全文获取类型
收费全文 | 2085篇 |
免费 | 30篇 |
国内免费 | 12篇 |
专业分类
化学 | 1127篇 |
晶体学 | 16篇 |
力学 | 32篇 |
数学 | 325篇 |
物理学 | 627篇 |
出版年
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 14篇 |
2016年 | 24篇 |
2015年 | 36篇 |
2014年 | 35篇 |
2013年 | 92篇 |
2012年 | 68篇 |
2011年 | 103篇 |
2010年 | 41篇 |
2009年 | 51篇 |
2008年 | 88篇 |
2007年 | 86篇 |
2006年 | 87篇 |
2005年 | 104篇 |
2004年 | 70篇 |
2003年 | 82篇 |
2002年 | 57篇 |
2001年 | 38篇 |
2000年 | 35篇 |
1999年 | 34篇 |
1998年 | 24篇 |
1997年 | 28篇 |
1996年 | 29篇 |
1995年 | 29篇 |
1994年 | 36篇 |
1993年 | 40篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 24篇 |
1989年 | 36篇 |
1988年 | 28篇 |
1987年 | 31篇 |
1986年 | 29篇 |
1985年 | 44篇 |
1984年 | 34篇 |
1983年 | 31篇 |
1982年 | 39篇 |
1981年 | 27篇 |
1980年 | 33篇 |
1979年 | 30篇 |
1978年 | 27篇 |
1977年 | 30篇 |
1976年 | 40篇 |
1975年 | 24篇 |
1974年 | 23篇 |
1973年 | 25篇 |
1972年 | 18篇 |
1971年 | 13篇 |
1968年 | 11篇 |
排序方式: 共有2127条查询结果,搜索用时 15 毫秒
91.
What is the most number of vectors inR d such that anyk+1 contain an orthogonal pair? The 24 positive roots of the root systemF 4 inR 4 show that this number could exceeddk. 相似文献
92.
93.
The compounds P2O3F4 and K2P2O5F2 have been studied by vibrational spectroscopy. From the shifts of the symmetric vibration of the terminal PO bonds it appears that the molecular structure contracts as the number of PO (bridged) and/or PF bonds increases. The changes occur at the expense of PO terminal bonds. In these tetrahedral phosphorus (V) compounds the PO (bridged) and PF bonding have a similar role, and the same phenomenon occurs in sulfur VI oxide fluoride compounds. 相似文献
94.
The dipeptide sweetener, aspartame, has been prepared in high yield via the coupling of L-phenylalanine methyl ester and L-aspartic acid N-thiocarboxyanhydride. 相似文献
95.
Denning MS Watts ID Moussa SM Durand P Rosseinsky MJ Tanigaki K 《Journal of the American Chemical Society》2002,124(19):5570-5580
The high symmetry and resulting electronic degeneracy of the C(60)(3)(-) anion is viewed as the key molecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerene systems. The experimental evaluation of this hypothesis requires the synthesis of face-centered cubic (fcc) trivalent fulleride anion salts derived from higher fullerenes such as C(70), which have thus far proved elusive with only stable A(1)C(70), A(4)C(70), and A(6)C(70) phases known. In this paper, we report the synthesis of fcc A(3)C(70) phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are strongly dependent on the cooling protocol due to the existence of metastable partially or completely orientationally disordered phases. EPR data indicate that the phases are metallic but not superconducting. The densities of states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recent observations that four electrons per C(70) anion are required for superconductivity. Size-matching on both the octahedral and tetrahedral sites is required for A(3)C(70) stability - K(2)CsC(70) is only stable at elevated temperature and Na(2)C(70) is unstable, the composition corresponding to C(70) and a sodium-rich trigonal phase. 相似文献
96.
Reactivite et distribution moleculaire dans les copolycondensats lineaires-I: Reactivite intrinseque
The effect of unequal intrinsic reactivity of functional groups on the molecular distribution in a linear step polymerization system is studied. In an R(A)2/R′(B)2 copolycondensate, the dependence of the polydispersity index on the relative reactivities of the two A-functional groups, at different extents of reaction, is examined in detail. 相似文献
97.
Wing W. Lam Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Charles M. Lindsay Keith Smith 《Journal of heterocyclic chemistry》1986,23(3):785-791
The synthesis of 2-azathianthrene ([1,4]benzodithiino[2,3-c]pyridine), the only remaining monoazathianthrene yet to be reported, is described. Attempts at the direct condensation of disubstituted pyridines with the dianion of 1,2-dimercaptobenzene were generally unsuccessful requiring that the alternative condensation of the dianion with disubstituted pyridine 1-oxides be employed. The title compound was characterized by physical means including 13C-nmr spectroscopy. One analog, 4-nitro-2-azathianthrene was also studied by X-ray crystallographic means; the molecule crystallized with two molecules in the asymmetric unit P21/n, a = 20.712(3), b = 7.8109(13), c = 13.720(2)Å, β = 107.880(11)°, Z = 8, the data refined to a final R = 0.051 for 3061 reflections. Dihedral angles between the planes of the phenyl rings were 135.00(13) and 132.52(13)° for the two independent molecules contained in the crystal. Close non? bonded S ?O intramolecular contacts were observed in both molecules between the sulfur and nitro-group oxygens. Both nitro groups are twisted out of the plane of the pyridine ring and are oriented at angles of 28.75 and 38.82° respectively. 相似文献
98.
Rotureau E Leonard M Dellacherie E Durand A 《Journal of colloid and interface science》2004,279(1):68-77
Ionic amphiphilic dextran derivatives were synthesized by the attachment of sodium sulfopropyl and phenoxy groups on the native polysaccharide. A family of dextran derivatives was thus obtained with varying hydrophobic content and charge density in the polymer chains. The surface-active properties of polymers were studied at the air-water and dodecane-water interfaces using dynamic surface/interfacial tension measurements. The adsorption was shown to begin in a diffusion-limited regime at low polymer concentrations, that is to say, with the diffusion of macromolecules in the bulk solution. In contrast, at long times the interfacial adsorption is limited by interfacial phenomena: adsorption kinetics or transfer into the adsorbed layer. A semiempirical equation developed by Filippov was shown to correctly fit the experimental curves over the whole time range. The presence of ionic groups in the chains strongly lowers the adsorption kinetics. This effect can be interpreted by electrostatic interactions between the free molecules and the already adsorbed ones. The adsorption kinetics at air-water and oil-water interfaces are compared. 相似文献
99.
When bromoform (CHBr3) is photolyzed at 266 or 303 nm in the presence of O2 and NO, the formation of secondary Br atoms is observed. By following the rate of growth of this secondary Br atom signal as a function of conditions, rate constants have been determined for the reactions CHBr2 + O2, CHBr2 + NO (both pressure-dependent), and CHBr2O2 + NO (k(2a) = (1.74 +/- 0.16) x 10(-11) cm3 molecule(-1) s(-1) at 23 degrees C). By measuring the amplitude of the secondary Br signal compared to the primary Br formed in the initial photolysis, it is established that the CHBr2O radical spontaneously decomposes to form CHBrO + Br at least 90%, and probably 100%, of the time, in agreement with previous work and with recent ab initio calculations. A survey of four other polybrominated methanes, CH2Br2, CHClBr2, CF2Br2, and CBr4, shows that they all generate secondary Br atoms when photolyzed at 266 nm in the presence of O2 and NO, suggesting that their reaction sequences are similar to that of bromoform. 相似文献
100.
Axel Schulz Thomas M. Klap tke
T. Stanley Cameron
P. K. Bakshi 《Journal of organometallic chemistry》1994,480(1-2):191-194The oxidation of Cp2NbCl2 with pure WF6 in SO2 solution yielded the cationic metallocene species [Cp2NbCl2]+[WF6]− essentially in quantitative yield. The same reaction carried out in the presence of either equimolar amounts or a two-fold excess of HCN led to the preparation of the new niobocenium salt [Cp2NbCl2]4+[WF6]2− which was studied by single crystal X-ray diffraction. This compound represents the first example of a structurally characterized metallocene-WF6− complex, and crystallizes in the tetragonal system: space group, P41212(No. 92), a = 11.083(8) Å, c = 48.285 (9) Å; Z = 8; R = 0.0759, RW = 0.0841. ab]Die Oxidation von Cp2NbCl2 mit reinem WF6 führt in SO2-Lösung zur Synthese von [Cp2NbCl2 ]+[WF6]− in nahezu quantitativer Ausbeute. Die analoge Reaktion führt unter Anwesenheit der äquimolaren Menge oder eines zweifachen Überschusses an HCN zur Ausbildung des Niobocenium-Komplexsalzes [Cp2NbCl2]4+ [WF6]2−[WCl6]2−, von dem eine Röntgenstrukturanalyse angefertigt wurde. Diese Verbindung repräsentiert den ersten structurell charakterisierten Vertreter eines Metallocen-WF6−-Komplexes und kristallisiert im tetragonalen System: Raumgruppe P41212 (Nr. 92), a = 11.083(8) Å, c = 48.285(9) Å; Z = 8; R = 0.0759, RW = 0.0841. kw]Niobium; X-ray diffraction; Oxidation; Metallocenes 相似文献