全文获取类型
收费全文 | 5925篇 |
免费 | 322篇 |
国内免费 | 25篇 |
专业分类
化学 | 4739篇 |
晶体学 | 20篇 |
力学 | 114篇 |
数学 | 574篇 |
物理学 | 825篇 |
出版年
2023年 | 61篇 |
2022年 | 79篇 |
2021年 | 108篇 |
2020年 | 135篇 |
2019年 | 154篇 |
2018年 | 124篇 |
2017年 | 105篇 |
2016年 | 241篇 |
2015年 | 186篇 |
2014年 | 196篇 |
2013年 | 306篇 |
2012年 | 523篇 |
2011年 | 596篇 |
2010年 | 269篇 |
2009年 | 204篇 |
2008年 | 444篇 |
2007年 | 440篇 |
2006年 | 447篇 |
2005年 | 412篇 |
2004年 | 291篇 |
2003年 | 253篇 |
2002年 | 209篇 |
2001年 | 61篇 |
2000年 | 33篇 |
1999年 | 29篇 |
1998年 | 31篇 |
1997年 | 32篇 |
1996年 | 28篇 |
1995年 | 23篇 |
1994年 | 16篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 18篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1971年 | 4篇 |
1967年 | 6篇 |
排序方式: 共有6272条查询结果,搜索用时 187 毫秒
191.
Tucker MJ Getahun Z Nanda V DeGrado WF Gai F 《Journal of the American Chemical Society》2004,126(16):5078-5079
We studied here the binding of the mastoparan X peptide to a zwitterionic lipid bilayer (POPC) and demonstrated that nitrile-derivatized amino acids can be used to determine the hydration state (or change in hydration state) of specific sites of membrane-interactive peptides (upon binding). We have also shown that polarized ATR-FTIR measurements can further be used to uncover information regarding the spatial orientation of individual side chains as well as their conformational preference within the lipid bilayer. 相似文献
192.
Tullio Caronna Bruno M. Vittimberga Matthew E. Kornn W. Grant McGimpsey 《Journal of photochemistry and photobiology. A, Chemistry》1995,90(2-3):137-140
The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λmax ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.
Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λmax ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10−8 M−1 s−1.
In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λmax ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies. 相似文献
193.
The electron distributions and bonding in Ru3(CO)9(
3-
2,
2,
2-C6H6) and Ru3(CO)9(
3-
2,
2,
2-C60) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru3(CO)9 inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru–Cfullerene bond lengths are shorter than the corresponding Ru–Cbenzene distances and the Ru–Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru3(CO)9(
3-
2,
2,
2-C60) than for Ru3(CO)9(
3-
2,
2,
2-C6H6). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru–Cfullerene than Ru–Cbenzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal–metal bonds to empty orbitals of C60 and C6H6. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C60 than to C6H6, thus accounting for the longer metal–metal bonds in the fullerene-bound cluster. The principal metal–arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C6H6 permits a greater degree of electron flow and stronger bonding between the Ru3(CO)9 and C60 fragments. Of significance to the reduction chemistry of M3(CO)9(
3-
2,
2,
2-C60) molecules, the HOMO is largely localized on the metal–carbonyl fragment and the LUMO is largely localized on the C60 portion of the molecule. The localized C60 character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C60. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C60. 相似文献
194.
An ab initio-based kinetic Monte Carlo algorithm was developed to simulate the direct decomposition of NO over Pt and different PtAu alloy surfaces. The algorithm was used to test the influence of the composition and the specific atomic surface structure of the alloy on the simulated activity and selectivity to form N2. The apparent activation barrier found for the simulation of lean NO decomposition over Pt(100) was 7.4 kcal/mol, which is lower than the experimental value of 11 kcal/mol that was determined over supported Pt nanoparticles. Differences are likely due to differences in the surface structure between the ideal (100) surface and supported Pt particles. The apparent reaction orders for lean NO decomposition over the Pt(100) substrate were calculated to be 0.9 and -0.5 for NO and O2, respectively. Oxygen acts to poison Pt. Simulations on the different Pt-Au(100) surface alloys indicate that the turnover frequency goes through a maximum as the Au composition in the surface is increased, and the maximum occurs near 44% Au. Turnover frequencies, however, are dictated by the actual arrangements of Pt and Au atoms in the surface rather than by their overall composition. Surfaces with similar compositions but different alloy arrangements can lead to very different activities. Surfaces composed of 50% Pt and 50% Au (Pt4 and Au4 surface ensembles) showed very little enhancement in the activity over that which was found over pure Pt. The Pt-Pt bridge sites required for NO adsorption and decomposition were still effectively poisoned by atomic oxygen. The well-dispersed Pt(50%)Au(50%) alloy, on the other hand, increased the TOF over that found for pure Pt by a factor of 2. The most active surface alloy was one in which the Pt was arranged into "+" ensembles surrounded by Au atoms. The overall composition of this surface is Pt(56.2%)Au(43.8%). The unique "+" ensembles maintain Pt bridge sites for NO to adsorb on but limit O2 as well as NO activation by eliminating next-nearest neighbor Pt-bridge sites. The repulsive interactions between two adatoms prevent them from sharing the same metal atoms. The decrease in the oxygen coverage leads to a greater number of vacant sites available for NO adsorption. This increases the NO coupling reaction and hence N2 formation. The inhibition of the rate of N2 formation by O2 is therefore suppressed. The coverage of atomic oxygen decreases from 53% on the Pt(100) surface down to 19% on the "+" ensemble surface. This increases the rate of N2 formation by a factor of 4.3 over that on pure Pt. The reaction kinetics over the "+" ensemble Pt(56.2%)Au(43.8%) surface indicate apparent reaction orders in NO and oxygen of 0.7 and 0.0, respectively. This suggests that oxygen does not poison the PtAu "+" alloy ensemble. The activity and selectivity of the PtAu ensembles significantly decrease for alloys that go beyond 60% Au. Higher coverages of Au shut down sites for NO adsorption and, in addition, weaken the NO and O bond strengths, which subsequently promotes desorption as well as NO oxidation. The computational approach identified herein can be used to more rapidly test different metal compositions and their explicit atomic arrangements for improved catalytic performance. This can be done "in silico" and thus provides a method that may aid high-throughput experimental efforts in the design of new materials. The synthesis and stability of the metal complexes suggested herein still ultimately need to be tested. 相似文献
195.
196.
197.
198.
199.
200.
Tetiana Krachko Mark Bispinghoff Dr. Aaron M. Tondreau Dr. Daniel Stein Matthew Baker Dr. Andreas W. Ehlers Prof. Dr. J. Chris Slootweg Prof. Dr. Hansjörg Grützmacher 《Angewandte Chemie (International ed. in English)》2017,56(27):7948-7951
Phosphinidenes [R-P] are convenient P1 building blocks for the synthesis of a plethora of organophosphorus compounds. Thus far, transition-metal-complexed phosphinidenes have been used for their singlet ground-state reactivity to promote selective addition and insertion reactions. One disadvantage of this approach is that after transfer of the P1 moiety to the substrate, a challenging demetallation step is required to provide the free phosphine. We report a simple method that enables the Lewis acid promoted transfer of phenylphosphinidene, [PhP], from NHC=PPh adducts (NHC=N-heterocyclic carbene) to various substrates to produce directly uncoordinated phosphorus heterocycles that are difficult to obtain otherwise. 相似文献