全文获取类型
收费全文 | 2835篇 |
免费 | 59篇 |
国内免费 | 41篇 |
专业分类
化学 | 1784篇 |
晶体学 | 15篇 |
力学 | 75篇 |
数学 | 364篇 |
物理学 | 697篇 |
出版年
2021年 | 28篇 |
2020年 | 24篇 |
2019年 | 22篇 |
2018年 | 19篇 |
2016年 | 36篇 |
2015年 | 32篇 |
2014年 | 41篇 |
2013年 | 107篇 |
2012年 | 102篇 |
2011年 | 138篇 |
2010年 | 82篇 |
2009年 | 57篇 |
2008年 | 124篇 |
2007年 | 151篇 |
2006年 | 127篇 |
2005年 | 114篇 |
2004年 | 112篇 |
2003年 | 107篇 |
2002年 | 105篇 |
2001年 | 80篇 |
2000年 | 76篇 |
1999年 | 57篇 |
1998年 | 42篇 |
1997年 | 43篇 |
1996年 | 51篇 |
1995年 | 39篇 |
1994年 | 51篇 |
1993年 | 52篇 |
1992年 | 47篇 |
1991年 | 29篇 |
1990年 | 44篇 |
1989年 | 41篇 |
1988年 | 26篇 |
1987年 | 21篇 |
1986年 | 25篇 |
1985年 | 53篇 |
1984年 | 38篇 |
1983年 | 26篇 |
1982年 | 40篇 |
1981年 | 32篇 |
1980年 | 42篇 |
1979年 | 36篇 |
1978年 | 48篇 |
1977年 | 40篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 21篇 |
1973年 | 43篇 |
1972年 | 16篇 |
1971年 | 17篇 |
排序方式: 共有2935条查询结果,搜索用时 15 毫秒
111.
112.
William J. Evans 《Journal of organometallic chemistry》1978,154(3):C50-C51
The (Me3Si)3C group causes very large steric hindrance to nucleophilic displacement at a silicon atom to which it is attached, and (Me3Si)3CSiMe2Cl is even less reactive than t-Bu3SiCl towards base. The compounds (Me3Si)3CSiMe2X (X = Cl, Br, or I) are cleaved by MeOH/MeONa to give (Me3Si)2CHSiMe2OMe, possibly via the silaolefin (Me3Si)2 CSiMe2, and the correspondLug (Me3Si)3 CSiPh2X compounds undergo the analogous reaction even more readily. The halides (Me3Si)3CSiR2X (X = Cl or Br) and (Me3Si)3CSiCl3 do not react with boiling alcoholic silver nitrate, but the iodides (Me3Si)3CSiR2I are rapidly attacked. 相似文献
113.
The ESR and ENDOR spectra of the radical cation of N,N′-bis-(4-fluoro-phenyl)-4-4′-bipyridylium dichloride (fluorophenylquat FPQ) in methanol was studied over a temperature range from +_40° to ?90°. The ENDOR technique was used to obtain accurately the splitting constans for a highly complicated ESR spectrum and computer simulation showed excellent agreement. Fluorine ENDOR resonance was clearly observed with a line width similar to that of the protons. On decreasing the temperature the concentration of the radical cation decreases until at ?90° the ESR intensity was very small. This process is reversible and concentration studies indicate that the radical cation is in equilibrium with a diamagnetic dimer species. The thermodynamic parameters ΔH°, ΔG° and ΔS° for the process are reported. 相似文献
114.
Using single-crystal, automated diffractometer techniques, the linear coefficient of thermal expansion has been determined for La0.62Pb0.38MnO3 from 298 to 627 K. The linear coefficient of thermal expansion is observed to undergo a change from 7.2 × 10?5 Å/K for T < Tc to 10.8 × 10?5 Å/K for T > Tc. It is concluded that while the rhombohedral distortion in the (La, Pb)MnO3 system can be understood qualitatively on the basis of ionic size and polarizability considerations alone, the quantitative systematics of the distortion parameters and the change in the linear thermal expansion coefficient at Tc indicate a significant coupling between the elastic and magnetic exchange forces. 相似文献
115.
Rodger D. Scurlock Christopher H. Evans Silvia E. Braslavsky Kurt Schaffner 《Photochemistry and photobiology》1993,58(1):106-115
The mechanism of I700 decay, representing an early event in the phytochrome Pr→ Pfr phototransformation, was reanalyzed in the microsecond range by conventional laser flash photolysis as well as by two-laser/two-color flash photolysis. Three kinetic models that might describe the I700 decay mechanism following Pr excitation were considered: a parallel, a sequential, and an equilibrium model. These models were used to mathematically simulate both the one- and two-laser flash experiments in an effort to select the model best describing the I700 decay. The sequential model could be excluded already on the basis of the one-laser flash photolysis results alone. Discussion of the two-laser/two-color flash rcsults in the context of the equilibrium and the parallel models is presented. 相似文献
116.
D.J. Evans L.A.P. Kane-Maguire D.A. Sweigart 《Journal of organometallic chemistry》1981,215(2):C27-C29
Kinetic results for the addition of OH? to [Mn(CO)3(η-C6H6)]+ (I) in water (eq. 1, X OH) obey the expression kobs kOH[OH?], and give a kOH value of 290 mol?1 dm3 s?1 at 20.0°C and ionic strength of 0.25 mol dm?3. The analogous reaction of NaCN with I in water fits the two-term expression kobs = kOH[OH?] + kCN[CN?], and leads to a kCN value of 0.8 mol?1 dm3 s?1 at 20.0°C and ionic strength of 0.25 mol dm?3. Interestingly, the related reaction (eq. 1, X N3) is too rapid to follow by stopped-flow spectrophotometry, indicating the overall rate trend N3? » OH? » CN?. This unusual nucleophilicity order, unexpected on the basis of both basicity and polarizability, is similar to that previously observed for anion addition to free carbonium ions. 相似文献
117.
M. B. Evans 《Chromatographia》1978,11(4):183-187
Summary The limitations of methods for the characterization of stationary phases, which require the use of a base or reference stationary
phase, are considered. In order to overcome these problems, a new method of column characterization, based upon measurements
of molecular retention index and ΔMe values for selected functional groups, is proposed. ΔMe values are shown to be dependent
upon liquid phase composition, nature of the support and liquid phase loading. 相似文献
118.
First, we evaluated the complete electric dipole moment matrixeZ using a multi- AO basis on Cr, C, and O centers fortheimportant, parent organometallic molecule -C6H6Cr(CO)3. Second, we generated ground state LCAO-MO eigenvectors employing five commonly used semi-empirical procedures for molecules of this size which contain transition metals. Transformation of operator matrixeZ into the MO space of each of the five methods then leads us to conclude that the empirical input of Basch, Viste and Gray is most suitable. Finally, we compare the merits of several single- basis orbitals with the multi- metal orbitals.[/p] 相似文献
119.
Narayani Ramakrishnan Marian E. Clay Liang-YAN Xue Helen H. Evans Antonio Rodriguez-Antunez Nancy L. Oleinick 《Photochemistry and photobiology》1988,48(3):297-303
Abstract— Chloroaluminum phthalocyanine (CAPC) is an efficient photosensitizer for the inactivation of Chinese hamster V79 cells. In order to investigate possible molecular mechanisms in the photo-dynamic action of CAPC and visible light, the induction and repair rate of two classes of DNA lesions have been determined, i.e. DNA single-strand breaks and DNA-protein cross-links. In cells pretreated with 1 μ.M CAPC, a fluence of 12 kJ/m2 of red light (>600 nm) kills approximately 50% of the cells and induces 3 to 3.5 Gy-equivalents of single-strand breaks. The repair of these breaks was slower than the repair of single-strand breaks induced by -irradiation. The photodynamic action of CAPC also induces a large number of DNA-protein cross-links which, in contrast to -radiation-induced DNA-protein cross-links, do not appear to be repaired during 4 h of post-treatment incubation in fresh medium. These studies suggest that DNA may be an important target for the cytotoxicity of CAPC + red light. 相似文献
120.
Deacon GB Evans DJ Junk PC Lork E Mews R Zemva B 《Dalton transactions (Cambridge, England : 2003)》2005,(13):2237-2238
Addition of 15-crown-5 to [GdF(AsF6)2], both dissolved in liquid SO2, and crystallisation at -30 degrees C has led to the isolation of the tetranuclear ionic complex [Gd4F7(15-crown-5)4][AsF6]5.6 SO2 which is stable up to--10 degrees C where SO2 loss leads to loss of crystallinity. 相似文献