首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   78052篇
  免费   345篇
  国内免费   383篇
化学   24259篇
晶体学   787篇
力学   6726篇
数学   31933篇
物理学   15075篇
  2020年   29篇
  2018年   10448篇
  2017年   10268篇
  2016年   6071篇
  2015年   857篇
  2014年   304篇
  2013年   342篇
  2012年   3777篇
  2011年   10499篇
  2010年   5628篇
  2009年   6033篇
  2008年   6582篇
  2007年   8749篇
  2006年   205篇
  2005年   1300篇
  2004年   1526篇
  2003年   1963篇
  2002年   999篇
  2001年   247篇
  2000年   295篇
  1999年   150篇
  1998年   190篇
  1997年   142篇
  1996年   198篇
  1995年   114篇
  1994年   74篇
  1993年   92篇
  1992年   54篇
  1991年   62篇
  1990年   51篇
  1989年   59篇
  1988年   62篇
  1987年   57篇
  1986年   58篇
  1985年   46篇
  1984年   44篇
  1983年   41篇
  1982年   44篇
  1981年   40篇
  1980年   48篇
  1979年   51篇
  1978年   37篇
  1973年   32篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
  1907年   32篇
  1904年   28篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
31.
32.
The complexing process proceeding in the NiII–thiocarbohydrazide (H2N–H–NC(=S)–NH–NH2)–propanone triple system in EtOH solution and nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix has been studied. It has been found that in the first case, template synthesis leading, as a minimum, to formation of three coordination compounds of NiII with (N,N,S,S)-donor tetradentate ligands having NiL1, NiL2 and NiL3compositions where L1 is 4,6,6-trimethyl-2,3,7,8-tetraazanonen-3-di(thiohydrazide)-1,9, L2 is 4,6,6,12-tetrametyl-1,9-dithio-2,3,7,8,10,11-hexaazatridekadien-3,11-hydrazide-1 and L3 is 2,8,10,10,16-pentamethyl-5,13-dithio-3,4,6,7,11,12,14,15-octaazaheptadekatrien-2,7,15 is observed, whereas in the gelatin-immobilized matrix, a complexing process in the system considered does not occur.  相似文献   
33.
The different aspects of Vitamin D3 determination and the data reported in the literature are discussed. The spectrophotometric determination of Vitamin D3 in cod-liver oil is carried out after the alkaline saponification, extraction of unsaponified parts, precipitation of accompanying sterols and the column- and thin-layer chromatographic purification and separation of vitamin D3 on 40 cm plates from other vitamins. The dyestuff α-naphtholbenzein is suited well as standard substance for the better location and identification of vitamin D3 zone on the thin-layer plate. The results obtained from the chemical method were checked through the simultaneous biological determination.  相似文献   
34.
The theory is presented for rotational excitation of 1Π molecules by collisions with rare gas atoms. It is shown that collision dynamics are described by the two electronic potential energy surfaces which correlate with the electronically degenerate Λ components (doublets) in the diatomic molecule. Because the ±Λ degeneracy can alternatively be described as being in the rotational rather than in the electronic degree of freedom, one can reformulate the collision dynamics in terms of a single effective electronic potential which then depends on Φ, the angle of rotation about the diatomic axis. The form of this Φ dependence is found to be identical to that postulated to explain experimental data for collisional transfer of laser-induced fluorescence.  相似文献   
35.
Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H2 X1Σg+ (ν = 0,J) + 2hv → E,F1Σg+E,JE = J) + hν → H2+X2Σg++) + e?, for νE = 0, 1, or 2 and for JE = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν+ = νE, the detailed H2+ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the JE value of the intermediate state and the ν+ value of the ion.  相似文献   
36.
A study is made of the visible chemiluminescence resulting from the reaction of an atomic beam of barium with IZ under single-collision conditions (~ 10?4 torr). The resulting spectrum consists of the BaI C2Π → X2Σ emission on top of an underlying “continuum”. The variation of the BaI emission intensity with Ba and I2 flux is investigated, and it is concluded that the reaction is bimolecular. The total phenomenological cross section for barium atom removal is determined to be 86 A2, which agrees well with the total reactive cross section calculated assuming an electron jump mechanism. The short wavelength cutoff is identified as the transition from the υ′ = 41 level of the BaI C2Π32 state to the υ′ = 41 level of the BaI X2Σ state. A strict lower bound Do0(BaI) ? 102 ± 0.7 kcal/mole for the ground state dissociation energy of BaI is obtained from this short wavelength cutoff. The value Do0(BaI) = 102 ± 1 kcal/mole is recommended, where the error estimate includes the possible contribution from the final relative translational energy of the products.  相似文献   
37.
Reactions of salicylaldehydes with boronate ester derivatives of aniline have been examined. Addition of these Schiff base ligands to palladium acetate or Na2PdCl4 afforded novel boron-containing trans-bis(N-arylsalicylaldiminato) palladium complexes.Condensation of salicylaldehyde (2-HOC6H4C(O)H) with H2NC6H4Bpin (pin=1,2-O2C2Me4) afforded the boron-containing Schiff bases, 2-HOC6H4C(H)=NC6H4Bpin (1–3a). Similar reactivity with 2-hydroxy-5-nitrobenzaldehyde and 2-hydroxy-1-naphthaldehyde gave the corresponding Schiff bases (1-3b) and (1-3c), respectively. Reaction of Schiff bases (2) and (3) with palladium acetate or Na2PdCl4 afforded complexes of the type PdL2 (4,5), where L=deprotonated Schiff base. The molecular structure of the nitro-salicylaldehyde 4-Bpin palladium complex (5b) was characterized by an X-ray diffraction study. All new palladium compounds have been characterized fully and tested for their antifungal activity against Aspergillus niger and Aspergillus flavus.  相似文献   
38.
An electrospray ionization (ESI) ambient pressure ion-mobility spectrometer (APIMS) interfaced to an orthogonal reflector time-of-flight mass spectrometer (TOFMS) was evaluated for the first time as a detector for the identification of phenylthiohydantoin (PTH)-derivatized amino acids, the final products in the Edman sequencing process of peptides and proteins. The drift and flight times of the twenty common PTH amino acids were characterized by a well-defined 2-D mobility/mass spectral pattern. The combination of mobility/mass modes of analysis gave rise to a unique trend-line formation for the series of PTH amino acids. In addition, each PTH amino acid had a unique reduced mobility constant K(o), thus enabling the differentiation of all the amino acid derivatives including the PTH-leucine and PTH-isoleucine isomers. More importantly it was shown that it was possible to resolve a complete reference mixture of PTH amino acids in a single experimental run in less than 1 min. Detection limits for the PTH amino acids were found to range from 1.04 to 3.52 ng; indicating that the limits of detection were less than 17.0 pmol for all of the PTH amino acids.  相似文献   
39.
The coordination compound [Co(C10H7COO)2(H2O)3]2n · 4nH2O was prepared by the reaction of 1-naphthoic acid and cobalt(II) acetate tetrahydrate in basic solution, and was fully characterized by X-ray diffraction, element analysis, FTIR, TG-DTA and luminescent spectra. In the crystal the six-coordinated Co(II) centers are linked into one-dimensional zigzag chains by water molecules, which are further assembled into a two-dimensional network through weak inter-chain C–H···π interactions. The solid complex exhibits favorable fluorescent properties similar to those of free ligand at room temperature, which can be assigned to the intraligand electronic transfer.  相似文献   
40.
It has been established that on heating, 3-benzoyl-4-hydroxy-1-methyl-4-phenylpiperidine is ring-opened in the presence of arylamines by a type of retroaldol reaction, with subsequent transamination of the intermediate Mannich base and the formation of 3-arylamino-1-oxo-1-phenylpropanes. When using arylhydrazines this γ-piperidol is recyclized with the formation of 1,3-diarylpyrazoles and their 4,5-dihydro derivatives. The mass spectral behavior of a series of 3-arylamino-substituted 1-phenylpropanones has been studied. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1486–1495, October, 2007.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号