全文获取类型
收费全文 | 2136篇 |
免费 | 46篇 |
国内免费 | 9篇 |
专业分类
化学 | 1415篇 |
晶体学 | 10篇 |
力学 | 37篇 |
数学 | 391篇 |
物理学 | 338篇 |
出版年
2021年 | 30篇 |
2020年 | 21篇 |
2019年 | 22篇 |
2018年 | 19篇 |
2017年 | 18篇 |
2016年 | 37篇 |
2015年 | 30篇 |
2014年 | 32篇 |
2013年 | 119篇 |
2012年 | 91篇 |
2011年 | 107篇 |
2010年 | 66篇 |
2009年 | 50篇 |
2008年 | 91篇 |
2007年 | 91篇 |
2006年 | 108篇 |
2005年 | 116篇 |
2004年 | 88篇 |
2003年 | 75篇 |
2002年 | 66篇 |
2001年 | 31篇 |
2000年 | 28篇 |
1999年 | 20篇 |
1998年 | 22篇 |
1997年 | 25篇 |
1996年 | 28篇 |
1995年 | 31篇 |
1994年 | 24篇 |
1993年 | 24篇 |
1992年 | 30篇 |
1991年 | 24篇 |
1990年 | 16篇 |
1989年 | 18篇 |
1988年 | 26篇 |
1986年 | 16篇 |
1985年 | 45篇 |
1984年 | 42篇 |
1983年 | 31篇 |
1982年 | 43篇 |
1981年 | 40篇 |
1980年 | 47篇 |
1979年 | 29篇 |
1978年 | 37篇 |
1977年 | 34篇 |
1976年 | 28篇 |
1975年 | 29篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 15篇 |
1969年 | 13篇 |
排序方式: 共有2191条查询结果,搜索用时 15 毫秒
991.
Richard D. Adams Burjor Captain Jack L. Smith Jr. 《Journal of organometallic chemistry》2004,689(1):65-70
Diacenaphtho[1,2-c:1′,2′-e]-1,2-dithiin 2 was synthesized in 23% yield by the reaction of acenaphthylene with elemental sulfur at 120 °C. This reaction also afforded either diacenaphtho[1,2-b:1′,2′-d]thiophene 1 or diacenaphtho[1,2-b:1′,2′-e]-dihydro[e]-1,4-dithiin 3 depending on the reaction time. Compound 2 was desulfurized and converted to 1 under UV-vis irradiation in a benzene solution. Reaction of 2 with Pt(COD)2 yielded the complex Pt(COD)(C24H12S2) 4 (COD=1,5-cyclooctadiene) by insertion of a Pt(COD) group into the S-S bond of 2. When heated, 4 was desulfurized and converted to 1 by elimination of a (COD)PtS grouping. Compounds 1-4 were characterized crystallographically. 相似文献
992.
Reactions of activated halo compounds XCH(2)-A (X = Br, I; A = ester, ketone) with C(60)(2-) anion give rise to C(60)(CH(2)-A)(2) adducts (major products) along with unexpected methanofullerenes C(60)>CH-A and monosubstituted dihydrofullerenes C(60)(H)(CH(2)-A) (minor products). Methanofullerenes are shown to come from side reactions with X(2)CH-A traces. [reaction: see text] 相似文献
993.
We have constructed a group of classical potentials based on ab initio density-functional theory (DFT) calculations to describe the chemical bonding between benzenedithiolate (BDT) molecule and gold atoms, including bond stretching, bond angle bending, and dihedral angle torsion involved at the interface between the molecule and gold clusters. Three DFT functionals, local-density approximation (LDA), PBE0, and X3LYP, have been implemented to calculate single point energies (SPE) for a large number of molecular configurations of BDT-1, 2 Au complexes. The three DFT methods yield similar bonding curves. The variations of atomic charges from Mulliken population analysis within the molecule/metal complex versus different molecular configurations have been investigated in detail. We found that, except for bonded atoms in BDT-1, 2 Au complexes, the Mulliken partial charges of other atoms in BDT are quite stable, which significantly reduces the uncertainty in partial charge selections in classical molecular simulations. Molecular-dynamics (MD) simulations are performed to investigate the structure of BDT self-assembled monolayer (SAM) and the adsorption geometry of S adatoms on Au (111) surface. We found that the bond-stretching potential is the most dominant part in chemical bonding. Whereas the local bonding geometry of BDT molecular configuration may depend on the DFT functional used, the global packing structure of BDT SAM is quite independent of DFT functional, even though the uncertainty of some force-field parameters for chemical bonding can be as large as approximately 100%. This indicates that the intermolecular interactions play a dominant role in determining the BDT SAMs global packing structure. 相似文献
994.
The reaction of Rh(4)(CO)(12) with Ph(3)GeH at 97 degrees C has yielded the first rhodium cluster complexes containing bridging germylene and germylyne ligands: Rh(8)(CO)(12)(mu(4)-GePh)(6), 9, and Rh(3)(CO)(5)(GePh(3))(mu-GePh(2))(3)(mu(3)-GePh)(mu-H), 10. When the reaction is performed under hydrogen, the yield of 9 is increased to 42% and no 10 is formed. Compound 9 contains a cluster of eight rhodium atoms arranged in the form of a distorted cube. There are six mu(4)-GePh groups bridging each face of this distorted cube. Four of the rhodium atoms have two terminal carbonyl ligands, while the remaining four rhodium atoms have only one carbonyl ligand. Compound 10 contains a triangular cluster of three rhodium atoms with one terminal GePh(3) ligand, three bridging GePh(2) ligands, and one triply bridging GePh ligand. There is also one hydrido ligand that is believed to bridge one of the Rh-Ge bonds. Compound 9 reacted with PPhMe(2) at 25 degrees C to give the tetraphosphine derivative Rh(8)(CO)(8)(PPhMe(2))(4)(mu(4)-GePh)(6), 11. The structure of 11 is similar to 9 except that a PPhMe(2) ligand has replaced a carbonyl ligand on each the four Rh(CO)(2) groups. Compound 10 reacted with CO at 68 degrees C to give the complex Rh(3)(CO)(6)(mu-GePh(2))(3)(mu(3)-GePh), 12. Compound 12 is formed by the loss of the hydrido ligand and the terminal GePh(3) ligand from 10 and the addition of one carbonyl ligand. All compounds were fully characterized by IR, NMR, elemental, and single-crystal X-ray diffraction analyses. 相似文献
995.
Jack M. Miller Kesgapillai Balasanmugam Adrian Fulcher 《Journal of mass spectrometry : JMS》1989,24(7):497-503
Some ion-formation processes during fast atom bombardment (FAB) are discussed, especially the possibility of reactions in the gas phase. Divided (two halves) FAB probe tips were used for introducing two different samples into the source at the same time. Our results showed [M + A]+ ions (where M = crown ethers and A = alkali metal ions), can be produced, at least in part, in the gas phase when crown ethers and sources of alkali metal ion are placed on two halves of the FAB probe tip. The extent of this ion formation depends on the volatility of the crown ether and on steric factors. Cluster ions such as (M + LiCl)Li+, (2M + LiCl)Li+, [2M + K]+ and [2M + Na]+ are also observed to form in the gas phase. Unimolecular decompositions contribute to some ions detected in FAB. When the alkali ion salt and the crown ether are mixed together the probability of [M + A]+ ion formation increases significantly, regardless of the volatility of the crown ether. 相似文献
996.
Fred L. Tobiason Greg H. Cain Jack W. Anderson 《Journal of polymer science. Part A, Polymer chemistry》1978,16(1):275-282
Dipole moments and their temperature dependence have been measured in p-dioxane for fractionated novolac phenol–, o-cresol–, and p-cresol–formaldehyde polymers. The phenol–formaldehyde fractions covered a molecular weight range of 200 to 6100, and the limiting dipole moment ratio 〈μ2〉/xm2 is 1.48. The p-cresol–formaldehyde dipole-moment ratio at a DP of 4 is 2.47, whereas the phenol–formaldehyde dipole-moment ratio is 1.40. That for o-cresol–formaldehyde is intermediate in value. The dipole-moment temperature coefficients are positive for p-cresol chains and negative for the phenol–formaldehyde chains. These results indicate that the hydroxyl groups along the p-cresol–formaldehyde polymer are highly ordered, with the aromatic rings closer to the sterically hindered planar position than in the phenol–formaldehyde polymers. 相似文献
997.
Araújo CS Drew MG Félix V Jack L Madureira J Newell M Roche S Santos TM Thomas JA Yellowlees L 《Inorganic chemistry》2002,41(8):2250-2259
The synthesis and properties of 3 new ligand-bridged bimetallic complexes, 1(2+), 2(2+), and 3(2+), containing [RuCl([9]aneS(3))](+) metal centers are reported. Each complex was bridged by a different ditopic ligand. 1(2+) is bridged by 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz), while 2(2+) and 3(2+) are bridged by 2,3-bis(2-pyridyl)pyrazine (dpp) and 2,2'-bipyrimidine (bpym), respectively. The Ru([II]) isovalent states of these complexes have been investigated using a variety of techniques. In the case of 3(2+), X-ray crystallography studies show preferential crystallization of an anti form with respect to coordinated chloride ligands (crystal data for [3][Cl(2)].4H(2)O: C(20)H(38)Cl(4)N(4)O(4)Ru(2)S(6), monoclinic, space group P2(1)/a, a = 10.929(14), b = 13.514(17), c = 11.299(16) A, beta = 90.52(1), V = 1669 A(3), Z = 2). UV/vis spectroscopy shows that spectra of these complexes are dominated by intraligand (pi-->pi) and metal-to-ligand Ru(d)-->L(pi) charge transfer transitions. Electrochemical studies reveal that metal-metal interactions are sufficiently intense to generate the Ru(III)/Ru(II) mixed valence [[RuCl([9]aneS(3))(2)](L-L)](3+) state, where L-L = individual bridging ligands. Although the 1(3+), 2(3+), and 3(3+) mixed valence states were EPR silent at room temperature and 77 K, isotropic solution spectra were observed for the electrochemically generated radical cations 1(+), 2(+), and 3(+), with 1(+) displaying well-resolved hyperfine coupling to bridging ligand nitrogens. Using UV/vis/NIR spectroelectrochemistry, we investigated optical properties of the mixed valence complexes. All three showed intervalence charge transfer (IVCT) bands that are much more intense than electrochemical data indicate. Indeed, a comparison of IVCT data for 1(3+) with an analogous structure containing [(NH3)(3)Ru](2+) metal centers shows that the IVCT in the new complex is an order of magnitude more intense. It is concluded that although the new complexes show relatively weak electrostatic interactions, they possess large resonance energies. 相似文献
998.
W.A. Shantha Nandana Jack Passmore D.C. Neil Swindells Peter S. White Chi-Ming Wong 《Journal of fluorine chemistry》1982,21(1):14
Antimony pentafluoride acts as a useful oxidising agent towards many non-metals, giving interesting cations, and in the process is itself reduced. It would be helpful to know what the reduced products are, and under what conditions they are formed. Therefore, SbF5 and the known SbF5·SbF3(1) in AsF3 solution were reduced by iodine and/or PF3 giving crystals of the new adduct, (SbF3)6(SbF5)5 [Monoclinic, a = 11.638(1), b = 8.995(1), c = 16.723(3) ā, β = 106.81(1)°, P21/c]; (SbF3)5(SbF5)3 [Orthorhombic, a = 19.187(9), b = 15.890(2), c = 15.713(3) ā, Pnma] and (SbF3)3SbF5 [Monoclinic, a = 10.895(3), b = 10.941(3), c = 4.772(1) ā, β = 96.66(3)°, P21/m]. (SbF3)3SbF5 seemed to be the most reduced adduct, no evidence was obtained for (SbF3)n(SbF5) n > 3, under these conditions. The (SbF3)6(SbF5)5 adduct has a Raman spectrum identical to that reported by Gillespie(2) and coworkers for an adduct of approximate composition SbF3·SbF5 and has a very different structure to that of (SbF3)6(SbF6)5 reported by Edwards.(3) The crystal structures of the new adducts will be discussed and the cations they contain compared with those found in SbF3·SbF5(1) and (SbF5)6(SbF5)5(3) (Edward's form). 相似文献
999.
Jack D. Anderson N. Kent Dalley Ganapathi R. Revankar Roland K. Robins 《Journal of heterocyclic chemistry》1986,23(6):1869-1878
Several 3-alkoxysubstituted pyrazolo[3,4-d]pyrimidine ribonucleosides structurally related to adenosine, inosine and guanosine have been prepared by the direct glycosylation of preformed aglycon precursor containing a 3-alkoxy substituent. Ring closure of 5(3)-amino-3(5)-ethoxypyrazole-4-carboxamide ( 6b ) with either formamide or potassium ethyl xanthate gave 3-ethoxyallopurinol ( 7b ) and 3-ethoxy-6-thioxopyrazolo[3,4-d]-pyrimidin-4(5H,7H)-one ( 10 ), respectively. Methylation of 10 gave the corresponding 6-methylthio derivative 15 . Similar ring annulation of 5(3)-methoxypyrazole-4-carboxamide ( 6a ) with formamide afforded 3-methoxyallopurinol ( 7a ). Treatment of 5(3)-amino-3(5)-methoxypyrazole-4-carbonitrile ( 5a ) with formamidine acetate furnished 4-amino-3-methoxypyrazolo[3,4-d]pyrimidine ( 4 ). High-temperature glycosylation of 7b with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose in the presence of boron trifluoride etherate gave a 2:1 mixture of N-1 and N-2 glycosyl blocked nucleosides 11b and 13b . Deprotection of 11b and 13b with sodium methoxide gave 3-ethoxy-1-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidin-4(5H)-one ( 12b ) and the corresponding N-2 glycosyl isomer 14b , respectively. Similar glycosylation of either 4 or 7a , and subsequent debenzoylation gave exclusively 4-amino-3-methoxy-1-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidine ( 9 ) and 3-methoxy-1-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidin-4-(5H)-one ( 12a ), respectively. The structural assignment of 12a was made on the basis of single-crystal X-ray analysis. Application of this general glycosylation procedure to 15 gave the corresponding N-1 glycosyl derivative 16 as the sole product, which on debenzoylation afforded 3-ethoxy-6-(methylthio)-1-(3-D-ribofuranosylpyrazolo[3,4-d]pyrimidin-4(5H)-one ( 17 ). Oxidation of 16 and subsequent ammonolysis furnished the guanosine analog 6-arnino-3-ethoxy-1-β-D-ribofuranosylpyrazolo[3,4-d]-pyrimidin-4(5H)-one ( 19 ). Similarly, starting from 3-methoxy-4,6-bis(methylthio)pyrazolo[3,4-d]pyrimidine ( 20 ), 6-amino-3-methoxy-1-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidin-4(5H)-one ( 23 ) was prepared. 相似文献
1000.
Forrest T. Smith Jack Deruiter Deborah Ann Carter 《Journal of heterocyclic chemistry》1989,26(6):1815-1817
Synthetic routes to 1,2,3,5,10,10a-hexahydropyrrolo[1,2,-b]isoquinolin-5-ones functionalized at C-10 were investigated. Attempts to prepare the ketone 2 , and subsequently introduce the desired C-10 hydroxymethylene group were unsuccessful due to the failure of 6 to cyclize and the unreactivity of 9 to relevant nucleophiles. Compound 1 was prepared by the condensation of 1-pyrroline with 7-methoxyisobenzopyran-1,3,-(4H)-dione to give 12 in quantitative yield. Acyl halide formation and subsequent reduction gave the desired compound 1. 相似文献