全文获取类型
收费全文 | 3866篇 |
免费 | 66篇 |
国内免费 | 44篇 |
专业分类
化学 | 2716篇 |
晶体学 | 28篇 |
力学 | 55篇 |
数学 | 673篇 |
物理学 | 504篇 |
出版年
2022年 | 18篇 |
2021年 | 36篇 |
2020年 | 21篇 |
2019年 | 44篇 |
2018年 | 24篇 |
2017年 | 31篇 |
2016年 | 76篇 |
2015年 | 52篇 |
2014年 | 63篇 |
2013年 | 194篇 |
2012年 | 171篇 |
2011年 | 201篇 |
2010年 | 101篇 |
2009年 | 82篇 |
2008年 | 239篇 |
2007年 | 256篇 |
2006年 | 262篇 |
2005年 | 262篇 |
2004年 | 204篇 |
2003年 | 140篇 |
2002年 | 134篇 |
2001年 | 47篇 |
2000年 | 60篇 |
1999年 | 54篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 74篇 |
1995年 | 53篇 |
1994年 | 57篇 |
1993年 | 40篇 |
1992年 | 43篇 |
1991年 | 30篇 |
1990年 | 32篇 |
1989年 | 27篇 |
1988年 | 32篇 |
1987年 | 35篇 |
1986年 | 46篇 |
1985年 | 60篇 |
1984年 | 63篇 |
1983年 | 51篇 |
1982年 | 43篇 |
1981年 | 47篇 |
1980年 | 53篇 |
1979年 | 34篇 |
1978年 | 49篇 |
1977年 | 37篇 |
1976年 | 30篇 |
1975年 | 26篇 |
1974年 | 26篇 |
1973年 | 34篇 |
排序方式: 共有3976条查询结果,搜索用时 15 毫秒
91.
In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K1 = 267 and K2 approximately 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)2] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930(4) A, beta = 116.246(1) degrees , V = 1992.95(10) A3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-Np distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A. 相似文献
92.
Andrew M. Poitras Sadie E. Knight Mark W. Bezpalko Bruce M. Foxman Christine M. Thomas 《Angewandte Chemie (International ed. in English)》2018,57(6):1497-1500
Addition of H2 across the cobalt–phosphorus bond of (PPP)CoPMe3 ( 3 ) is demonstrated, where PPP is a monoanionic diphosphine pincer ligand with a central N‐heterocyclic phosphido (NHP?) donor. The chlorophosphine CoII complex (PPClP)CoCl2 ( 2 ) can be generated through coordination of the chlorophosphine ligand (PPClP, 1 ) to CoCl2. Subsequent reduction of 2 with KC8 in the presence of PMe3 generates (PPP)CoPMe3 ( 3 ), in which both the phosphorus and cobalt centers have been reduced. The addition of 1 atm of H2 to complex 3 cleanly affords (PPHP)Co(H)PMe3 ( 4 ), in which H2 has ultimately been added across the metal–phosphorus bond. Complex 4 was characterized spectroscopically and using computational methods to predict its geometry. 相似文献
93.
We report the feasibility of multistage fragmentation in combination with a fast background subtraction method, yielding the equivalent of MS3. The first quadrupole selects an ion of interest, and the ion is axially accelerated into Q2 to generate fragment ions. Subsequent stages of mass selection and fragmentation are obtained by quadrupolar resonant excitation within the Q2 collision cell. The fragments are analyzed downstream by either a resolving quadrupole or a time-of-flight (TOF) mass spectrometer, and multistage spectra are obtained by subtraction (MS(n) - MS(n-1)) for n = 3 or 4. We discuss the characterization of this method, including product ion arrival times, fragmentation efficiencies, and ion selectivity. We report accurate TOF mass spectra of background-subtracted MS3 for protonated molecules reserpine (m/z 609), bosentan (m/z 1552), and taxol (m/z 854). 相似文献
94.
95.
Luis F. Lafuerza Louise Dyson Bruce Edmonds Alan J. McKane 《The European Physical Journal B - Condensed Matter and Complex Systems》2016,89(7):159
A recently proposed model of social interaction in voting is investigated by simplifyingit down into a version that is more analytically tractable and which allows a mathematicalanalysis to be performed. This analysis clarifies the interplay of the different elementspresent in the system – social influence, heterogeneity and noise – and leads to a betterunderstanding of its properties. The origin of a regime of bistability is identified. Theinsight gained in this way gives further intuition into the behaviour of the originalmodel. 相似文献
96.
97.
Guillaume Berionni Bruce Pégot Régis Goumont 《Magnetic resonance in chemistry : MRC》2010,48(2):101-110
The 15N as well as 1H and 13C chemical shifts of nine substituted tetrazolopyridines and their corresponding tetrazolopyridinium salts have been determined by using NMR spectroscopy at the natural abundance level of all nuclei in CD3CN. In this paper, we report, for the first time, the N‐alkylation reaction of electron deficient tetrazolopyridines. The treatment of tetrazolopyridines 5–13 with one equivalent of trialkyloxonium tetrafluoroborate leads to a mixture of two isomers, i.e. N3‐ and N2‐alkyl tetrazolo[1,5‐a]pyridinium salts. It has been observed that the N3‐isomer is always the major isomer, except in the case of the CF3 substituent, where the two isomers are obtained in the same amount. The quaternary tetrazolopyridinium nitrogen N3 is shielded by around 100 ppm (parts per million) with respect to the parent tetrazolopyridine. Experimental data are interpreted by means of density functional theory (DFT) calculations, including solvent‐induced effects, within the conductor‐like polarizable continuum model (CPCM). Good agreements between theoretical and experimental 1H, 13C and 15N NMR were found. The combination of multinuclear magnetic resonance spectroscopy with gauge including atomic orbital (GIAO) DFT calculations is a powerful tool in the structural elucidation for both neutral and cationic heterocycles and in the determination of the orientation of N‐alkylation of tetrazolopyridines. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
98.
99.
The standard reduction potential of the Pb2+ (aq)|Pb(s) half-cell at 298 K has been re-determined and found to be ? 126.7 ± 0.2 mV, 0.4 mV more negative than the currently accepted value. The new value is used to calculate a self-consistent set of solubility products for several sparingly soluble lead salts using the known E0s of the appropriate lead-lead salt electrodes. Activity. Activity coefficients of lead chloride in aqueous solution are also given. 相似文献
100.
Michael I. Bruce Omar bin Shawkataly Brian K. Nicholson 《Journal of organometallic chemistry》1985,286(3):427-437
H2Ru3(μ3-S)(CO)9 is deprotonated by K[HBBus3] to give cluster anions which react with [O{Au(PPh3)}3]+ or with AuCl(PPh3)/T1+ to give HRu3Au(μ3-S)(CO)9(PPh3) (1) and Ru3Au2(μ3-S)(CO)9(PPh3)2 (3). A similar sequence with HRu3(μ3-SBut)(CO)9 leads to Ru3Au(μ3-SBut)(CO)9(PPh3) (2) as the main product although some 1 also forms, indicating SC cleavage competes with deprotonation of HRu3(μ3-SBut)(CO)9 by [HBBus3]?. The X-ray crystal structures of 1, 2 and 3 are described; (1) and (2) have “butterfly” AuRu3 cores with markedly different hinge angles of 119 and 148° respectively, while 3 has a trigonal-bipyramidal Au2Ru3 skeleton. All three clusters have the sulphur atom symmetrically bridging the Ru3 triangular face. 相似文献