponding saturated derivatives in chemical yields varying from 65 to 98%. Mechanistic rationale of this reaction has been also demonstrated. 相似文献
42.
Clémence Guillaume Jacques Le Calvé Iliana Dimicoli Michel Mons 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,32(1):157-165
The intensity distribution of benzene+-Arn cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers atn=20 and 45) have been observed in the 15–60 size range. The evaporation dynamics of these species in the 2–50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflectron time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyondn=20, a sudden decrease of the evaporation energy is observed; in then=45–47 size range, the magic number is accounted for by the specific dynamics of then=46 and 47 clusters, in particular the possible loss of two argon atoms forn=47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters. 相似文献
43.
Interchange reactions involving esters were used in reactive processing with a view to obtaining polymer blends with stabilized morphology. Dibutyltinoxide (DBTO) appeared to be an excellent catalyst for these reactions. In fact, it was shown that the true catalytic entity is a dimeric alkoxy, acyloxy distannoxane entity formed in situ, during processing, by the reaction of the DBTO with the polymer ester groups. This compound was first obtained with model esters and characterized by multinuclear NMR analysis (1H, 13C, 119Sn). The catalytic efficiency of the in situ polymeric distannoxane was compared with other added parent distannoxanes. Later on – still with model compounds – ligand exchanges at the tin sites were investigated and it was shown that these exchanges are not essential to the catalyst activity, but when they take place it increases the catalytic efficiency. Then, this type of catalysis was successfully used for interchange reactions in reactive extrusion of different polymer blends, some applications are briefly presented. To cite this article: M.-F. Llauro and A. Michel, C. R. Chimie 9 (2006). 相似文献
44.
Abdeloihid Mellouki Robert Georges Michel Herman Deanne L. Snavely Svetlana Leytner 《Chemical physics》1997,220(3):443-322
We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm−1 spectral resolution using a Fourier transform interferometer. The rotational analysis of the fundamental N---H stretch (110) at 3530.811343(82) cm−1 was performed. A set of 13 upper state rovibrational parameters was determined, allowing the 2715 assigned rovibrational lines to be reproduced with a standard deviation of 1.3 10−3 cm−1. An attempt to record the fundamental band under slit-jet conditions is reported. The role of hot bands accompanying the series of the N---H stretch excitation is investigated. Effective vibrational parameters — ω01, X011, Y111, X1,24 — are obtained. The lower level in the hot band series is unambiguously identified as the V24 = 1 level, by retrieving X1,24 independently, from other spectral data. The observation of the complex band pattern accompanying the N---H series in the higher overtone range is discussed with the help of new data, recorded around the 150 band at different temperatures using intracavity laser optoacoustic spectroscopy. 相似文献
45.
Diblock copolymers composed of poly(oxy-ethylene) (POE) and poly(dl-lactic acid) segments were synthesized by anionic polymerization of d,l-lactide using the oxyanion formed by reaction of the monohydroxyl monomethoxy-poly(ethylene glycol) on sodium hydride. For comparison, a similar copolymer was prepared by using tin octoate to catalyze the lactide polymerization. The copolymers were used to make nanoparticles, which were stored at 4 °C. After a few months under these storage conditions, a dramatic decrease of the poly(ethylene glycol) content was observed, however, the mean diameter of the nanoparticles was not affected. The degradation of the nanoparticles was investigated in vitro under conditions selected to mimic physiological conditions. Changes of characteristics were monitored by 1H NMR, SEC, DLLS and CZE on nanoparticles and/or on the degradation by-products dissolved in the ageing medium. According to their nanometric dimensions, the microparticles degraded very slowly and there was no difference in behaviour between the sodium hydride and the stannous octoate-derived copolymers. 相似文献
46.
Cataldo L Dutan C Misra SK Loss S Grützmacher H Geoffroy M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(11):3463-3468
The EPR spectrum of the novel radical Mes*(CH3)P--PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P--P bond. 相似文献
47.
Reaction of benzotriazol‐1‐yl acetone 1 with phenyl isothiocyanate followed with α‐chloroacetone or ethyl‐α‐chloroacetate afforded 2‐anilinothiophenes 3 or 4 , respectively. Treatment of 3 with malononitrile at different reaction conditions afforded 6 or 7 . Reaction of 1 with CS2 in DMF and phenacylbromide afforded S‐alkylated thiophene 10 . Reactions of the latter compound with different active methylene nitriles afforded thienylthiopyridine derivatives 14 and 15 . Condensation of 10 with hydrazine hydrate afforded hydrazon derivative 16 . Reaction of thiophene 17 with formamide in DMF afforded 19 which converted to N‐thienylpyrimidine 20 when treated with malononitrile. The structure of the newly synthesized compounds has been established on the basis of their analytical and spectral data. The compounds were also investigated for antibacterial and antifungal activities. 相似文献
48.
Pedro de Lima Neto Mohamed Atik Luis A. Avaca Michel A. Aegerter 《Journal of Sol-Gel Science and Technology》1994,1(2):177-184
ZrO2 coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC3H7)4 as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm3 of ZrO2. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35–0.75 m. The influence of the ZrO2 coatings on the corrosion behavior of stainless steel substrates in aqueous NaCl was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments. 相似文献
49.
In this work we introduce an electron localization function describing the pairing of electrons in a molecular system. This function, called "electron pair localization function," is constructed to be particularly simple to evaluate within a quantum Monte Carlo framework. Two major advantages of this function are the following: (i) the simplicity and generality of its definition; and (ii) the possibility of calculating it with quantum Monte Carlo at various levels of accuracy (Hartree-Fock, multiconfigurational wave functions, valence bond, density functional theory, variational Monte Carlo with explicitly correlated trial wave functions, fixed-node diffusion Monte Carlo, etc). A number of applications of the electron pair localization function to simple atomic and molecular systems are presented and systematic comparisons with the more standard electron localization function of Becke and Edgecombe are done. Results illustrate that the electron pair localization function is a simple and practical tool for visualizing electronic localization in molecular systems. 相似文献
50.
Rodella Cristiane B. Franco Roberto W.A. Magon Claudio J. Donoso Jose P. Nunes Luis A.O. Saeki Margarida J. Aegerter Michel A. Sargentelli Vagner Florentino Ariovaldo O. 《Journal of Sol-Gel Science and Technology》2002,25(1):83-88
Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V2O5/TiO2 catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm–1 and 770 to 850 cm–1. The Raman spectra do not exhibit characteristic peaks of crystalline V2O5. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V4+ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V4+ ions are located in sites with octahedral symmetry substituting for Ti4+ ions in the rutile structure. Magnetically interacting V4+ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V2O5, a partial oxidation of V4+ to V5+ is apparent from the EPR results. 相似文献