全文获取类型
收费全文 | 4567篇 |
免费 | 174篇 |
国内免费 | 21篇 |
专业分类
化学 | 3175篇 |
晶体学 | 27篇 |
力学 | 94篇 |
数学 | 589篇 |
物理学 | 877篇 |
出版年
2023年 | 23篇 |
2022年 | 31篇 |
2021年 | 36篇 |
2020年 | 94篇 |
2019年 | 63篇 |
2018年 | 36篇 |
2017年 | 48篇 |
2016年 | 104篇 |
2015年 | 124篇 |
2014年 | 143篇 |
2013年 | 229篇 |
2012年 | 277篇 |
2011年 | 277篇 |
2010年 | 198篇 |
2009年 | 158篇 |
2008年 | 270篇 |
2007年 | 248篇 |
2006年 | 253篇 |
2005年 | 230篇 |
2004年 | 190篇 |
2003年 | 158篇 |
2002年 | 182篇 |
2001年 | 116篇 |
2000年 | 91篇 |
1999年 | 89篇 |
1998年 | 60篇 |
1997年 | 74篇 |
1996年 | 92篇 |
1995年 | 57篇 |
1994年 | 50篇 |
1993年 | 51篇 |
1992年 | 37篇 |
1991年 | 37篇 |
1990年 | 26篇 |
1989年 | 45篇 |
1988年 | 26篇 |
1987年 | 31篇 |
1986年 | 31篇 |
1985年 | 32篇 |
1984年 | 24篇 |
1983年 | 17篇 |
1982年 | 35篇 |
1981年 | 31篇 |
1980年 | 17篇 |
1979年 | 23篇 |
1978年 | 28篇 |
1977年 | 29篇 |
1976年 | 22篇 |
1974年 | 24篇 |
1971年 | 17篇 |
排序方式: 共有4762条查询结果,搜索用时 31 毫秒
91.
Sulfur-containing polymethine dyes exhibit anomalous chromophoric properties. The vinylene shift of the first two homologues is often considerably larger or smaller than the shift between higher members of the series. Moreover, the absorption wavelengths of non-symmetrical dyes with sulfur-containing groups, as calculated by the mean value rule differ from the experimental data, but not in the predicted manner.This deviation is caused by an interannular no-bond SS-interaction in the monomethine ion. This conclusion is supported by the spectra of appropriately-substituted dyes and is substantiated by theoretical considerations. Starting from the well-known symmetry properties of polymethine MO's the simple PMO-method reproduces correctly the effects of the pπ-type overlap between atomic orbitals of adjacent S atoms at long wavelength absorptions: The SS-interactions lead to a hypsochromic shift in monomethines with (4N + 2)π-electrons (N = 1,2…) and to a bathochromic shift in monomethines with (4N)π-electrons. 相似文献
92.
S. Hartmann 《Fresenius' Journal of Analytical Chemistry》1956,151(5):332-336
Zusammenfassung Es wird ein neues Verfahren zur Bestimmung geringer P2O5-Mengen neben großen Mengen V2O5 beschrieben. Die Abtrennung des Vanadins geschieht dabei durch Ionenaustausch, während die Bestimmung des Phosphors in den vanadinfreien Eluaten colorimetrisch erfolgt. Das Verfahren wurde vorzugsweise an P2O5-haltigen Vanadinpentoxyden, Ammoniummetavanadaten und Natriumorthovanadaten erprobt und hat sich hierbei bestens bewährt.Herrn Ing.-Chem. G. Martens, der mir bei der Beschaffung der erforderlichen Vanadinverbindungen jederzeit in großzügiger Weise behilflich war, sei an dieser Stelle herzlichst gedankt. 相似文献
93.
In this study we have compared different ligands containing three or more hetero-atoms (N, O and/or S) with respect to their ability to form tridentate complexes with a Tc-tricarbonyl moiety. Comparison of each ligand in a competition reaction with histidine first and then with each other compound allowed to rank the ligands according to their ability of complex formation with the [99mTc(CO)3]+ precursor from diethylenetriamine (most efficient of the studied ligands) to nitrilotriacetic acid (weakest complexing properties). The results provide insight in the structural requirements for the formation of stable Tc-tricarbonyl complexes and suggest preferred combinations and arrangements of the hetero-atoms involved in the complex formation. They also give a good indication which type of ligand is most appropriate to modify biomolecules for an efficient and stable labelling with a Tc-tricarbonyl moiety. 相似文献
94.
Diekers M Luo C Guldi DM Hirsch A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(4):979-991
For the first time several T(h)-symmetrical hexakisadducts of C(60) bearing up to six electro- and photoactive o-phenylene diamine or 9,10-dialkoxyanthracene moieties were synthesized and subjected to photoinduced electron/energy-transfer studies. Both donors form a densely packed pi-donor shell surrounding the fullerene core. In these novel core-shell ensembles (7 and 19), either an efficient energy transfer from the dialkoxyanthracene periphery, or an electron transfer from the o-phenylene diamine periphery transduces the flow of excited-state energy or electrons, respectively, to the fullerene moiety, which resides in the central core. Due to the relatively high reduction potential of the fullerene core, which is anodically shifted by approximately equal to 0.7 V, compared with that of pristine C(60), the outcome of these intramolecular reactions depends mainly on the donor ability of the peripheral system. Interestingly, the charge-separated state in the o-phenylene diamine heptad (7; tau=2380 ns in benzonitrile) is stabilized by a factor of 20 relative to the corresponding o-phenylene diamine dyad (6; tau=120 ns in benzonitrile), an effect that points unequivocally to the optimized storage of charges in this highly functionalized fullerene ensemble. 相似文献
95.
Using ring closing metathesis (RCM) as the key operation, a rapid access to β-lactams fused to a sultam moiety of variable ring size was developed from low cost, commercially available starting materials. An efficient RCM of 4-vinyl-azetidin-2-ones to give 1-aza-bicyclo[4.2.0]oct-4-en-8-ones is also reported. 相似文献
96.
Sandra Hübner Dirk Michalik Dr. Haijun Jiao Dr. Helfried Neumann Dr. Stefan Klaus Dirk Strübing Dr. Anke Spannenberg Dr. Matthias Beller Prof. Dr. 《化学:亚洲杂志》2007,2(6):734-746
Chiral N‐dienyl lactams are crucial building blocks for the synthesis of complex organic compounds. However, their generation is rather challenging. This paper reports the novel one‐pot reaction of (S)‐methyl pyroglutamate as the a mide component with different a ldehydes and d ienophiles (AAD reaction) to give novel chiral 1‐amido‐2‐cyclohexenes. The corresponding N‐dienyl lactams generated in situ undergo subsequent Diels–Alder reactions in good yield and diastereoselectivity. The scope and limitations of the three‐component protocol were investigated. X‐ray and NMR spectroscopic analysis of the products as well as DFT calculations of the intermediates were also performed to explain the observed stereoselectivity and structural features. 相似文献
97.
Substituted 2-aminoindenes have been synthesized in almost quantitative yields by reactions of amines such as methylpiperazine, trimethylethylenediamine, 1,4-diaza-cycloheptane and N,N′-dimethylethylenediamine with 2-indanone. The 2-aminoindenes can be deprotonated and reacted with BrMn(CO)3(Py)2 to produce the respective aminoindenyl-cymantrenes in yields between 55–70%. The X-ray crystal structures of 2-(methylpiperazine)indenyl-cymantrene 5 (P1 , a = 12.667(3) Å, b = 16.630(3) Å, c = 17.382(3) Å, α = 72.70(3)°, β = 74.59(3)°, γ = 88.66(3)°, V = 3364.1(12) Å 3, Z = 8, R1(2σ(I)) = 4.02%, wR2(2σ(I)) = 10.30%) and the HClO4 adduct of 2-(trimethylethylenediamine)-indenyl-cymantrene 6 (Cc, a = 23.722(5) Å, b = 6.9080 Å, c = 13.264 Å, β = 111.77(3)°, V = 2018.6(7) Å 3, Z = 4, R1(2σ(I)) = 2.94%, wR2(2σ(I)) = 7.90%) were determined. In both complexes the indenyl-carbon bonded to nitrogen displays significantly longer bonds to manganese [223.5(3)–225.8(3) pm] than the other four carbon atoms [213.3(3)–219.1(3) pm]. The short indenyl-nitrogen bonds of 136.2(4) and 137.8(4) pm are indicative of a substantial multiple bond character. The complexation of Zn2+ by the nitrogen atoms of 6 results in significant shifts of the CO stretching frequencies. 相似文献
98.
Bouchoux G Defaye D McMahon T Likholyot A Mó O Yáñez M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(13):2900-2909
The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol. 相似文献
99.
J. -M. Welter D. Richter R. Hempelmann O. Hartmann E. Karlsson L. O. Norlin T. O. Niinikoski 《Hyperfine Interactions》1984,17(1-4):117-124
We have performed transverse fieldSR experiments on several different samples of copper in the temperature range below 2 K, including isotope separated Cu and impurity doped polycrystalline Cu.We do not observe any strong effect of the isotope composition of the sample. A63Cu and a natural Cu sample of identical purity both yield 0.16s–1 for the low-temperature plateau, while an increased linewidth in the65Cu case may be related to the strong effects of Fe impurities.Careful transverse field measurements on large single crystals at 0.08 K reveal non-Gaussian lineshapes in accordance with the picture of diffusing muons at this temperature. This allows us to reject several of the existing models for muon behaviour in copper below 2 K. 相似文献
100.
M. Althoff W. Braunschweig F.J. Kirschfink H.-U. Martyn P. Rosskamp D. Schmitz H. Siebke W. Wallraff J. Eisenmann H.M. Fischer H. Hartmann A. Jocksch G. Knop H. Kolanoski H. Kück V. Mertens R. Wedemeyer G. Zobernig 《Physics letters. [Part B]》1984,149(6):524-532
We have determined the average lifetime of hadrons containing b quarks produced in e+e? annihilation to be . Our method uses charged decay products from both non-leptonic and semileptonic decay modes. 相似文献