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   1805篇
  免费   57篇
  国内免费   13篇
化学   1378篇
晶体学   8篇
力学   41篇
综合类   1篇
数学   148篇
物理学   299篇
  2022年   20篇
  2021年   19篇
  2020年   34篇
  2019年   25篇
  2018年   26篇
  2017年   23篇
  2016年   58篇
  2015年   51篇
  2014年   45篇
  2013年   84篇
  2012年   113篇
  2011年   95篇
  2010年   71篇
  2009年   57篇
  2008年   118篇
  2007年   123篇
  2006年   111篇
  2005年   124篇
  2004年   93篇
  2003年   70篇
  2002年   68篇
  2001年   26篇
  2000年   25篇
  1999年   24篇
  1998年   23篇
  1997年   13篇
  1996年   20篇
  1995年   18篇
  1993年   10篇
  1992年   7篇
  1991年   10篇
  1988年   12篇
  1986年   6篇
  1985年   18篇
  1984年   18篇
  1982年   22篇
  1981年   16篇
  1980年   11篇
  1979年   24篇
  1978年   13篇
  1977年   19篇
  1976年   15篇
  1975年   12篇
  1974年   8篇
  1972年   7篇
  1970年   8篇
  1969年   10篇
  1968年   6篇
  1967年   7篇
  1966年   6篇
排序方式: 共有1875条查询结果,搜索用时 15 毫秒
11.
l-Menthyl vinyl ether (l-MVE) was homopolymerized and copolymerized with the monomers indene (IN) and acenaphthylene (ANp) by BF3OEt2 as a catalyst. The chiral menthyl substituent was cloven from the homopolymers and copolymers using dry-hydrogen bromide gas. After the removal of optically active menthyl group, poly(vinyl alcohol) (PVA) from l-MVE homopolymer was optically inactive, and copolymers (VA-IN, VA-ANp) from l-MVE-IN and l-MVE-ANp copolymers were still optically active. Hence, in the case of l-MVE homopolymer, it was concluded that asymmetric induction in the polymer main chain can only produce pseudoasymmetry. In the case of l-MVE-IN and l-MVE-ANp copolymers, it was found that asymmetric induction proceeded in the copolymer main chain and was caused by the influence of chiral menthyl group.  相似文献   
12.
The copolymerizations of l-menthyl vinyl ether (l-MVE) with the monomers vinylene carbonate (VCA) and indene (IN) were carried out in benzene with azobisisobutyronitrile (AIBN) as an initiator to obtain optically active copolymers. The optically active l-menthyl residue from the copolymer main chain was removed using dry hydrogen bromide gas. After the ether cleavage reaction, the copolymers prepared (VA–VCA and VA–IN) were still optically active, and hence it was found that asymmetric induction had taken place in the copolymer main chain. The optical rotatory dispersion (ORD) and circular dichroism (CD) data of the original and ether-cloven copolymers were also determined.  相似文献   
13.
The photo‐initiated cationic polymerization (PCP) of epoxides using diaryliodonium salt photoacid generators (PAGs) bearing alkyl groups and anions was investigated. The properties and reactivities of a series of iodonium salts containing various cations and anions were compared in the context of a PCP reaction. The products from the decomposition of the cations of these salts were analyzed using gas chromatography‐mass spectrometry (GC‐MS) spectra. The relationship between the molecular structure of the salts and their reaction mechanism in the PCP reaction was investigated. Based on the results of the investigation, it was concluded that the structures of the cations and anions of theiodonium salts affect the PCP reaction rate, which was controlled by the products from the diaryliodonium salts. As part of an additional investigation, the diaryliodonium salts‐epoxide materials were applied to 254 nm‐photo‐patterning. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
14.
A new family of indoaniline-derived calix[4]arenes has been synthesized for the purpose of developing a new chromogenic receptor. A condensing reaction of calix[4]arene (1) with 4-(diethylamino)-2-methylaniline hydrochloride (2) in the presence of an oxidizing agent under alkaline conditions affords mono- (3), 1,2-bis- (4), 1,3-bis- (5), and tetrakisindoaniline-derived (6) calix[4]arenes after careful column chromatography. Compound 3 is crystallized from a CHCl(3)-MeOH solution, and the crystal structure was determined by X-ray analysis. The crystal is monoclinic, space group P2(1)/n, Z = 4, a = 19.507(6) ?, b = 18.591(6) ?, c = 8.524(2) ?, beta = 94.69(2) degrees. The final R value for 2406 reflections of F(o) > 3sigma(F(o)) is 0.085. A unique intramolecular hydrogen-bonding network involving the carbonyl oxygen of indoaniline for 3 implied that the quinone carbonyl group as an acceptor of the chromophore can easily be subjected to an electrostatic interaction in the lower rim. Indeed, 1,3-bis(indoaniline)-derived 2,4-bis((ethoxycarbonyl)methoxy)calix[4]arene 7, prepared by the reaction of 5 with ethyl bromoacetate in the presence of NaH, is capable of undergoing an efficient ion-dipole interaction between the binding cation and the two quinone carbonyl groups of the chromophores, so that a selective Ca(2+)-induced pronounced color change (wavelength change > 100 nm) occurs with an association constant on the order of 10(6) in 99% EtOH, making 7 of potential use as an optical sensor for Ca(2+) detection. The IR and NMR studies have indicated that Ca(2+) is encapsulated in the cavity made by the distally located OCH(2)CO(2) groups on the lower rim of the cone-shaped calix[4]arene segment. Interestingly, however, the shape of the cavity in which Ca(2+) has been encapsulated does not have a C(2) axis of symmetry, as inferred from the (1)H-(1)H COSY experiment. On the other hand, 1,2-bis(indoaniline)-derived analogue 8 shows no response with metal ions, which can be interpreted to mean the absence of a cavity for encapsulation on the lower rim.  相似文献   
15.
Cycloheptatrienes were obtained by the reaction of 2-substituted allylic alcohols with alkynes in the presence of catalytic amounts of palladium complexes and p-toluenesulfonic acid.  相似文献   
16.
[Ni(cyclam)](ClO(4))(2)-catalyzed indirect electroreduction of olefinic bromides produced six-membered compounds in low to high yields. The synthetic intermediate 49 of Ipecac and Corynanthe alkaloids was obtained in 88% yield in a highly stereoselective manner. Lactam 66, the synthetic precursor of tacamonine, was prepared in 49% yield as a mixture of two diastereoisomers. The electrolysis of the bromoacetates gave the debrominated compounds in good yields.  相似文献   
17.
All the three possible rotamers of the title compound were separated by chromatography, and unambiguously identified by NMR and X-ray analysis. One of the isomers was optically inactive Ci conformation. The other optical active forms were resolved to give a pair of enantiomers, which were characterized by optical rotation and CD spectra. Thus the optical inactivity of a compound such as meso-tartaric acid that can take Ci conformation in solution, is now ascribed to that the molecule has an optically inactive Ci conformer and equal amounts of optically active conformers, that are enantiomers, in solution.  相似文献   
18.
Polymerization of vinyl monomers having large negative e values in the presence of phosphorus compounds containing halogens was studied in order to examine the cationic initiation ability of phosphorus compounds. N-Vinylcarbazole was effectively polymerized by phosphorus compounds such as PCI3, PBr3, PCI2C6H5, PCI(C6H5)2, POCI3, and POCI2C6H5 even in benzene. The initiation ability of phosphorus compounds decreased in the order; PCI2 ? PBr3 > PCI2C6H5 > PCI(C6H5)2, and also POCI3 > POCI265. On the other hand, PCI3 and PBr3, which were less effective than POCI3, showed initiation ability for the polymerizations of styrene and α-methylstyrene in nitrobenzene as a solvent. The results of the copolymerization of styrene with methyl methacrylate by PCI3 in nitrobenzene, the result of solvent dielectric constant effect, and the effect of additives such as water, tert-butyl chloride, triethylamine, and hydroquinone, indicate the polymerization of styrene by PCI3 to proceed by a cationic mechanism.  相似文献   
19.
The alignment of nematic liquid crystalline (LC) molecules with respect to the LC cell surface was regulated between the homeotropic (H) state and the parallel (P) state by the photochromic trans—cis isomerization of surface-attached azobenzene (Az) moieties. By bringing the angular-selective photoreaction into the system, the orientational order in the P state was significantly improved to attain a universal orientational axis over the entire irradiated area, which was rotatable and erasable photochemically. Laser pulse experiments revealed that the allignment relaxation time τ was reduced from 1000 to 30 ms by increasing the pulse intensity from 10 to 20 mJ cm−2. Strong coupling between the LC and Az molecules was indicated.  相似文献   
20.
Benzaldehyde are reduced to benzyl alcohol by a model compound of NAD(P)H almost quantitatively. Reductions of some other aldehydes are also mentioned.  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