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   2069篇
  免费   88篇
  国内免费   18篇
化学   1415篇
晶体学   4篇
力学   69篇
数学   436篇
物理学   251篇
  2022年   16篇
  2021年   15篇
  2020年   19篇
  2019年   34篇
  2018年   27篇
  2017年   20篇
  2016年   48篇
  2015年   56篇
  2014年   46篇
  2013年   143篇
  2012年   119篇
  2011年   119篇
  2010年   92篇
  2009年   75篇
  2008年   120篇
  2007年   125篇
  2006年   109篇
  2005年   99篇
  2004年   116篇
  2003年   95篇
  2002年   97篇
  2001年   43篇
  2000年   36篇
  1999年   26篇
  1998年   27篇
  1997年   22篇
  1996年   42篇
  1995年   21篇
  1994年   24篇
  1993年   13篇
  1992年   23篇
  1991年   14篇
  1990年   7篇
  1989年   23篇
  1988年   15篇
  1987年   9篇
  1986年   11篇
  1985年   20篇
  1984年   20篇
  1983年   15篇
  1982年   19篇
  1981年   19篇
  1980年   17篇
  1979年   15篇
  1978年   16篇
  1977年   16篇
  1976年   10篇
  1975年   12篇
  1974年   14篇
  1973年   17篇
排序方式: 共有2175条查询结果,搜索用时 15 毫秒
941.
Polyphenolic nonahydroxyterphenoyl-containing C-glycosidic oak ellagitannins are found in wine as a result of the aging of this beverage in oak-made barrels. Once in the slightly acidic wine (pH approximately 3-4), some of these complex natural products such as (-)-vescalagin (1), but not its C-1 epimer (-)-castalagin (2), can capture grape-derived nucleophilic entities such as ethanol, the flavanols catechin (10a) and epicatechin (10b), the anthocyanin oenin (13b), and the thiolic glutathione (16) to furnish condensation products with retention of configuration at the C-1 locus. A computer-aided rationale of this high diastereoselectivity is given. These condensation products can contribute to the modulation of organoleptic properties of the wine, as evidenced by the 23 nm bathochromic shift color absorbance observed with the novel oenin-based anthocyano-ellagitannin (15b). Hydrolysis of 1 under solvolytic conditions furnished another novel compound that we refer to as vescalene (21), in addition to the known (-)-vescalin (18). Of pharmacological importance is the fact that most of these found-in-wine water-soluble ellagitannin derivatives are much more potent than etoposide (VP-16) at inhibiting top2-mediated DNA decatenation in vitro (top2=topoisomerase II)). The known (-)-vescalin (18) and the novel vescalene (21) fully inhibited top2 at 10 microM concentration!  相似文献   
942.
A systematic study on the Stille and Sonogashira cross‐coupling of iodinated imidazo[1,2‐a]pyridines was performed, permitting the preparation of various vinyl‐, ethynyl‐, and allenyl‐substituted derivatives. These methods are particularly valuable, given their experimental simplicity and high degree of flexibility with regard to functional groups that can be introduced in positions 3, 6, or 8 of the imidazo[1,2‐a]pyridine core. Effects concerning different substitution positions and the nature of the 2‐substituent under various reaction conditions are reported in detail for the above types of unsaturated groups introduced.  相似文献   
943.
The half-projected Hartree–Fock function (HPHF ) for singlet states is defined as a linear combination of two Slater determinants which contains only spin eigenstates with even spin quantum numbers. The possible uses of such an approach for determining molecular properties are investigated computing the potential energy curve, binding energy, force constant, and dipole moment variation corresponding to the lithium hydride ground state. Full projected and restricted Hartree–Fock calculations (PHF and RHF ) are performed simultaneously for comparison purposes. It is found that the HPHF model yields very satisfactory results, very close to those of the PHF scheme. Both models predict properly the molecular behavior as a function of nuclear separation, whereas the RHF one fails. A discussion is given in terms of configuration equivalents. It is concluded that the HPHF scheme seems to be useful for determining molecular properties specially in the case of large systems in which the more sophisticated methods are unmanageable.  相似文献   
944.
Synthesis of linear polyphosphonates and polyphosphonamides is achieved by polycondensation in solution of aromatic diols and diamines with alkyldiphosphonyl tetrachlorides, in the presence of a hydrochloric acid trapping agent (triethylamine). The main limiting factors for chain growth are the insolubility of the polymers and the occurrence of side reactions.  相似文献   
945.
946.
In this communication, the structural analysis of six synthetic O-Linked amphiphilic cholesteryl polyethoxy neoglycolipids containing N-acetyl-D-glucosamine was performed by electrospray ionization mass spectrometry in the positive ion mode, with a QqTOF-MS/MS hybrid instrument. The MS/MS analyses provided evidence for the "in situ" formation, in the collision cell of the tandem mass spectrometer, of an unexpected and unique [C-glycoside]+ product ion, resulting from an ion-molecule reaction between the N-acetyl-D-glucosamine oxonium ion and the neutral cholesta-3,5-diene molecule. Quasi MS3 analysis of this ion resulted in the dissociation of the precursor [C-glycoside]+ ion, which produced the expected third generation N-acetyl-D-glucosamine oxonium and the protonated cholesta-3,5-diene product ions.  相似文献   
947.
Oriented stable binding of functional proteins on surfaces is of fundamental interest for receptor/ligand studies in atomic force microscopy (AFM) and surface plasmon resonance (SPR) experiments. Here we have chosen the His6-tagged carboxyl-tail (C-tail) of the alpha1c-subunit of the L-type Ca2+ channel and calmodulin (CaM) as its cognitive partner as a model system to develop a new functional surface. Covalently attached self-assembled monolayers on ultraflat gold containing NTA-thiols to which the His6-tagged C-tail was bound and thiols with triethylene-glycol groups as matrix-thiols represented the system of choice. The topography of this surface was characterized using AFM; its ability to bind C-tail proteins oriented and stable was confirmed by SPR measurements and by complementary force spectroscopy experiments with a CaM4-construct covalently attached to the tip. The developed anchoring strategy can now be used to study receptor/ligand interactions in general applying force spectroscopy and SPR on His6-tagged proteins oriented immobilized onto this new NTA-functionalized self-assembled monolayer.  相似文献   
948.
Derivatives of new polydienic systems have been isolated. Two [2]benzothienophthalazines, nitrogenous analogues of unknown anthra[2,3-c]thiophens are described. A furo[3,4-g]-phtalazine and a thieno[3,4-g]phthalazine are isolated. These different compounds react with philodienes. The first tetraza-2,3,8,9-naphthacenes have been synthesized. Structures of these compounds are proved by chemical and spectrographical methods.  相似文献   
949.
Crystal structures of three Ni(CN)(4)(2)(-) salts all with eclipsed ligands and varying axial stacking arrangements are presented. The absorption spectra of all three salts show a slight red shift in the x,y-polarizations and a large red shift in their z-polarizations upon crystallization from solution. Semiempirical ZINDO calculations provide a good model of the solid state, even with only a three-molecule segment, allowing reproduction of the red-shifting and intensity increase upon crystallization found experimentally. The modified nickel beta(s,p) bonding parameter of -5 found appropriate for Ni coordination in our previous studies of single Ni(CN)(4)(2-) planes and a helically stacked Cs(2)[Ni(CN)(4)].H(2)O crystal was changed to -3 for the more parallel-stacked Ni(CN)(4)(2-) planes in this case, while beta(d) was retained at -41. Crystal data are as follows: Na(2)[Ni(CN)(4)].3H(2)O, triclinic space group P1, a = 7.2980(10) A, b = 8.8620(10) A, c = 15.132(2) A, alpha = 89.311(5) degrees, beta = 87.326(5) degrees, gamma = 83.772(6) degrees, V = 971.8(2) A(3), T = 100 K, Z = 4, R = 0.024, R(w) = 0.064; Sr[Ni(CN)(4)].5H(2)O, monoclinic space group C2/m, a = 10.356(2) A, b = 15.272(3) A, c = 7.1331(10) A, beta = 98.548(12) degrees, V = 1115.6(3) A(3), T = 100 K, Z = 4, R = 0.024, R(w) = 0.059; Rb(2)[Ni(CN)(4)].1.05H(2)O, triclinic space group P1, a = 8.6020(10) A, b = 9.6930(10) A, c = 12.006(2) A, alpha = 92.621(6) degrees, beta = 94.263(6) degrees, gamma = 111.795(10) degrees, V = 924.0(2) A(3), T = 100 K, Z = 4, R = 0.034, R(w) = 0.067.  相似文献   
950.
Partial methylation of methyl N-acetyl-β-d-neuraminate methyl glycoside using methyl iodide and silver oxide gives a mixture of methyl ethers of methyl N-acetyl-N-methyl-β-d-neuraminate methyl glycoside, which was fractionated by chloroform-water partition followed by preparative column chromatography on silica gel. After trimethylsilylation of the fractions, gas-liquid chromatography on OV-101 and mass spectrometry facilitated the identification of 13 methyl ethers.  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