全文获取类型
收费全文 | 11513篇 |
免费 | 414篇 |
国内免费 | 47篇 |
专业分类
化学 | 7569篇 |
晶体学 | 79篇 |
力学 | 297篇 |
数学 | 2030篇 |
物理学 | 1999篇 |
出版年
2023年 | 70篇 |
2022年 | 141篇 |
2021年 | 179篇 |
2020年 | 233篇 |
2019年 | 239篇 |
2018年 | 262篇 |
2017年 | 247篇 |
2016年 | 405篇 |
2015年 | 379篇 |
2014年 | 398篇 |
2013年 | 754篇 |
2012年 | 837篇 |
2011年 | 853篇 |
2010年 | 533篇 |
2009年 | 483篇 |
2008年 | 673篇 |
2007年 | 635篇 |
2006年 | 611篇 |
2005年 | 559篇 |
2004年 | 448篇 |
2003年 | 394篇 |
2002年 | 342篇 |
2001年 | 189篇 |
2000年 | 158篇 |
1999年 | 119篇 |
1998年 | 99篇 |
1997年 | 88篇 |
1996年 | 115篇 |
1995年 | 66篇 |
1994年 | 91篇 |
1993年 | 57篇 |
1992年 | 72篇 |
1991年 | 75篇 |
1990年 | 75篇 |
1989年 | 67篇 |
1988年 | 54篇 |
1987年 | 47篇 |
1986年 | 38篇 |
1985年 | 63篇 |
1984年 | 80篇 |
1983年 | 42篇 |
1982年 | 56篇 |
1981年 | 49篇 |
1980年 | 54篇 |
1979年 | 40篇 |
1978年 | 40篇 |
1977年 | 51篇 |
1976年 | 40篇 |
1975年 | 40篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The reaction of (R(2)PCH(2)SiMe(2))(2)NM (PNP(R)M; R = Cy; M = Li, Na, MgHal, Ag) with L(2)ReOX(3) [L(2) = (Ph(3)P)(2) or (Ph(3)PO)(Me(2)S); X = Cl, Br] gives (PNP(Cy))ReOX(2) as two isomers, mer,trans and mer,cis. These compounds undergo a double Si migration from N to O at 90 degrees C to form (POP(Cy))ReNX(2) as a mixture of mer,trans and fac,cis isomers. Additional thermolysis effects migration of CH(3) from Si to Re, along with compensating migration of halide from Re to Si. DFT calculations on various structural isomers support the greater thermodynamic stability of the POP/ReN isomer vs PNP/ReO and highlight the influence of the template effect on the reactivities of these species. 相似文献
992.
Gaël Rouillé Dr. Cornelia Jäger Dr. Mathias Steglich Friedrich Huisken Prof. Dr. Thomas Henning Prof. Dr. Gabriele Theumer Ingmar Bauer Dr. Hans‐Joachim Knölker Prof. Dr. 《Chemphyschem》2008,9(14):2085-2091
The spectroscopic characterization of corannulene (C20H10) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography‐mass spectrometry (GC‐MS). During a high‐performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time‐dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium. 相似文献
993.
Theab initio SCF method is used for computing the main electronic properties of the ribose unit of the nucleic acids. The present study is devoted to the ribose in the C3-endo,gg conformation. The properties investigated include the distribution of the electronic charges, the electrostatic molecular potential around the four oxygens of the unit, the hydration and the Na+ binding schemes studied in the supermolecule approximation. The possibilities of through-water binding of the cation to the sugar are also explored. The predictions of the computation in particular with regard to cation binding to the ribose ring are correlated with recent experimental results. 相似文献
994.
We propose an SQP algorithm for mathematical programs with vanishing constraints which solves at each iteration a quadratic program with linear vanishing constraints. The algorithm is based on the newly developed concept of \({\mathcal {Q}}\)-stationarity (Benko and Gfrerer in Optimization 66(1):61–92, 2017). We demonstrate how \({\mathcal {Q}}_M\)-stationary solutions of the quadratic program can be obtained. We show that all limit points of the sequence of iterates generated by the basic SQP method are at least M-stationary and by some extension of the method we also guarantee the stronger property of \({\mathcal {Q}}_M\)-stationarity of the limit points. 相似文献
995.
Cu(II) and Zn(II) complexes were synthesized from equimolar amounts of carboxamides; 1,8-bis(2-thiophenecarboxamido)-p-menthane (tkdam) and 1,8-bis(2-furancarboxamido)-p-menthane (furdam). The structure of the carboxamides were determined by elemental analysis; 1H NMR, 13C NMR, FT-IR and LC-MS spectra. The relative energies and the electronic properties (LUMO, HOMO, LUMO-HOMO gap) of the ligands
were investigated theoretically by performing Semi-empirical molecular orbital theory PM3 method in Hyperchem 7 (Release).
Carboxamide complexes having general formula as; monomeric, [ML]Cl2 and dimeric [Cu(tkdam)Cl]2Cl2 · 5H2O were synthesized and characterized by using element analysis; FT-IR, LC-MS spectra; magnetic susceptibility, molar conductivity
and thermal (TGA/DTA curve) studies. It was found that the coordination number of the monomeric complexes is four whereas
dimeric’s is six. The changes in the selected vibration bands in FT-IR indicate that, carboxamides behave as tetradentate
ligands and coordinate to metal ions from acyl ring (through S/O) and amide carbonyl (C=O). Molar conductivity measurements
indicate the 1: 2 ionic nature of the carboxamide complexes. 相似文献
996.
Ferenc F. Gaál Biljana F. Abramovi Ferenc B. Szebenyi Velimir D. Cani 《Fresenius' Journal of Analytical Chemistry》1977,286(3-4):222-225
Summary Amperometric and potentiometric methods at a small constant current were developed to follow the course of catalytic titrations. In ammoniacal medium EDTA was determined by titration with 0.1 M copper(II) chloride in the presence of hydrogen peroxide as the indicator. Amounts of 42.26–126.87 mg of EDTA were determined with a maximal average deviation of 0.55 %. The results obtained are in good agreement with those of comparable methods.
Bestimmung von ÄDTA mit Hilfe der katalytischen amperometrischen und der katalytischen potentiometrischen Titration bei kleinem konstantem Strom
Zusammenfassung Die amperometrische und die potentiometrische Methode bei kleinem konstantem Strom wurden für die Verfolgung katalytischer Titrationen ausgearbeitet. ÄDTA wurde in Ammoniakmedium mit 0,1 M Kupfer(II)-chlorid in Anwesenheit von Wasserstoffperoxid als Indicator titriert. Mengen von 42,26–126,87 mg ÄDTA wurden mit einer mittleren Abweichung von 0,55% bestimmt. Die Ergebnisse stimmen gut mit denen vergleichbarer Methoden überein.
The authors thank the SIZ for researchs of SAP Vojvodina for the partial financial support of the present work. 相似文献
997.
P. Rajec L'. Mátel J. Orechovská J. Šúcha I. Novák 《Journal of Radioanalytical and Nuclear Chemistry》1996,208(2):477-486
Inorganic sorbents are often used in separation of metals and radionuclides in radioanalytical application and they were also used in technological scale for separation of radionuclides in cleanup of Three Mile Island NPP. Inorganic sorbents become popular in the last years because no problem with organic contamination, there are stable against radiation, sorption efficiency can be tailor made for selective separation of chosen metal. Contrary to the organic sorbents they have usually lower capacity and chemical stability is limited to narrower pH. Nevertheless of some problems, many good properties of inorganic sorbents make them very attractive for sorption study. 相似文献
998.
Basallote MG Estevan F Feliz M Jesús Fernández-Trujillo M Hoyos DA Llusar R Uriel S Vicent C 《Dalton transactions (Cambridge, England : 2003)》2004,(4):530-536
The novel incomplete cuboidal cluster [W3Se4H3(dmpe)3](PF6), [1](PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)3(dmpe)3](PF6), [2](PF6), was obtained by reacting [W3Se4Br3(dmpe)3](PF6) with NaOH in MeCN-H2O solution. The complexes [1](PF6) and [2](PF6) were converted to their BPh4- salts by treatment with NaBPh4. Recrystallisation of [1](BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)3](BPh4). The crystal structures of [1](BPh4), [2](BPh4) and [W3Se4H2(OH)(dmpe)3](BPh4) have been resolved. Although the [1]+ trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)3]+ (X = Cl, Br). The kinetics of this reaction has been studied at 25 degrees C in MeCN-H2O solution (1:1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X(-) anion but shows a first order dependence on the concentration of acid (k1 = 12.0 mol(-1) dm(3) s(-1)), whereas the second one is independent of the nature and concentration of both the acid and added salts (k2 = 0.024 s(-1)). In contrast, the reaction of [2]+ with acids occurs in a single step with kobs = 0.63 s(-1)(HCl) and 0.17 s(-1)(HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)3]+ cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved. 相似文献
999.
Brunner H Cattey H Meier W Mugnier Y Stückl AC Wachter J Wanninger R Zabel M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(16):3796-3802
The reaction of the cluster salts [Cp(2*) Nb(CO)(2)](n)[Co(11)Te(7)(CO)(10)] (Cp*=C(5)Me(5); n=1, 2) with excess PMe(2)Ph gave the neutral, dark brown clusters [Co(11)Te(7)(CO)(6)(PMe(2)Ph)(4)] (5) and [Co(11)Te(7)(CO)(5)(PMe(2)Ph)(5)] (6) with 147 metal valence electrons. The new compounds were characterized by IR spectroscopy, elemental analyses, and mass spectrometry. The molecular structure of 6 was determined by X-ray crystallography. Like its precursor anion, it consists of a pentagonal-prismatic [Co(11)Te(7)] core, but with a ligand sphere composed of five CO and five PMe(2)Ph ligands. Detailed electrochemical studies of both reactions reveal that a stepwise substitution of CO ligands in the initial cluster anions takes place leading to intermediate [Co(11)Te(7)(CO)(10-m)(PMe(2)Ph)(m)](n-) ions (m=1-5; n=1, 2). Each of these intermediates is distinguished by at least one oxidation and two reduction waves, giving rise to a total of 21 redox couples and 27 electroactive species. The electron sponge character of the new compounds is particularly pronounced in 5, which exhibits charges n between +1 and -4 corresponding to metal valence electron counts of between 146 and 151. 相似文献
1000.
Dr. Gaël Rouillé Mathias Steglich Dr. Cornelia Jäger Prof. Dr. Friedrich Huisken Prof. Dr. Thomas Henning Gabriele Theumer Dr. Ingmar Bauer Prof. Dr. Hans‐Joachim Knölker 《Chemphyschem》2011,12(11):2131-2137
We report on the characterization of dibenzo[cde,opq]rubicene (C30H14). The molecule was studied in solution at room temperature with absorption spectroscopy in the visible (vis) and ultraviolet (UV) wavelength ranges, and with emission spectroscopy. The infrared (IR), visible, ultraviolet, and vacuum ultraviolet (VUV) absorption spectra of a thin film were measured also at room temperature. In addition, the UV/vis absorption spectrum was measured at cryogenic temperatures using the matrix isolation spectroscopy technique. The interpretation of spectra was supported by theoretical calculations based on semiempirical and ab initio models, as well as on density functional theory. Finally, the results of the laboratory study were compared with interstellar spectra. 相似文献