全文获取类型
收费全文 | 84225篇 |
免费 | 441篇 |
国内免费 | 391篇 |
专业分类
化学 | 28696篇 |
晶体学 | 801篇 |
力学 | 6780篇 |
数学 | 32837篇 |
物理学 | 15943篇 |
出版年
2020年 | 64篇 |
2018年 | 10452篇 |
2017年 | 10274篇 |
2016年 | 6170篇 |
2015年 | 926篇 |
2014年 | 394篇 |
2013年 | 551篇 |
2012年 | 3975篇 |
2011年 | 10694篇 |
2010年 | 5762篇 |
2009年 | 6160篇 |
2008年 | 6762篇 |
2007年 | 8935篇 |
2006年 | 431篇 |
2005年 | 1521篇 |
2004年 | 1715篇 |
2003年 | 2111篇 |
2002年 | 1177篇 |
2001年 | 375篇 |
2000年 | 379篇 |
1999年 | 251篇 |
1998年 | 291篇 |
1997年 | 250篇 |
1996年 | 310篇 |
1995年 | 209篇 |
1994年 | 185篇 |
1993年 | 198篇 |
1992年 | 176篇 |
1991年 | 146篇 |
1990年 | 139篇 |
1989年 | 163篇 |
1988年 | 158篇 |
1987年 | 160篇 |
1986年 | 202篇 |
1985年 | 189篇 |
1984年 | 148篇 |
1983年 | 133篇 |
1982年 | 128篇 |
1981年 | 130篇 |
1980年 | 164篇 |
1979年 | 117篇 |
1978年 | 140篇 |
1977年 | 109篇 |
1976年 | 109篇 |
1975年 | 81篇 |
1974年 | 90篇 |
1973年 | 100篇 |
1972年 | 84篇 |
1971年 | 59篇 |
1970年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Gerhard Gritzner 《Monatshefte für Chemie / Chemical Monthly》1976,11(9):1499-1512
The redox behaviour of hexakismethylisonitrilmanganese(I) [MnL
6
+] has been studied in acetic acid, dichloromethane, 1,2-dichloroethane, propylenecarbonate, butyrolactone, methanol, ethanol, nitromethane, acetonitrile, N-methyl-2-pyrrolidinone, dimethylformamide, dimethyl sulfoxide and water. The reversible diffusion-controlled oxidation MnL
6
+/MnL
6
2+ could be observed in all solvents studied, on both the dropping mercury electrode and the stationary platinum electrode. Employing tetrabutylammonium perchlorate as supporting electrolyte, the oxidation MnL
6
2+/MnL
6
3+ was observable only in acetic acid, nitromethane, 1,2-dichloroethane, dichloromethane, propylenecarbonate, butyrolactone and acetonitrile. In all other solvents oxidation of the solvent preceded the oxidation MnL
6
2+/MnL
6
3+. Poorly defined polarographic waves attributable to the one electron reduction of the MnL
6
+ were observed in butyrolactone, propylenecarbonate, acetonitrile, dimethylformamide, N-methyl-2-pyrrolidinone and dimethyl sulfoxide. All potential values were recorded versus bisbiphenylchromium(I)-iodide [BBCr(I)J], the problems of measuring against external aqueous reference electrodes are discussed. The redox potential of the process MnL
6
+/MnL
6
2+ was found to be a function of the donor properties of the solvents used; the effects of outer sphere coordination on the redox behaviour of this couple are discussed. No effect of the supporting electrolytes tetrabutylammonium perchlorate, tetraethylammonium nitrate and tetraethylammonium perchlorate on the redox behaviour of MnL
6
+ was found. The UV-spectrum of MnL
6(PF6)2 has been recorded. 相似文献
72.
Zusammenfassung Zur Berechnung der Polarisationsenergie nach dem Konzept eines starren Gitters wird zuerst die Polarisierbarkeit des N3
– in NaN3 berechnet. Mit den bekannten Daten für die Brechungsindizes in den Hauptachsen kann nach einer Methode vonMott der erforderliche Polarisierbarkeitstensor aufgestellt werden. Um Argumente für die Berechtigung des Verfahrens zu erhalten, wird die Polarisierbarkeit des Chlorions in NaCl analog berechnet. Unter Zugrundelegung der vonPauling gegebenen Strukturdaten des NaN3 wird dann die Polarisationsenergie mit Hilfe verallgemeinerter Grundlagen für den Ausbau eines Ionenpaares aus NaN3 und NaCl berechnet, während Dipol—Dipol-Wechselwirkungen abgeschätzt werden. Vernachlässigt man den Beitrag der Verschiebungspolarisation, so erhält man für die gesamte Polarisationsenergie bei NaN3 3,4 eV und bei NaCl 4,2 eV, wasSchottky-Fehlordnung vermuten läßt. Der Zusammenhang mit der Fehlerbildungsenergie wird im Anschluß daran diskutiert.
Mit 6 Abbildungen 相似文献
For the determination of the polarization energy based upon the model of a rigid lattice, the polarizability of the azide ion in sodium azide is calculated. WithMott's method and the known data for the refractive indices along the main directions the polarizability tensor can be constructed. An analogous calculation is performed for the polarizability of the chlorine ion in NaCl in order to test arguments of our method. WithPauling's data of the NaN3 structure, the polarization energy for the removal of an ion pair from both NaN3 and NaCl is calculated by means of a generalized basis estimating dipole—dipole interactions. Neglecting the displacement polarization, one obtains for the total polarization energy a value of 3,4 eV for NaN3, and 4,2 eV for NaCl, resp., pointing atSchottky-disorder. The connection with the energy of defect formation is discussed hereupon.
Mit 6 Abbildungen 相似文献
73.
The intermolecular cyclotrimerisation of terminal and internal alkynes can be catalysed by simple cobalt complexes such as a CoBr2(diimine) under mild reaction conditions when treated with zinc and zinc iodide with high regioselectivity in excellent yields. 相似文献
74.
T. Bernard Tate 《The Journal of the Operational Research Society》1977,28(4):765-779
Good technique, i.e. competence in carrying out operational research, is as vital for the practitioner to acquire as competence in the traditional ‘techniques’. It is suggested that universities should teach students how to conduct simple projects prior to alerting them to more sophisticated methodological issues.As an aid to this task, the traditional five or six parts of an operational research project are broken down and extended into 39 steps, and regrouped into three phases. Also, five skills are identified as being conceptually essential to "good" operational research, which students can develop through tackling suitable case studies. 相似文献
75.
Furancarboxylic Acids in Cattle Urine The urine of cattle contains large amounts of furancarboxylic acids. The same acids were isolated from rats after feeding with methyl esters of the so-called F-acids. Since the F-acids were not detected in cattle-food, they must be produced somewhere in the body of cattle. 相似文献
76.
Cornelissen C Erker G Kehr G Fröhlich R 《Dalton transactions (Cambridge, England : 2003)》2004,(23):4059-4063
Treatment of the complexes [(C(5)H(4)PR(2))(2)Zr(CH(3))(2)](b: R = isopropyl; c: R = cyclohexyl) with the reagent HIr(CO)(PPh(3))(3) (2b) yield the heterobimetallic complexes [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Ir(CO)(PPh(3)))] (4b, 4c) with evolution of methane. The reaction of the -PPh(2) substituted analogue with initially yields an intermediate [(H(3)C)(2)Zr(mu-C(5)H(4)PPh(2))(2)Ir(H)(CO)(PPh(3))] 5a, that still contains both methyl groups at zirconium and does not contain a metal-metal bond. At room temperature, the intermediate reacts further with methane formation to eventually yield the (Zr-Ir) complex 4a. The corresponding [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Rh(CO)(PPh(3)))] complexes 3a (R = Ph) and 3b (R = isopropyl) react cleanly with isopropyl alcohol to liberate methane and yield the corresponding [mu-C(5)H(4)PR(2))(2)(Me(2)CHO-Zr-Rh(CO)(PPh(3)))] products (7a, 7b). Carefully monitoring the reaction of with Me(2)CHOH by NMR revealed that the Zr-Rh functionality is attacked first to give the intermediate [Me(Me(2)CHO)Zr([micro sign]-C(5)H(4)PR(2))(2)Rh(H)(CO)(PPh(3))] (6b). This intermediate then reacts further to cleave off methane and re-form the (Zr-Rh) metal-metal bond to yield the product 7b. The tetrametallic mu-oxo-(Zr-Rh) metallocene derivate 11a was obtained starting from the (Zr-Rh) complex 3a and it was characterized by X-ray diffraction. It may be that this reaction is also initiated by H-OH addition to the [Zr-Rh] metal-metal bond. 相似文献
77.
The total synthesis of rhodoxanthin according to the scheme C14 + C12 + C14 is reported. 相似文献
78.
79.
Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions. 相似文献
80.
Zusammenfassung Flüssige und feste Legierungen im System Au–Cd wurden von 60 bis 90 At% Cd zwischen 653° und 873°K mit derEMK-Methode untersucht und thermodynamische Eigenschaften berechnet. Die Aktivitäten von Cd zeigen stark negative Abweichungen vom idealen Verhalten. Die partiellen molaren Enthalpien sind negativ und verschieben sich mit zunehmender Temperatur zu weniger negativen Werten, die partiellen molaren Entropien sind positiv und nehmen mit zunehmender Temperatur zu. Aus eigenen Werten und Literaturdaten wurden integrale thermodynamische Größen für die Schmelzen im gesamten System Au–Cd bei 1000°K berechnet. Für einige feste Cd-reiche Phasen wurden ebenfalls Aktivitäten erhalten und aus den Knickpunkten derEMK-Temperatur-Kurve Temperaturen von Phasenübergängen bestimmt.
Mit 5 Abbildungen
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献
Thermodynamic investigations in theAu–Cd-system
Liquid and solid alloys in the Au–Cd system were investigated between 60 and 90 at % Cd and 653°–873°K by theemf method, and thermodynamic properties were calculated. The activities of Cd exhibit strong negative deviations from ideal behavior. Partial molar enthalpies are negative and become less so with increasing temperature; partial molar entropies are positive and increase with increasing temperature. A consistent set of integral thermodynamic data for liquid solutions of the entire Au–Cd system at 1000°K were calculated using values taken from the literature. Activities for some solid Cd-rich phases were also obtained. From discontinuities in theemf vs. temperature curve temperatures of phase transitions were determined.
Mit 5 Abbildungen
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献